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Journal of Geographical Sciences >

2019 , Vol. 29 >Issue 5: 791 - 802

DOI: https://doi.org/10.1007/s11442-019-1628-8

Evaluating the soil quality of newly created farmland in the hilly and gully region on the Loess Plateau, China

  • CHEN Yiping , 1 ,
  • WU Junhua 1, 2 ,
  • WANG Hong 1, 2 ,
  • MA Jifu 1, 2 ,
  • SU Cuicui 1, 2 ,
  • WANG Kaibo 1 ,
  • WANG Yi 1
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  • 1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS, Xi’an 710061, China;
  • 2. University of the Chinese Academy of Sciences, Beijing 100049, China

Author: Chen Yiping (1968-), Professor, specialized in environmental biology science and ecological process management. E-mail:

Received date: 2018-07-11

  Accepted date: 2018-12-15

  Online published: 2019-04-19

Supported by

National Key Research and Development Program of China, No.2017YFD0800500

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Journal of Geographical Sciences, All Rights Reserved
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Abstract

In order to better understand the quality of newly created farmland (NF) as well as slope and check-dam farmland (CF) soil quality, two typical traditional farmland areas within the hilly and gully region of Chinese Loess Plateau were selected as controls in this analysis. The results of this study initially reveal that pH values for slope farmland (SF) and CF were significantly lower (P < 0.05) than those for NF and that SF values were always greater than those of their CF counterparts. At the same time, cation exchange capacity (CEC) and soil organic matter (SOM) occurred in the following size-decreasing sequence, i.e., CF, SF, and NF. Data also show that long-term planting has resulted in increases in total nitrogen (N), available N, total phosphorus (P), available P, total potassium (K) and available K and has also enhanced concentrations of soil copper (Cu), zinc (Zn), iron (Fe), and manganese (Mg) as well as soil cadmium (Ca), lead (Pb), chromium (Cr), and mercury (Hg). These latter elements occur in the concentration sequence of SF > CF > NF. Overall, results indicate that long-term planting negatively impacts soil health because of the accumulation of toxic heavy metals. This means that farmland planted over longer timescales needs to lie fallow to enable it to rehabilitate while NF requires the addition of organic matter as well as rotational utilization in order to facilitate the development of green agriculture.

Key words： Loess Plateau; land use; soil quality; heavy metals

Cite this article

CHEN Yiping , WU Junhua , WANG Hong , MA Jifu , SU Cuicui , WANG Kaibo , WANG Yi . Evaluating the soil quality of newly created farmland in the hilly and gully region on the Loess Plateau, China[J]. Journal of Geographical Sciences, 2019 , 29(5) : 791 -802 . DOI: 10.1007/s11442-019-1628-8

1 Introduction

The Loess Plateau covers approximately 62.4 × 104 km2 within China and comprises one of the most severe regions for soil erosion globally because of numerous gullies. The dominant form of agricultural farmland in this region is found on slopes less than 15°, encompassing 56.3% of the total on the plateau. A second abundant form of farmland is of a check-dam type, the most widespread structures used for controlling soil and water loss in this region, which encompass about 9% of the overall total. Check-dam farming is, however, responsible for 20.5% of the total food production derived from the hilly and gully region (Chen et al., 2015; Liu et al., 2018).
The Chinese government implemented the Grain for Green (GFG) project in 1999 in order to reduce soil erosion, the largest vegetation ecological restoration program put in place since the founding of the People’s Republic of China in 1949. Vegetation coverage on the Loess Plateau has greatly increased from 31.6% in 1999 to 59.6% in 2013 (Wang et al., 2011). Regional soil erosion has therefore been effectively reduced and annual sediment discharge into the Yellow River declined to 1.73 Gt in 2013, close to primordial agricultural period levels (Chen et al., 2015). This largest vegetation restoration program also resulted in a grain deficit in partial regions on the Loess Plateau because this area exceeded the 105% of the upper limited area set by the GFG program (2.36 million ha) in 2008. As a result, 4.83 million ha of cropland was converted to forest and grassland on the Loess Plateau between 2000 and 2008, which doubled the limitation area for the GFG Program (Tang et al., 1998; Lu et al., 2012).
Liu et al. (2015) analyzed the benefits of useful agricultural productivity to meet the food supply needs of an expanding population. In order to address this contradiction between population and farmland shortages, the Chinese government implemented a program entitled ‘Filling gullies to create farmland on the Loess Plateau’ within the city of Yan’an on the Loess Plateau hilly and gully region in Shaanxi Province. This area within northern China includes 12.68% of the Loess Plateau area. Around 800 million USD dollars were invested in this project which created 333 million hectares of farmland between 2013 and 2017 (He, 2015; Liu et al., 2015). This approach, filling gullies to create farmland, involves the removal of soil from surrounding hills and then using this to infill channels. The flat fields created within these channels are then used for cultivation and ultimately increase the area of farmland on the Loess Plateau.
The completion of this gully filling project is one desirable outcome for regional sustainable development. It is clear that soil fertility plays a key role in both the growth of crops and their productivity. At the same time, however, little information is currently available about whether the fertility of new farmland will meet the needs for crop growth. In order to assess this outstanding question, a series of soil samples were collected from newly created (NF), slope (SF), and check-dam (CF) farmland and a range of physical and chemical parameters (i.e., pH, cation exchange capacity (CEC), organic matter content, concentrations of total nitrogen (N), available N, total phosphorus (P), available P, total potassium (K), and available K as well as heavy metals were measured in order to evaluate both the quality of NF and agricultural sustainability. Typical and primary farmland types (i.e., SF and CF lands) were selected as comparative samples.

2 Materials and methods

2.1 Study area and sample site

The soil samples evaluated in this study were collected from within the Liangjiahe (110°43'E, 36°50'N) and Nangou watersheds (109°17'E, 36°35'N), examples of typical hilly and gully regions on the Loess Plateau (Figure 1). These watersheds encompass the towns of Wen’anyi and Gaoqiao, respectively, within the city of Yan’an in Shaanxi Province. The average annual temperature in this region ranges between 8.5℃ and 9.5℃, accumulated temperature (greater than or equal to 10℃) ranges between 3100℃and 3300℃, the frost-free period ranges between 162 days and 190 days, and average annual precipitation ranges between 520 mm and 550 mm, all indicating a warm-temperate, arid and semi-arid climate zone (Zhang et al., 2016). Almost 70% of regional precipitation is concentrated in summer months when there is a high probability of heavy rains, floods, and droughts (Zhou et al., 1997).
Figure 1 Maps showing the sample collection site. The five-pointed star denotes the sampling sites within Shaanxi Province, China.
Nine sample plots of 10 m × 10 m each were selected from NF, SF, and CF regions, respectively. Specifically, NF areas within the Liangjiahe and Nangou watersheds were created in 2015 and have subsequently not been cultivated, while CF areas were implemented in the 1970s, and SF areas have been cultivated for more than 40 years. Maize (Zea mays L.) and potato (Solanum tuberosum L.) are cultivated extensively in both SF and CF areas. A total of five sample points were selected in each case using the ‘diagonal’ method during winter 2017; plough layer soil was collected from the surface to a depth of 20 cm and was mixed evenly before approximately 1 kg of loess was placed in polythene bags and brought to the laboratory for analysis. The soil type within these sampling regions is Huangmian Soil (Calcaric Cambisols, FAO) developed from loess parent material.

2.2 Sample analysis

Composite soil samples were sieved through a 2 mm nylon griddle to completely remove sundries, pebbles, roots, and litter before being air dried at room temperature. Subsamples of less than 2 mm air-dried soil were again ground to pass through 0.149 mm, 0.25 mm, and 1.00 mm nylon sieves for further soil chemical analyses. The fraction less than 1.00 mm in size was used for soil pH as well as available N, P, K, and trace element analyses. The fraction less than 0.25 mm in size was then also used for soil organic matter as well as total N, P, K, and soil CEC capacity analyses, while the fraction less than 0.149 mm in size was used for total trace elements and heavy metal analyses.
Soil pH values were determined using the potential method (i.e., soil:water ratio 1:2.5) (GB7859-1987), while organic matter content was determined via hot oxidation with potassium dichromate and sulfuric acid (Yeomans et al., 1988). Soil total N and P contents were determined using the Kjeldahl digestion procedure (Bremner et al., 1972) and molybdenum antimony blue colorimetry (Murphy et al., 1962), respectively, while total K was determined using the HF-HClO4 digestion method (Jackson, 1969). The amount of available N was released and transformed to NH3 using 1.07 M NaOH and FeSO4 powder at 40℃for 24h, and then absorbed with 2% (w/v) H3BO3, and finally titrated with 0.005M H2SO4 (Lu, 2000). The amounts of available phosphorus and potassium were then determined by extracting samples with 0.5 M NaHCO3 (pH = 8.5) at a solution:solid ratio of 20:1 for 30 minutes (Olsen et al., 1982) and 0.2 M cold HNO3 at a solution:solid ratio of 20:1 for 30 minutes (Bao, 2002), respectively. Soil CEC was measured following soil treatment with NaOAc (pH = 8.2) and NH4OAc solutions (pH = 7.0) (Lu, 2000).
Values for available iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) in soils were determined using the diethylenetriaminepentaacetic acid (DTPA) micronutrient extraction method (Lindsay et al., 1978). In order to determine contents of total Fe, Mn, Zn, Cu, chromium (Cr), cadmium (Cd), lead (Pb), and mercury (Hg) in soils, samples were digested with a typical concentrated acid mixture (HCl-HNO3-HF-HClO4) (Zheng et al., 2016) and metal concentrations were analyzed using atomic absorption spectroscopy (AAS; ZEEnit 700 P, Analytik Jena, Germany). Concentrations of Fe, Cu, Zn, Mn, and Cr were measured using the AAS air acetylene flame method applying an electrically modulated deuterium-HCL background correction. Hydride-forming Hg elements were measured using the AAS HS55 Hydride System, while concentrations of Pb and Cd were measured using the AAS graphite furnace coupled with a MPE 60 graphite autosampler, applying two-field mode Zeeman effects background correction. All metal concentrations were tested three times in this study and expressed in micrograms per gram on a dry weight basis (mg•kg-1 dw).

2.3 Statistical analysis

One-way analysis of variance was used to determine the effects of land use on soil quality indicators, while the least significant difference (LSD) method was applied for multiple comparisons. All statistical analyses were performed using the software SPSS 20.0 (IBM SPSS Statistics, IBM Corp., USA), and a significance level P < 0.05 was applied unless stated otherwise.

3 Results

3.1 Soil pH, CEC, and organic matter

The results of this analysis reveal that different types of farmland are characterized by a range of pH values (Figure 2). Data show that pH values for loess were significantly higher than the national average values (pH ≈ 6.8) from a second soil survey (1979-1983), while compared with NF, pH values for SF and CF were significantly lower (P < 0.05) than those for NF but for SF pH values were greater than those recorded for CF soils.
Figure 2 Comparisons between soil pH values in three loess types. Bars (means values ± SD from six independent experiments) with different letters are significantly different at the 0.05 level (n = 6) based on LSD multiple comparisons. The dotted line denotes soil pH background levels (1979-1983).
The data presented in Figure 3 highlight changes in CEC (Figure 3a) and organic matter content (Figure 3b) in the different farmland types. The CEC size sequence recovered in this analysis (from largest-to-smallest) was CF, SF, and NF; differences between these three types were statistically significant (P < 0.05) (Figure 3a). Indeed, compared with NF samples, long-term planting has resulted in an increase in soil organic matter concentration (Figure 3b). The organic matter order recovered here (from largest-to-smallest) was CF, SF, and NF; data show that the NF content is lower than values seen in nine grade standards from Shaanxi Province (less than 6.00 g•kg-1) (Guo, 1992) and therefore extremely deficient and unfavorable for crop growth and development.
Figure 3 Comparisons between concentrations of soil CEC (a) and organic matter (b) in three types of loess soils. Bars (means values ± SD from three independent experiments for CEC or six independent experiments for organic matter) with different letters are significantly different at the 0.05 level based on LSD multiple comparisons.

3.2 Soil macro elements

The data presented in Figure 4 illustrates changes in macroelement content within different types of farmland. Results show that compared with NF values, long-term planting has resulted in increases in total (Figure 4a) and available N (Figure 4b) as well as total (Figure 4c) and available P (Figure 4d), and total (Figure 4e) and available K (Figure 4f) concentrations. The order of change recovered in concentrations of total and available N as well as total and available K (from largest-to-smallest) were CF, SF, and NF, while concentrations of total (greater than 0.6 g•mg-1) and available N (greater than 30 g•mg-1) were all below basic six grade criteria for national nutrient levels on NF. These data show that NF soils within this study are unhealthy even though these regions are characterized by abundant potassium, at higher levels than those required by loess criteria (i.e., greater than 6 mg•kg-1 for total and greater than 50 mg•kg-1 for available K, respectively) (Guo, 1992). The concentration order of change for total and available P (from largest-to-smallest) was SF, CF, and NF; total and available P concentrations did not, however, attain necessary loess criteria in any sample (i.e., greater than 0.6 mg•kg-1 and greater than 5 mg•kg-1, respectively) (Guo, 1992).
Figure 4 Comparisons between concentrations of total N (a), available N (b), total P (c), available P (d), total K (e) and available K (f) in three loess types. Bars (mean values ± SD from six independent experiments) with different letters are significantly different at the 0.05 level (n = 6) based on LSD multiple comparisons.

3.3 Soil microelements

The data presented in Figures 5 and 6 show changes in microelement contents given different patterns of soil utilization. These results reveal that compared with NF (Figure 5),concentrations of total Cu (Figure 5a), Zn (Figure 5b), and Mg (Figure 5c) have all been enhanced by long-term planting (P > 0.05), an order of change from largest-to-smallest of CF, SF, and NF. It is also the case that NF long-term planting has enhanced concentrations of soil available Cu (P < 0.05), Zn (P < 0.05), Fe (P < 0.05), and Mg (P < 0.05) even though values for these parameters were significantly lower in CF cases than in SF (P < 0.05), with the exception of the latter element. Available Fe and Mg contents in all farmland types did not attain levels required by loess criteria, greater than 5.0 mg•kg-1 and greater than 10 mg•kg-1, respectively. Values for available Zn content also did not reach the necessary level, greater than 1.0 mg•kg-1, with the exception of the SF case (Guo, 1992).
Figure 5 Comparisons between contents of total Cu (a), Zn (b), and Mn (c) in three loess types. Bars (mean values ± SD from three independent experiments) with different letters are significantly different at the 0.05 level (n = 3) based on LSD multiple comparisons.
Figure 6 Comparisons between contents of soil available Cu (a), Zn (b), Fe (c), and Mn (d) in three loess types. Bars (mean values ± SD from three independent experiments) with different letters are significantly different at the 0.05 level (n = 3) based on LSD multiple comparisons.

3.4 Soil toxic elements

The data presented in Figure 7 highlight trends in concentrations of soil Cd (Figure 7a) as well as total Pb (Figure 7b), Cr (Figure 7c), and Hg (Figure 7d) in different farmland types. Although long-term planting has increased the concentrations of all these toxic elements in soil, the effects of different farmland types are significantly different with respect to Cd and Pb concentrations, in sequence from largest-to-smallest SF, CF, and NF. The concentration order for Cr and Hg from largest-to-smallest was CF, SF, and NF. Data show that Hg concentrations in all farmland types exceeded the safe background value of 0.06 mg•kg-1 (Xue, 1986).
Figure 7 Comparisons between concentrations of total Cd (a), Pb (b), Cr (c), and Hg (d) in three loess types. Bars (mean values ± SD from three independent experiments) with different letters are significantly different at the 0.05 level (n = 3) on the basis of LSD multiple comparisons.

4 Discussion

4.1 Loess pH values

The physicochemical and biological properties of soils have always been influenced by pH values. Indeed, soil pH is well known to play a significant role in fertility formation and evolution (Brady et al., 1999; Aciego et al., 2008). Previous research performed as part of the second soil survey (1979-1983) has shown that loess pH values for NF, SF and CF lands all tend to be significantly higher than the national average (pH ≈ 6.8), being 22%, 20%, and 18%, respectively. It is also the case that long-term planting acts to significantly decrease loess pH values but there is nevertheless an insignificant difference between SF and CF lands in this analysis. In terms of plant growth, NF lands require amelioration via acidic comprehensive fertility because a neutral pH is advantageous to both crop growth and development; a range between pH 5.5 and pH 6.5 has been shown to be best for potato and tomato growth (Islam et al., 1980).

4.2 Loess organic matter

The quality and fertility of soils can be characterized on the basis of numerous basic chemical parameters, including CEC, organic matter, and total N. The first of these variables, CEC, is an indicator of soil quality and fertility; thus, a higher CEC concentration equates to better soil quality and fertility (Brady et al., 2002). The results of this analysis show that different farmland types have significantly different effects on CEC capacity (P < 0.05) (Figure 2a); results reveal a CEC size sequence from largest-to-smallest of CF, SF, and NF. The data suggest that the fertility of CF lands is better than that of their SF counterparts, while NF areas are the worst of the three loess types.
Organic matter content is the dominant index for assessing soil quality in agricultural practice (Bellamy et al., 2005). Long-term cultivation has resulted in an increase in the concentration of soil organic matter (Wang et al., 2015), from largest-to-smallest CF, SF, and NF. Soil CEC is closely correlated with pH values and SOM content (Caravaca et al., 1999; Von Oheimb et al., 2008); results show that CEC is negatively related to soil pH values (r = -0.579) and positively related to SOM (r = 0.737), consistent with previous research (Qiu et al., 2010). Results show that soil organic matter content does not reach the necessary nine grade standard within Shaanxi Province (greater than 6.00 g•kg-1) (Guo, 1992) for NF lands and is therefore extremely deficient and unfavorable for the growth and development of crops. Appropriate amounts of organic matter must therefore be added to NF lands and pH values must be decreased in order to increase both CEC capacity and soil quality.

4.3 Loess macroelements

Chemical elements play very important roles in plant growth and development. At the same time, however, different elements perform varied physiological functions within plant lifecycles and can be classified as either macroelements (e.g., N, P, and K), medium elements, (e.g., Ca, Mg, and S), or trace elements (e.g., Fe, Mn, Cu, Zn, Mo, and Cl) according to vital growth requirements (Kalavrouziotis et al., 2008; Li et al., 2010).
N retention and supply are thought to be influenced by soil organic matter (Brubaker et al., 1993). This element is the key raw material for amino acids and also performs a very important function in soil fertility. The results of this study show that the concentration of total N was insignificantly higher (P < 0.05) in both SF and CF lands compared to NF lands. Available N is directly absorbed by crops and influences both growth and yield (Macdonald et al., 2009); data show that this element occurs at significantly higher levels in both SF and CF lands compared to NF areas. It is also the case that fertility in SF and CF cases tends to be higher in terms of soil organic matter and N when compared to NF situations. This result implies that NF lands require a supply of both soil organic matter and N.
The element P is an important nutrient necessary for both plant growth and development. However, although soil contains a large proportion of total P, just a relatively small component is immediately available to plants (Richardson et al., 2009); this means that ensuring a suitable volume of this element in soils is critical for both crop growth and agricultural productivity. The results of this study also reveal that the concentration change order of total and available phosphorus was SF, CF, and NF (from largest-to-smallest) but that total phosphorus concentration did not reach the required loess criterion level in all cases (greater than 0.6 mg•kg-1). As this was also the case for available phosphorus concentrations (greater than 5 mg•kg-1) (Guo, 1992) it is clear that deficiencies decrease crop yields as available P is almost universally required to maintain production (Sundara et al., 2002). The results of this analysis therefore suggest that phosphorus fertilization should be carried out to improve crop yields on NF lands.
The element K is also key for plant growth and development; the presence of this metal enhances saccharide transport ratios and photosynthesis in natural ecosystems (Tripler et al., 2006), while the presence of soil available K trapped between interlayers of clay minerals is an important resource for plant growth in many soils (Mengel et al., 1993). The results of this analysis demonstrate that long-term planting enhances concentrations of both total and available K; indeed, available contents of this metal equaled (or exceeded) loess criteria in all cases (greater than 50 mg•kg-1) (Guo, 1992). It is therefore clear that NF lands do not require the addition of this element.

4.4 Loess microelements

Trace micronutrient elements in field crops (e.g., Fe, Mn, Cu and Zn) are essential for growth and development although concentrations at one in a thousand levels are required (Zou et al., 2008). Earlier research has estimated (Sillanpää, 1982, 1990) that agricultural soils globally are 49% deficient in Zn, 14% deficient in Cu, 10% deficient in Mn, and 3% deficient in Fe. The results of this analysis also suggest that concentrations of total Cu (Figure 5a), Zn (Figure 5b), and Mn (Figure 5c) have been enhanced by long-term planting (P > 0.05), from largest-to-smallest change in the order CF, SF, and NF. This result implies that NF lands require the addition of reasonable levels of supplementary trace micronutrients.
Availability in this context can be defined as the quantity of a soil nutrient that is accessible to plant roots over a useful time period such as one growing season (White et al., 1999). Crop roots can also absorb micronutrients directly from soils in solution, which results in a decrease in the total pool available. Previous research has shown that available concentrations of Fe, Mn, Cu, and Zn are related to plant growth responses to micronutrients (White et al., 1999); this means that deficiencies in available micronutrients can lead to physiological sickness. Long-term planting enhances the concentrations of soil available Cu (P < 0.05), Zn (P < 0.05), Fe (P < 0.05), and Mn (P < 0.05). However, available contents of Fe and Mn in all farmland types assessed here did not attain necessary loess criteria values greater than 5.0 mg•kg-1 and greater than 10 mg•kg-1, respectively. This was also the case for available Zn (criterion greater than 1.0 mg•kg-1), with the exception of SF lands (Guo, 1992). It is therefore clear that NF lands will require the addition of fertilizer or micronutrient supplements for plant growth because of low Fe, Mn, Cu, and Zn levels.

4.5 Non-essential loess elements

The pollution of soils with heavy metals has become a severe problem in many parts of the world (Facchinelli et al., 2001; Solgi et al., 2012). As a direct result of rapid socioeconomic development over recent decades, soil pollution by heavy metals has become both serious and widespread in China because of the overuse of chemical fertilizers (Chen et al., 1999; Wang et al., 2001). Although some heavy metals are present in soils, additional contributions also come from anthropogenic activities including agriculture, urbanization, industrialization, and mining (Facchinelli et al., 2001). This kind of pollution not only degrades soil quality and results in a decrease in crop growth, but threatens human health via the food chain (Wang et al., 2001; Nabulo et al., 2010; Dong et al., 2011). The elements Pb, Cd, Cr, and Hg are the most toxic, while the main sources of heavy metals in soil are from various agrochemicals, fertilizers, pesticides, and atmospheric deposition (Berthelsen et al., 1995; Gray et al., 2003; Huang et al., 2007). They influence crop growth and damage soil ecosystem health. Although concentrations of Pb, Cd, Cr, and Hg do not reach ecological risk levels in any of the land types studied here, Cd concentrations did exceed criteria in all cases (less than, or equal to, 1 mg•kg-1) (Xue, 1986). The results of this analysis are therefore particularly relevant to SF and CF lands, while NF areas appear to be the most suitable for green agriculture.

5 Conclusions

The results of this study show that different farmland types encompass different nutrient levels in the context of loess physicochemical parameters. In the first place, NF lands tend to have higher pH values which should be decreased artificially, while the soil organic matter content of these areas is lower than the nine grade standard in Shaanxi Province (less than 6.00 g•kg-1). These lands are therefore extremely deficient and unfavorable for crop growth and development. The data show that concentrations of total (greater than 0.6 g•mg-1) and available N (greater than 30 g•mg-1) fall below the basic line of six grade criteria for national nutrient levels in NF lands while at the same time this farmland contains abundant K. Total P concentrations do not attain loess criteria values in all soil samples (greater than 0.6 mg•kg-1) as is also the case for available concentrations of this element (greater than 5 mg•kg-1). The data also show that NF lands do tend to lack some useful microelements but contain some toxic components. The results of this study suggest that NF lands are currently unhealthy and require increases in fertility for effective agricultural practice. Results indicate that long-term planting is a risk to soil health because of the accumulation of toxic heavy metals. Long-term planting therefore also requires fallow and rehabilitation to ensure food security.

The authors have declared that no competing interests exist.

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Chen H M, Zheng C R, Tu Cet al., 1999. Heavy metal pollution in soils in china: Status and countermeasures.Ambio, 28(2): 130-134.The pedosphere lies at the juncture of the atmosphere, biosphere, hydrosphere and lithosphere. Heavy-metal pollution of soil greatly affects not only the yield and quality of crops, animal and human health, but also the quality of the whole environment. The current status and the effects of heavy-metal pollution in the soils of China are reviewed in this paper. Soil contamination by heavy metals from sewage irrigation and metal mining, smelting, and processing activities was seen to be serious. Urban enterprises also contribute to heavy-metal pollution of soils in China. The effects of soil pollution on plants, animals, and human beings are discussed. Effective countermeasures for pollution control are also presented.

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[11]
Chen Y, Wang K, Lin Yet al., 2015. Balancing green and grain trade.Nature Geoscience, 8(10): 739-741.Since 1999, China's Grain for Green project has greatly increased the vegetation cover on the Loess Plateau. Now that erosion levels have returned to historic values, vegetation should be maintained but not expanded further as planned.

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[12]
He C X, 2015. The situation, characteristics and effect of the gully reclamation project in Yan’an.Journal of Earth Environment, 6(4): 255-260.

[13]
Dong J, Yang Q W, Sun L Net al., 2011. Assessing the concentration and potential dietary risk of heavy metals in vegetables at a Pb/Zn mine site, China.Environmental Earth Sciences, 64(5): 1317-1321.Cd, Pb, Cu and Zn were measured in vegetables in Xiguadi village around Lechang Pb/Zn mine in Guangdong province, South China. The daily intake (DI) of these metals from vegetables by local people was also determined. The respective Cd, Pb, Cu and Zn concentration was 0.05–0.90 (mean 0.25), 1.04–5.82 (2.64), 0.53–7.07 (2.00) and 3.87–25.20 (11.68)02mg02kg 611 , of which Cd concentration in all vegetables exceeded the safe limit given by FAO/WHO. The DI was found to be 49.76, 475.56, 360.36 and 2,102.6302μg, respectively. The present results indicated local mining activity caused vegetable heavy metal contamination and Cd concentration exceeding the stipulated standards for all vegetables indicating potentially serious dietary risks for local people.

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[14]
Facchinelli A, Sacchi E, Mallen L, 2001. Multivariate statistical and GIS-based approach to identify heavy metal sources in soils.Environmental Pollution, 114(3): 313-324.

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[15]
Gray C, McLaren R, Roberts A, 2003. Atmospheric accessions of heavy metals to some New Zealand pastoral soils.The Science of The Total Environment, 305(1-3): 105-115.

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[16]
Guo Z Y, 1992. Soil of Shaanxi. Beijing: Science Press (in Chinese).

[17]
Huang S S, Liao Q L, Hua Met al., 2007. Survey of heavy metal pollution and assessment of agricultural soil in Yangzhong District, Jiangsu Province, China.Chemosphere, 67(11): 2148-2155.

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[18]
Islam A, Edwards D G, Asher C J, 1980. pH optima for crop growth: Results of a flowing solution culture experiment with 6 species.Plant and Soil, 54(3): 339-357.Ginger, cassava, maize, wheat, french bean and tomato were grown for periods up to six weeks in continuously flowing nutrient solutions at seven constant pH values ranging from 3.3 to 8.5. All species achieved maximum or near-maximum growth in the pH range 5.5 to 6.5. However, there were substantial differences in the ability of species to grow outside this range. Ginger and cassava were the most tolerant species to low solution pH, while ginger and tomato were the only species to show no yield depression at the highest solution pH. Roots of all species at pH 3.3 and some species at pH 4.0 exhibited symptoms of hydrogen ion injury. In addition, the concentrations of magnesium in the tops of all six species, of nitrogen in the tops of tomato and cassava, and of manganese in the tops of maize at these pH values were inadequate for optimal growth. Growth depression at high solution pH was associated with iron deficiency in maize and wheat and with nitrogen and/or copper deficiency in cassava.The relevance of the present results to crop growth under field conditions is discussed. The complex interplay of plant and soil characteristics militates against precise definition of an optimum pH range for the growth of a particular crop unless the soil is also specified.

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[19]
Jackson M L, 1969. Soil Chemical Analysis: Advanced Course. University of Wisconsin, Madison, Wisconsin.

[20]
Kalavrouziotis I K, Robolas P, Koukoulakis P Het al., 2008. Effects of municipal reclaimed wastewater on the macro- and micro-elements status of soil and of brassica oleracea var. Italica, and b-oleracea var. Gemmifera.Agricultural Water Management, 95(4): 419-426.An experiment was conducted in a greenhouse, located in Agrinion, Greece, where the effect of treated municipal wastewater (TMWW), compared to the ordinary irrigation water, was studied by means of a randomized block statistical design, on the macro- and micro-element and heavy metal content of Brassica oleracea var. Italica (Broccoli), and B. oleracea var. Gemmifera (Brussels sprouts) plants, as well as on the physical and chemical properties of the clay loam (CL) soil, and its inorganic composition, in order to examine the possibility of TMWW reuse for the irrigation of the above vegetables. The transplanting was done on December 2005, and harvesting of the heads and sprouts 16 weeks later, i.e. at the end of April. Ordinary irrigation water, TMWW, soil, and plant samples, were analyzed, and the data obtained were statistically processed. The following were found: applied TMWW increased significantly, in comparison to control, the content of some macro- and micro-elements in the soil, but the concentrations of most of them were generally within the accepted critical levels, except for P and Zn and Cd whose concentration varied as follows: in Brocoli soil P 18.36–41.16 mg/kg, Zn 3.61–4.64 mg/kg, and Cd 0.065–1.20 mg/kg, while in Brussels sprouts soil P 20.6–36.32 mg/kg, Zn 2.87–4.83 mg/kg and Cd 0.06–1.45 mg/kg. These results showed that TMWW had a residual effect with respect to these elements. Similarly, the TMWW increased significantly the heavy metal content in the dry matter of the roots as follows: in Brussel sprouts Cd varied from 0.0083 to 0.78, Co 0.029 to 3.38 and Ni from 4.83 to 7.27 μg/g, respectively, and in Broccoli Ni varied from 4.20 to 10.13 μg/g. TMWW also increased the accumulation of Fe in the roots of Broccoli from 379.5 to 1022.0 mg/kg. However, the levels of the heavy metals in the edible plant parts (heads and sprouts) were very high, varying as follows: in Broccoli Ni 3.91–4.15 μg/g, and Pb 9.82–10.40 μg/g, while in Brussels sprouts Cd 0.8–1.17 μg/g, Co 2.35–2.70 μg/g, and Ni 5.70–6.17 μg/g. These increased heavy metal contents in the edible plant parts, and the heavy FC and E. coli load of the TMWW, constitute a high health risk factor, and therefore the TMWW studied, cannot be used at the present time for the irrigation of these vegetables, unless it is subjected in the future, to a secondary or advanced primary treatment (APT).

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[21]
Li J W, Richter D D, Mendoza Aet al., 2010. Effects of land-use history on soil spatial heterogeneity of macro- and trace elements in the southern piedmont USA.Geoderma, 156(2): 60-73.To quantify effects of land-use history on soil spatial heterogeneity, we sampled surficial mineral soils (0 7.5 cm depth) using a spatially-explicit design within three 0.09-ha plots in each of three ecosystems in the Southern Piedmont area of USA. The three ecosystems were old hardwood forests, cultivated agricultural fields, and old-field pine forests, three common ecosystems in the region that represent the common trajectory of nearly two centuries of land-use history. In total, 243 soil samples were collected and 12 soil properties were measured on each sample. Results indicated that: (i) land-use history altered soil properties' central tendencies and their spatial heterogeneities; (ii) within-plot variations indexed by coefficients of variation and Cochran's C tests of within-plot variances were generally much higher in hardwood and pine-forest soils than in cultivated soils; (iii) for soil C, and major and trace elements spatial patterns as indicated by trend-surface analysis, correlograms, and interpolation maps were evident under hardwood and pine forests and much less so under cultivation. We document cases in which land use alters both the soil property's central tendencies and their heterogeneity (C, N, C:N ratio, Ca, and K), and cases in which changes are apparent in central tendency but not in heterogeneity (bulk density, D b). Relative to soils that have never been cultivated, spatial heterogeneity is greatly reduced in many soil properties by plowing, fertilization, and other practices associated with agricultural crop production, but that successional forest growth on previously cultivated soils re-establishes heterogeneity of soil properties within a few decades. Overall, within-plot variances were very high for most properties especially of the forested soils and indicate that researchers should better match sample sizes and sample designs with the variability of the soil properties they are studying.

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[22]
Lindsay W L, Norvell W A, 1978. Development of a dtpa soil test for zinc, iron, manganese, and copper.Soil Science Society of America Journal, 42(3): 421-428.

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[23]
Liu Q, Wang Y, Zhang Jet al., 2013. Filling gullies to create farmland on the Loess Plateau.Environmental Science & Technology, 47(14): 7589-7590.Environ Sci Technol. 2013 Jul 16;47(14):7589-90. doi: 10.1021/es402460r. Epub 2013 Jul 1.

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[24]
Liu Y S, Guo Y J, Li Y Ret al., 2015. GIS-based effect assessment of soil erosion before and after gully land consolidation: A case study of Wangjiagou project region, Loess Plateau.Chinese Geographical Science, 25(2): 137-146.The Loess Plateau is one typical area of serious soil erosion in the world. China has implemented ′Grain for Green′(GFG) project to restore the eco-environment of the Loess Plateau since 1999. With the GFG project subsidy approaching the end, it is concerned that farmers of fewer subsidies may reclaim land again. Thus, ′Gully Land Consolidation Project′(GLCP) was initiated in 2010. The core of the GLCP was to create more land suitable for farming in gullies so as to reduce land reclamation on the slopes which are ecological vulnerable areas. This paper aims to assess the effect of the GLCP on soil erosion problems by studying Wangjiagou project region located in the central part of Anzi valley in the middle of the Loess Plateau, mainly using the revised universal soil loss equation(RUSLE) based on GIS. The findings show that the GLCP can help to reduce soil shipment by 9.87% and it creates more terraces and river-nearby land suitable for farming which account for 27.41% of the whole study area. Thus, it is feasible to implement the GLCP in places below gradient 15°, though the GLCP also intensifies soil erosion in certain places such as field ridge, village land, floodplain, natural grassland, and shrub land. In short, the GLCP develops new generation dam land and balances the short-term and long-term interests to ease the conflicts between economic development and environmental protection. Furthermore, the GLCP and the GFG could also be combined preferably. On the one hand, the GFG improves the ecological environment, which could offer certain safety to the GLCP, on the other hand, the GLCP creates more farmland favorable for farming in gullies instead of land reclamation on the slopes, which could indirectly protect the GFG project.

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[25]
Lu R K.2000. Soil Analytical Methods of Agronomic Chemicals. Beijing: China Agricultural Science and Technology Press (in Chinese).

[26]
Lu Y, Fu B, Feng Xet al., 2012. A policy-driven large scale ecological restoration: Quantifying ecosystem services changes in the Loess Plateau of China.PLoS One, 7(2): 31782.As one of the key tools for regulating human-ecosystem relations, environmental conservation policies can promote ecological rehabilitation across a variety of spatiotemporal scales. However, quantifying the ecological effects of such policies at the regional level is difficult. A case study was conducted at the regional level in the ecologically vulnerable region of the Loess Plateau, China, through the use of several methods including the Universal Soil Loss Equation (USLE), hydrological modeling and multivariate analysis. An assessment of the changes over the period of 2000-2008 in four key ecosystem services was undertaken to determine the effects of the Chinese government's ecological rehabilitation initiatives implemented in 1999. These ecosystem services included water regulation, soil conservation, carbon sequestration and grain production. Significant conversions of farmland to woodland and grassland were found to have resulted in enhanced soil conservation and carbon sequestration, but decreased regional water yield under a warming and drying climate trend. The total grain production increased in spite of a significant decline in farmland acreage. These trends have been attributed to the strong socioeconomic incentives embedded in the ecological rehabilitation policy. Although some positive policy results have been achieved over the last decade, large uncertainty remains regarding long-term policy effects on the sustainability of ecological rehabilitation performance and ecosystem service enhancement. To reduce such uncertainty, this study calls for an adaptive management approach to regional ecological rehabilitation policy to be adopted, with a focus on the dynamic interactions between people and their environments in a changing world.

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[27]
Macdonald C A, Thomas N, Robinson Let al., 2009. Physiological, biochemical and molecular responses of the soil microbial community after afforestation of pastures with Pinus radiata. Soil Biology and Biochemistry, 41(8): 1642-1651.Afforestation and deforestation are key land-use changes across the world, and are considered to be dominant factors controlling ecosystem functioning and biodiversity. However, the responses of soil microbial communities to these land-use changes are not well understood. Because changes in soil microbial abundance and community structure have consequences for nutrient cycling, C-sequestration and long-term sustainability, we investigated impacts of land-use change, age of stand and soil physico-chemical properties on fungal and bacterial communities and their metabolic activities. This study was carried out at four sites in two geographical locations that were afforested on long-established pastures with Pinus radiata D. Don (pine). Two of the sites were on volcanic soils and two on non-volcanic soils and stand age ranged from 5 to 2002y. Microbial communities were analysed by biochemical (phospho-lipid fatty acids; PLFA) and molecular (multiplex-terminal restriction fragment length polymorphism; M-TRFLP) approaches. Both site and stand age influenced microbial properties, with changes being least detectable in the 5-y-old stand. Land use was a key factor influencing soil metabolic activities as measured by physiological profiling using MicroResp. Pasture soils had higher microbial biomass ( P02<020.001), and metabolic activities ( P02<020.001), and basal respiration rates were up to 2.8-times higher than in the pine soils. Microbial abundance analysis by PLFA showed that the fungal to bacterial ratio was higher in the pine soils ( P02<020.01). Community analysis suggested that soil bacterial communities were more responsive to site (principal component 1; P02<020.001) than to land use (principal component 5; P02<020.001). In contrast, the fungal community was more affected by land-use change (principal component 1; P02<020.001) than by site, although site still had some influence on fungal community structure (principal component 2; P02<020.001). Redundancy analysis also suggested that bacterial and fungal communities responded differently to various soil abiotic properties, land-use change and location of sites. Overall, our results indicate that the change in land use from pasture to P. radiata stands had a direct impact on soil fungal communities but an indirect effect, through its effects on soil abiotic properties, on bacterial communities. Most of the changes in bacterial communities could be explained by altered soil physico-chemical properties associated with afforestation of pastures.

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[28]
Mengel K, Uhlenbecker K, 1993. Determination of available interlayer potassium and its uptake by ryegrass.Soil Science Society of America Journal, 57(3): 761-766.Abstract The objective was to test whether the K+ release rate of nonexchangeable K+ can be measured with a modified electro-ultrafiltration technique. The cumulative K+ extraction curves obtained with an extraction at 400 V and 80 C were described by the first-order equation, the parabolic diffusion equation, the power function, and the Elovich equation, with the last of these showing the best fit. The b values obtained decreased with the depletion of nonexchangeable K+ brought about by the uptake of K+ by ryegrass. Findings show that the b value represents a reliable indicator for the availability of nonexchangeable K+. The same is true for the NH4+ exchangeable K+, provided the soil is low in exchangeable K+. -from Authors

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[29]
Murphy J, Riley J P, 1962. A modified single solution method for the determination of phosphate in natural waters.Analytica Chimica Acta, 27: 31-36.

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[30]
Nabulo G, Young S D, Black C R, 2010. Assessing risk to human health from tropical leafy vegetables grown on contaminated urban soils.Science of the Total Environment, 408(22): 5338-5351.Fifteen tropical leafy vegetable types were sampled from farmers' gardens situated on nine contaminated sites used to grow vegetables for commercial or subsistence consumption in and around Kampala City, Uganda. Trace metal concentrations in soils were highly variable and originated from irrigation with wastewater, effluent discharge from industry and dumping of solid waste. Metal concentrations in the edible shoots of vegetables also differed greatly between, and within, sites. Gynandropsis gynandra consistently accumulated the highest Cd, Pb and Cu concentrations, while Amaranthus dubius accumulated the highest Zn concentration. Cadmium uptake from soils with contrasting sources and severity of contamination was consistently lowest in Cucurbita maxima and Vigna unguiculata, suggesting these species were most able to restrict Cd uptake from contaminated soil. Concentrations of Pb and Cr were consistently greater in unwashed, than in washed, vegetables, in marked contrast to Cd, Ni and Zn. The risk to human health, expressed as a azard quotient (HQ M), was generally greatest for Cd, followed successively by Pb, Zn, Ni and Cu. Nevertheless, it was apparent that urban cultivation of leafy vegetables could be safely pursued on most sites, subject to site-specific assessment of soil metal burden, judicious choice of vegetable types and adoption of washing in clean water prior to cooking.

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[31]
Olsen S, Sommers L, 1982. Methods of soil analysis. Part 2: Phosphorus methods of soil analysis.ASA Monograph, 9: 403-430.

[32]
Qiu L P, Zhang X C, Cheng J Met al., 2010. Effects of black locust (Robinia pseudoacacia) on soil properties in the loessial gully region of the Loess Plateau, China. Plant and Soil, 332(2): 207-217.Black locust (Robinia pseudoacacia) has been widely planted in the Loess Plateau for soil and water conservation. The effects of black locust on soil properties has significant role in land use and ecosystem management. However, this beneficial effect has been little studied in the Loess Plateau. The soil properties below black locust and native grass growing in Nanxiaohe and Wangdonggou watersheds, located in the loessial gully region of the Loess Plateau, were studied for changes in soil properties after establishment of black locust. The black locust significantly increased soil cation exchange capacity, organic carbon, total nitrogen, nitrate, and carbon:nitrogen and carbon:phosphorus (P) ratios, as well as some enzymes like alkaline phosphatase and invertase in 0 20 cm or 0 80 cm depths of soil compared to the native grassland in Nanxiaohe and Wangdonggou watersheds. However, the effects on ammonium, total P, and extractable P and potassium were not consistent in both watersheds. There were more obvious differences in soil properties between black locust land and grassland for Nanxiaohe watershed than for Wangdonggou watershed, suggesting that the effects of black locust on most soil properties increase with black locust age. The results indicate that black locust has potential to improve soil properties in the loessial gully region of the Loess Plateau and the improvements were greater in long-term than middle-term black locust stands.

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[33]
Richardson A E, Hocking P J, Simpson R Jet al., 2009. Plant mechanisms to optimise access to soil phosphorus.Crop & Pasture Science, 60(2): 124-143.

[34]
Sillanpää , M, 1982. Micronutrients and the nutrient status of soils: A global study. Soils Bulletin 48, FAO, Rome.

[35]
Sillanpää , M, 1990. Micronutrient assessment at country level: An international study. Soils Bulletin No. 63, FAO, Rome.The results of the project "Micronutrient Assessment at Country Level" are reported. The objective was to quantify the effects of various micronutrients on yields, especially on those of economically important food and cash crops (including barley, groundnut, cotton, sunflower, cowpea, wheat, maize and rice). Field trials were carried out in 15 countries (including Ethiopia, Malawi, Mexico, Nep...

[36]
Solgi E, Esmailisari A, Riyahibakhtiari Aet al., 2012. Soil contamination of metals in the three industrial estates, Arak, Iran.Bulletin of Environmental Contamination and Toxicology, 88(4): 634-638.Abstract, respectively. Pearson correlation indicated that Arsenic, Cadmium and Lead were mainly derived from anthropogenic inputs, and Chromium and Nickel were controlled by natural source, whereas Nickel appeared to be affected by both anthropogenic and natural sources. The geo-accumulation Index (Igeo) calculated in three industrial estates gave values indicating unpolluted to strongly polluted.

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[37]
Sundara B, Natarajan V, Hari K, 2002. Influence of phosphorus solubilizing bacteria on the changes in soil available phosphorus and sugarcane and sugar yields.Field Crops Research, 77(1): 43-49.Influence of application of phosphorus solubilizing bacteria (PSB), Bacillus megatherium var. Phosphaticum, at 10 kg ha 1 of lignite based culture with and without varying amounts of P fertilizer was studied on soil available P changes and sugarcane growth and yield. The PSB application increased the PSB population in the rhizosphere and the plant available P status in the soil. It also enhanced tillering, stalk population and stalk weight, and led to a cane yield increase of 12.6% over no application. When used in conjunction with P fertilizers, PSB reduced the required P dosage by 25%. In addition, it was found that 50% of the costly super phosphate could be replaced by rock phosphate (RP), a cheap source of P, when applied in conjunction with PSB. The PSB improved juice quality and sugar yields. The influence of PSB was greatest when RP constituted a part of the P fertilizers applied.

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[38]
Tang K L, Zhang K L, Lei A L, 1998. Critical slope gradient for compulsory abandonment of farmland on the hilly Loess Plateau.Chinese Science Bulletin, 43(5): 409-412.Scientific evidence for the upper limit of farmland is proposed. Shallow gully erosion is one of erosion types distributed extensively on sloping farmland in the hilly regions of the Loess Plateau. Field observation and aerial photos interpretation, as well as laboratory experiments show that the shallow gully erosion occurring on the steeper farmland in the hilly regions of the Loess Plateau is an important factor leading to intensive erosion on slope, because of its extensive distribution and intensive runoff collection. The data on the formation, development and distribution of shallow gullies on sloping farmland indicate that critical slope gradient for shallow gully initiation ranges from 15 to 20 degrees with an average of 18.2 degrees. Therefore, it is suggested that critical slope gradient for compulsory abandonment of farmland on the hilly Loess Plateau should be kept within the critical slope gradient for shallow gully initiation to prevent shallow gully formation in order to control soil loss more effectively. But as the first step, the cultivation on the slopes with slope larger than 25 degrees where the maximum erosion occurs should be strictly forbidden.

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[39]
Teng Y, Wu J, Lu Set al., 2014. Soil and soil environmental quality monitoring in China: A review.Environ. Int., 69: 177-199.

[40]
Tripler C E, Kaushal S S, Likens G Eet al., 2006. Patterns in potassium dynamics in forest ecosystems.Ecology Letters, 9(4): 451-466.The biotic cycling of potassium (K) in forest systems has been relatively understudied in comparison with nitrogen (N) and phosphorus (P) despite its critical roles in maintaining the nutrition of primary production in forests. We investigated the ecological significance of K in forests from a literature review and data synthesis. We focused on (1) describing patterns of the effects of K availability on aboveground growth and change in foliar tissue of tree species from a variety of forests; and (2) documenting previously unreported relationships between hydrologic losses of K and N in forested watersheds from the Americas. In a review of studies examining tree growth under K manipulations/fertilizations, a high percentage (69% of studies) showed a positive response to increases in K availability in forest soils. In addition, 76% of the tree studies reviewed showed a positive and significant increase in K concentrations in plant tissue after soil K manipulation/fertilization. A meta-analysis on a subset of the reviewed studies was found to provide further evidence that potassium effects tree growth and increased tissue [K] with an effect size of 0.709 for growth and an overall effect size of 0.56. In our review of watershed studies, we observed that concentrations of K typically decreased during growing seasons in streams draining forested areas in the Temperate Zones and were responsive to vegetation disturbance in both temperate and tropical regions. We found a strong relationship (r2 = 0.42 0.99) between concentrations of K and N (another critical plant nutrient) in stream water, suggesting that similar mechanisms of biotic retention may control the flow of these nutrients. Furthermore, K dynamics appear to be unique among the base cations, e.g. calcium, magnesium, and sodium, because the others do not show similar seasonal patterns to K. We suggest that K may be important to the productivity and sustenance of many forests, and its dynamics and ecological significance warrant further study. We suggest that knowledge about the dynamics of this understudied element is imperative for our understanding patterns and processes in forest ecosystems.

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[41]
Von Oheimb G, Hardtle W, Naumann P Set al., 2008. Long-term effects of historical heathland farming on soil properties of forest ecosystems.Forest Ecology and Management, 255(6): 1984-1993.Much of Europe's landscape has been converted from agricultural land to forest over the last 150 years. Previous land-use activities may have persistent effects on forest ecosystem properties, yet there is scant information on the long-term recovery of forest soils from agriculture. In this study we examined the effects of historical heathland farming on forest soil properties by comparing (i) ancient oak forests with oak stands afforested more than 115 years ago on (ii) former heaths and (iii) formerly cultivated fields in NW Germany. Our results showed that former fields had significantly lower C, N, and P stores and lower C/N- and C/P-ratios in the organic layer than former heaths and ancient oak forests, but revealed more plant-available P, higher total P contents and low C/P-ratios in the A-horizon. We concluded that higher plant-available P and P stores in the A-horizon reflected well the former manuring of fields, while lower C, N, and P stores in the O-horizon were related to the age of the forest floor. Differences between former heaths and ancient forests were less pronounced and characterized by a low base saturation (BS) and high C/N-ratio in the organic layer of ancient forests, attributable to nutrient sequestration in forest biomass and to changes in microbial communities, respectively. In our study, organic layer and A-horizon served as complementary tracers for land-use legacies, since some of the effects of historical heathland farming were only detectable by means of properties of one of the humus horizons. Our findings demonstrate that heathland farming needs to be considered as a istorical site factor when analysing forest ecosystem processes, since soil legacies resulting from heathland farming may still impact present-day patterns of biodiversity and tree growth in lowland forest ecosystems.

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[42]
Wang F, Mu X, Li Ret al., 2015. Co-evolution of soil and water conservation policy and human-environment linkages in the Yellow River Basin since 1949.Science of the Total Environment, 508: 166-177.61Outlining complex problems and policy responses in China's soil erosion hotspot61Detecting policy co-evolution with human–environment linkages using DPSIR61Policy addressing real conditions mainly affected the environment initially.61Policy improved the rural economic and ecosystem when solving river's problems.61Providing a historical perspective on resource management with an actual story

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[43]
Wang Q, Dong Y, Cui Yet al., 2001. Instances of soil and crop heavy metal contamination in China.Soil and Sediment Contamination: An International Journal, 10(5): 497-510.Both general and specific investigations of soil and crop heavy metal contamination were carried out across China. The former was focused mainly on Cd, Hg, As, Pb, and Cr in soils and vegetables in suburbs of four large cities; the latter investigated Cd levels in both soils and rice or wheat in contaminated areas throughout 15 provinces of the country. The results indicated that levels of Cd, Hg, and Pb in soils and some in crops were greater than the Governmental Standards (Chinese government limits for soil and crop heavy metal contents). Soil Cd ranged from 0.46 to 1.04 mg kg0908081, on average, in the four cities and was as high as 145 mg kg0908081 in soil and 7 mg kg0908081 in rice in the wide area of the country. Among different species, tuberous vegetables seemed to accumulate a larger portion of heavy metals than leafy and fruit vegetables, except celery. For both rice and wheat, two staple food crops, the latter seemed to have much higher concentrations of Cd and Pb than the former grown in the same area. Furthermore, the endosperm of both wheat and rice crops had the highest portion of Cd and Cr. Rice endosperm and wheat chaff accumulated the highest Pb, although the concentrations of all three metals were variable in different parts of the grains. For example, 8.3, 6.9, 1.4, and 0.6 mg kg0908081 of Pb were found in chaff, cortex, embryo, and endosperm of wheat compared with 0.11, 0.65, 0.71, and 0.19 mg kg0908081 in the same parts of rice, respectively. Untreated sewage water irrigation was the major cause of increasing soil and crop metals. Short periods of the sewage water irrigation increased individual metals in soils by 2 to 80% and increased metals in crops by 14 to 209%. Atmospheric deposition, industrial or municipal wastes, sewage sludge improperly used as fertilizers, and metal-containing phosphate fertilizers played an important role as well in some specific areas.

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[44]
Wang X, Tang C, Baldock J Aet al., 2015. Long-term effect of lime application on the chemical composition of soil organic carbon in acid soils varying in texture and liming history.Biology and Fertility of Soils, 52(3): 295-306.There is ample evidence to suggest that liming can regulate soil organic carbon (SOC) pools either directly through influencing the solubility of SOC or indirectly by altering total organic C input...

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[45]
Wang Y, Fu B, Chen Let al., 2011. Check dam in the Loess Plateau of China: Engineering for environmental services and food security.Environ. Sci. Technol., 45(24): 10298-10299.Author information: (1)State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, CAS, Beijing 100085, P. R. China. yfwang@rcees.ac.cn

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[46]
White J G, Zasoski R J, 1999. Mapping soil micronutrients.Field Crops Research, 60(1): 11-26.

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[47]
Xue Chengze, Xiao Ling, Wu Qianfenget al., 1986. Study on the background value of ten elements in the main agricultural soil of Shaanxi Province.Journal of Northwest Sci-tech University of Agriculture and Forestry (Natural Science Edition), 14(3): 30-53 (in Chinese).

[48]
Yeomans J C, Bremner J M, 1988. A rapid and precise method for routine determination of organic carbon in soil 1.Communications in Soil Science & Plant Analysis, 19(13): 1467-1476.

[49]
Zhang B Q, He C S, Burnham Met al., 2016. Evaluating the coupling effects of climate aridity and vegetation restoration on soil erosion over the Loess Plateau in China.Science of the Total Environment, 539: 436-449.Abstract In this study, the coupling effects of climate aridity and vegetation restoration on runoff and sediment yield over the Loess Plateau were examined and characterized. To take into consideration the complexity of drought, as well as the varied strengths and weaknesses of different drought measures, two drought indices are selected to identify and evaluate drought variability. The Normalized Difference Vegetation Index (NDVI) data were obtained to monitor and express spatiotemporal variations in vegetation cover. The results show that most regions of the Loess Plateau experienced increasingly severe droughts over the past 40years, and these regions comprise the major source of the Yellow River sediment. Climatic drying initially occurred in the 1990s, and became statistically significant in 2000s. The increasingly severe droughts could negatively impact surface and groundwater supplies as well as soil water storage, but may also minimize surface runoff yield, which is one of the major causes of soil erosion on the Loess Plateau. Vegetation cover on the Loess Plateau was significantly improved after the implementation of "Grain for Green" project, which were helpful for controlling severe soil erosion. With the impacts of the construction of check dams, terraces and large reservoirs, runoff and sediment yield over the Loess Plateau initially exhibited downward trends between 1970 and 1990. After 1990, with the effects of the climate warming and drying, a second sharp reduction in runoff and sediment yield occurred. The coupling effects of climate aridity and vegetation restoration have led to a third significant decrease in runoff and sediment yield over the Loess Plateau after 2000. Copyright 2015 Elsevier B.V. All rights reserved.

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[50]
Zheng Y J, Chen Y P, Maltby Let al., 2016. Highway increases concentrations of toxic metals in giant panda habitat.Environ. Sci. Pollut. Res. Int., 23(21): 21262-21272.The Qinling panda subspecies (Ailuropoda melanoleuca qinlingensis) is highly endangered with fewer than 350 individuals inhabiting the Qinling Mountains. Previous studies have indicated that giant pandas are exposed to heavy metals, and a possible source is vehicle emission. The concentrations of Cu, Zn, Mn, Pb, Cr, Ni, Cd, Hg, and As in soil samples collected from sites along a major highway bisecting the panda's habitat were analyzed to investigate whether the highway was an important source of metal contamination. There were 11 sites along a 30-km stretch of the 108th National Highway, and at each site, soil samples were taken at four distances from the highway (0, 50, 100, and 300 m) and at three soil depths (0, 5, 10 cm). Concentrations of all metals except As exceeded background levels, and concentrations of Cu, Zn, Mn, Pb, and Cd decreased significantly with increasing distance from the highway. Geo-accumulation index indicated that topsoil next to the highway was moderately contaminated with Pb and Zn, whereas topsoil up to 300 m away from the highway was extremely contaminated with Cd. The potential ecological risk index demonstrated that this area was in a high degree of ecological hazards, which were also due to serious Cd contamination. And, the hazard quotient indicated that Cd, Pb, and Mn especially Cd could pose the health risk to giant pandas. Multivariate analyses demonstrated that the highway was the main source of Cd, Pb, and Zn and also put some influence on Mn. The study has confirmed that traffic does contaminate roadside soils and poses a potential threat to the health of pandas. This should not be ignored when the conservation and management of pandas is considered.

DOI PMID

[51]
Zhou Z S, Zheng S Q, Wu P Tet al., 1997. The study on anti-scourability of soil in Loess Plateau.Research of Soil & Water Conservation, 4(5): 47-58 (in Chinese).Based on the history of the research on anti-scourability of soil in Loess Plateau, provide two kinds of methods to analysing and testing the anti-scourability of soil, runoff plot information and field scouring. 168 times field scouring tests have been carried out in Liquan, Changwu, Ausai, Zhidan, Jiaxian, and analysed 98 runoff plots information over the years of Tianshui, Xifeng, Suide, Lishi and Changwu. Based on work above,put forward specific index to measure the anti-scourability of soil, and analysing the effect of the ground slope, the land utilization to the anti-scourability of soil. At last classification scheme of the anti-scourability of soil in Loess Plateau has been drawn up initially.

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