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Applied Radiation and Isotopes 68 (2010) 1650–1655

Contents lists available at ScienceDirect

Applied Radiation and Isotopes

0969-80

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/apradiso

137Cs tracing dynamics of soil erosion, organic carbon and nitrogen in slopingfarmland converted from original grassland in Tibetan plateau

Nie Xiaojun a,n, Wang Xiaodan b, Liu Suzhen b, Gu Shixian b, Liu Haijun c

a School of Surveying and Land Information Engineering, Henan Polytechnic University, Jiaozuo 454000, Chinab Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservation, Chengdu 610041, Chinac Institute of Water Resources Planning, Surveying, Design, and Research, Lhasa 850000, China

a r t i c l e i n f o

Article history:

Received 28 January 2010

Received in revised form

2 April 2010

Accepted 4 April 2010

Keywords:137Cs technique

Land-use change

SOC

Water erosion

Tillage erosion

Tibetan plateau

43/$ - see front matter & 2010 Elsevier Ltd. A

016/j.apradiso.2010.04.017

esponding author. Tel./fax: +86 391 3983693

ail address: niexj2005@126.com (N. Xiaojun).

a b s t r a c t

There is a shortage of research concerning the relationships between land-use change, soil erosion, and

soil organic carbon (SOC) and nitrogen (N) dynamics in alpine environments such as those found in the

Tibetan plateau. In this paper, typical sloping farmlands converted from grassland 50 years ago in

eastern Tibet were selected to determine dynamics of soil erosion, SOC, and total N associated with

land-use change. Soil samples were collected from sloping farmland and control fields (grassland). The137Cs, SOC, total N contents, and soil particle size fractions were analyzed in these samples. As

compared with the control fields, 137Cs, SOC, and total N inventories in the sloping farmlands decreased

by 30%, 27%, and 33%, respectively. Meanwhile variations in the three parameters were enhanced in the

sloping farmlands, with coefficients of variation (CVs) of 38%, 23%, and 20%, respectively, for 37Cs, SOC,

and total N. In addition, SOC and total N inventories significantly decreased with increasing soil erosion

in the sloping farmland. In a sloping farmland with a steep 241 gradient, the 137Cs inventory gradually

increased along a downslope transect with its lowest value at 0 Bq m�2 in the top-slope position (0 m).

The soil clay (o0.002 mm) content in such an area increased with decreasing elevation (r¼�0.95,

p¼0.001). Significant correlations between 137Cs and clay (r¼0.92, p¼0.003), SOC (r¼0.96, p¼0.001),

or total N (r¼0.95, p¼0.001) were also found in the farmland. These results showed that converting

alpine grassland to sloping farmland accelerates soil erosion, losses in SOC and N, and increases the

soil’s spatial variability. The combined impacts of tillage and water erosion contributed a significant

decrease in the soil’s organic carbon and N storages. Particularly in steep sloping farmlands, tillage

erosion contributed for severe soil loss, but the soil redistribution pattern was dominated by water

erosion, not tillage erosion, due to the lack of boundaries across the field patches. It was also found that137Cs, SOC, and total N moved along the same pathway within these sloping farmlands, resulting in net

C and N losses during soil redistribution. The negative influences of land-use conversion from grassland

to farmland in sloping areas in the Tibetan plateau should be addressed.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Tibet, known as the ‘‘World’s Ridge,’’ features one of the mostunique regional ecology areas in the world. Numerous glaciers,lakes, wetlands, and large international rivers are distributed inthe area, earning Tibet another distinction as the ‘‘Asian Tower.’’The Tibetan plateau exerts a profound impact on eastern Asiangeographical environments by providing an important barrier forthe stability of climate systems in this region. It is also animportant gene pool for global biological species and a key areafor global biodiversity conservation. However, Tibetan ecosystemsare fragile and sensitive to incompatible exogenic actions, such as

ll rights reserved.

.

rapid industrial development, intensive agriculture, and globalclimate change.

Alpine grassland is a major type of ecosystem in the Tibetanplateau, covering an area of 7.848�107 ha, which is approxi-mately 65.6% of the land area of Tibet. This ecosystem playsimportant barrier functions in Tibet in terms of ecologicalsecurity, such as climate regulation, water and soil conservation,and biodiversity maintenance. These functions, however, areweakening due to unreasonable land uses in the region. Thedecline in climate regulation and soil erosion control functions isespecially severe. Soils under alpine grasslands are generally highin SOC content because of low temperature and well-developedvegetation as a result of natural succession (Institute of SoilScience, Academia Sinica, 1986), which contributes an importantC pool in sustaining global C circulation balance. Previous studiesshowed that ecologically incompatible disturbances, such asovergrazing, cultivation, and global warming have resulted in

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Fig. 1. Study area location.

N. Xiaojun et al. / Applied Radiation and Isotopes 68 (2010) 1650–1655 1651

decreasing large sections of organic C storage in alpine grasslandsoils (Kang, 1996; Cheng et al., 1997; Wu and Tiessen, 2002; Jian,2002). Without effective measures to prevent these disturbances,the C sink function of alpine grasslands could be lost ortransformed into a C source, further accelerating global warming.Investigations on soil erosion conducted in the ‘‘Three RiversWatershed’’ (Jinsha River, Lancang River, and Nujiang River)showed that these regions are undergoing widespread soilerosion, especially in sloping farmlands, resulting in an increasein sediment content in rivers and a decrease in water quality(Zhong et al., 2008). To pursue economic profits, local residentsreclaimed forests and grasslands, even those on steep slopes, andturned them into croplands (Zhou et al., 2005), which often bringsmajor and negative influences on soil quality and function(Bossuyt et al., 1999; Dinesh et al., 2003; Elberling et al., 2003;Walker and Desanker, 2004; Merino et al., 2004; Guo and Gifford,2002; Su et al., 2004; Islam and Weil, 2000; Li et al., 2007; Yanget al., 2008; Mostafa et al., 2008). Recognized as a primary soilquality constituent, soil organic matter is important in maintain-ing soil structure stability, aiding in the soil infiltration of air andwater, promoting water retention, and enhancing soil fertility(Gregorich et al., 1994; Roose and Barthes, 2001; Zaujec, 2001).Reduction in SOC induced by cultivation change the distributionand stability of soil aggregates, thus increasing the likelihood ofsoil erosion (Cambardella and Elliott, 1993; Six et al., 2000). Forexample, Mostafa et al. (2008) reported that the conversion ofgrassland into cropland during an 18 year-period decreased SOCand total N by 50% each, and increased soil erodibility by 51% inthe highlands of Iran. As such, soil erosion, accelerated by theexpansion of sloping farmlands in the Tibetan plateau, could be aserious issue to the sustainable development of local agriculture.

The satellite remote sensing technique is a major method usedto investigate soil erosion in the Tibetan plateau (Zhong et al.,2008). However, it can only provide a qualitative description, nota quantitative evaluation. On the other hand, the Universal SoilLoss Equation (USLE) model can assess soil erosion quantitatively,but some parameters in the model, such as the rainfall factor(USLE-R) and soil erodibility factor (USLE-K), are difficult todetermine due to the lack of long-term field observations. Incomparison, the 137Cs technique can overcome the limitationsassociated with the above mentioned traditional approaches andhas been proven effective in assessing soil erosion intensities andprocesses (Ritchie and McHenry, 1990; Walling and Quine,1990,1991; Li et al., 2003; Zhang et al., 2004). Aside from 137Cs being awell-established tracer for soil erosion, it also provides a valuabletool for investigating medium-term erosion–carbon relationships.Recently, studies concerning 137Cs application to evaluate soilerosion (Yan et al., 2003) and erosion–carbon relationships(Li et al., 2003; Zhang et al., 2006a; Wei et al., 2008) wereconducted in the alpine grasslands of Qinghai–Tibetan plateau.Unfortunately, the dynamics of soil erosion, SOC, and N in land-use change from grassland to sloping farmland remain unclear.

Against this background, the current study aims to understand theeffects of the conversion of grassland to sloping farmland on soilerosion, SOC, and total N, as well as to determine the erosion-inducedsoil redistribution pattern in sloping farmlands at an alpine site inJinsha River Watershed, eastern Tibet. The results may provideinsights for promoting ecological security in the Tibetan Plateau.

2. Materials and methods

2.1. Study area

The study area lies in Jiangda County (301 210-32 1 000N, 971210-98 1530E), at the upper reaches of the Jinsha River, eastern

Tibet Autonomous Region, China (Fig. 1). With an average altitudeof 3650 m, the area is of typical alpine gorge geomorphology andhas a semi-humid temperate climate. Temperature per annumaverages 4.5 1C, with a maximum of 28 1C and a minimum of�15 1C. Annual precipitation is between 520 and 610 mm, andabout 77.9–95.8% of the rain occurs between May and September.Annual evaporation reaches 1600–1700 mm. Land use types inthe area mainly include forests, grasslands, and sloping farmlands.Most of the sloping farmlands had been converted from theoriginal grasslands 50 years ago. Soils in the study area, for bothgrassland and farmland, are derived from slope materials ofsandstone, limestone, metamorphic rock, and magmatic rock, andare classified as alpine meadow soil in Chinese soil taxonomy oras Calciudolls in US soil taxonomy. The dominant vegetationincludes alpine meadow and alpine steppe, and the major cropsare wheat (Triticum aestivum L.) and highland barley (Semen

Avenae Nudae).

2.2. Soil sampling and laboratory analysis

In the study area, sloping farmlands with an approximately 50-year cultivation history are mainly distributed in the lower partsof hillslopes between a.s.l 3400 and 3900 m. The sloping farm-lands are characterized by steep slopes (varying mainly from 101to 251) and short lengths (varying mainly from 10 to 40 m).Within the 3400–3900 m elevation interval, seven sloping farm-lands (i.e., 71, 111, 141, 161, 181, 191, and 241) at different locationswere selected to conduct soil sampling for 137Cs and otherphysical and chemical determinations. Meanwhile, seven controlfields were established for soil sampling, each in small patches ofunbroken and irregular grasslands neighboring the selectedsloping farmlands. The vegetation coverage was over 70% in thecontrol fields. Except for the 241 sloping farmland, samples fromthe upper, middle, and lower positions of each of the other sixfarmlands were collected. In the 241 sloping farmland, seven soilsampling points were set at 5 m intervals along the down-slopetransect and two replicates for each were collected.

Unlike the sampling for sloping farmlands, soil samples foreach control field were collected randomly in three replicates. Allsoil samples were collected to a depth of 23–37 cm depending onthe soil thickness (i.e., up to the bedrock) using a 6.8-cm diameterhand-operated core sampler. The core sample was subsequentlydivided into two segments: the surface (0–20 cm) and the subsoil(420 cm). For the six sloping farmlands (i.e., 71, 111, 141, 161, 181,and 191), the three surface and subsoil subsamples from theupper, middle, and lower positions of each farmland werecombined in bulk to create a composite sample. For the 241sloping farmland, the two replicated soil cores were combined in

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N. Xiaojun et al. / Applied Radiation and Isotopes 68 (2010) 1650–16551652

bulk to create a composite sample. For the seven control fields,three surface-and subsoil subsamples from three random posi-tions were combined in bulk to create a composite sample.Samples for the 137Cs reference could not be acquired because itwas difficult to find an appropriate reference site in the alpinegorge geomorphology, that is, an area where soil erosion,deposition, and human disturbance had not occurred. Overall,26 and 14 composite samples were collected from the slopingfarmlands and control fields, respectively.

Soil samples were air-dried, crushed, passed through a 2-mmmesh sieve to remove coarse material, and then divided into two,one for the measurement of 137Cs activity, and another for themeasurements of SOC, total N, and soil particle-size fraction.Samples for the o2 mm fraction were sealed in a 200-cm3

(fl0l mm�25 mm) plexiglass box. Twenty-eight days later,137Cs activity of these samples was measured using a hyper-pure lithium-drifted germanium detector linked to a LabSOCS(CANBERRA, USA) with a counting time of 36,000–86,400 s and ameasurement precision of ca.74.7% (95% confidence). Theoriginal measurements of 137Cs activity, expressed in terms ofper unit mass (Bq kg�1), were converted into the inventory(Bq m�2) by means of the total weight of the bulked core sampleand the cross-sectional area of the sampling device. The SOC wasdetermined using wet oxidation with K2Cr2O7, and the measure-ment of total nitrogen followed the classical Kjeldahl digestionmethod (Liu, 1996). Soil particle-size fractions were determinedby pipette method following H2O2 treatment to destroy organicmatter and dispersion of soil suspensions in Na-hexametapho-sphate (Liu, 1996).

38880

Horizontal length (m)

05 10 15 20 25 30

Fig. 2. 137Cs distribution across the 241 sloping farmland.

20.5%

2.3. Statistical analysis

One-way analysis of variance (ANOVA) was conducted to testthe significance of differences found between the sloping farm-lands and the control fields. Linear regression analysis was used totest the correlation between SOC, total N, and 137Cs.

y = -0.0022 x + 8.5644

r = -0.95, p = 0.001

17.0%

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19.5%

20.0%

3888

Cla

y co

nten

t (%

)

3891 3894 3897 3900 3903 3906Elevation (m)

Fig. 3. Relationship of clay content to elevation in the 241 sloping farmland.

3. Results

3.1. Soil erosion dynamics

In each of the seven sloping farmlands, 137Cs inventory waslower than that of its control field (Table 1). The 137Cs inventoryaveraged 1263 Bq m�2 (SE7178) in the seven farmlands,showing a 30% decrease compared to the mean of 1798 Bq m�2

(SE767) in the control fields. What’s more, coefficient ofvariation (CV) in 137Cs inventory was nearly 4 times as great inthe sloping farmlands (38%) as in the control fields (10%). Theseresults suggest that the conversion from alpine grassland tosloping farmland remarkably accelerated soil erosion intensityand increased its spatial variation.

Table 1Comparisons of the profile contents of 137Cs, SOC and total N (TN) between sloping far

Slope gradient 241 191

Sloping farmlands 137Cs (Bq m�2) 8407219 809

SOC (kg m�2) 4.2270.33 3.73

TN (kg m�2) 0.4070.03 0.40

Grasslands 137Cs (Bq m�2) 1712 1881

SOC (kg m�2) 6.03 6.59

TN (kg m�2) 0.54 0.64

Taking the 241 sloping farmland as a specific case, the soilerosion process can be tracked according to the 137Cs distributionalong the down-slope transect. Compared with a 137Cs inventoryof 1712 Bq m�2 in the control field, lower 137Cs inventories werefound within the 241 sloping farmland (Fig. 2). This implies a netsoil loss in steep sloping farmland. The 137Cs inventory increasedalong the down-slope transect, (Fig. 2), suggesting that soil lossgradually declined along this direction. In particular, 137Csinventory in the top-slope position (0 m) was the lowest, with avalue of 0 Bq m�2 (Fig. 2). This shows that tillage erosion inducedthe most severe soil loss in the positions. Soil clay (o0.002 mm)content increased with decreasing elevations in the farmlands(Fig. 3) and a significant negative correlation was found betweenthe two factors (r¼�0.95, p¼0.001). Soil clay content was alsosignificantly related to 137Cs inventory (r¼0.92, p¼0.003). The

mland and control fields (grassland).

181 161 141 111 71

1689 1331 677 1621 1870

5.10 3.63 2.90 5.08 5.72

0.51 0.39 0.31 0.49 0.56

1976 1438 1813 1876 1901

6.70 5.72 6.80 6.75 6.88

0.65 0.54 0.70 0.62 0.67

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809 840 1331 1621 1689 1870

Fig. 5. SOC and total N pools versus 137Cs inventory along the downward slope.

N. Xiaojun et al. / Applied Radiation and Isotopes 68 (2010) 1650–1655 1653

relationships between elevation, 137Cs, and soil clay indicate thatsoil fine particles are selectively transported by water erosion.

3.2. SOC and N dynamics

Similar to the results found for 137Cs inventory, contents ofSOC and total N in each sloping farmland were lower than those intheir respective control fields (Table 1). The inventories of SOCand total N averaged 4.34 (SE70.37) and 0.44 kg m�2 (SE70.03)in the seven farmlands, respectively, showing a significantdecrease of 27% (po0.001) and 33% (p¼0.001) compared tomeans of 6.49 kg m�2 SOC (SE70.16) and 0.62 kg m�2 total N(SE70.02) for their respective control fields. In addition, largevariations in SOC (CV, 23%) and total N (CV, 20%) were found in theseven sloping farmlands, while small variations were found (CV:7% for SOC; 10% for total N) in the control fields. These resultssuggest that the conversion from alpine grassland to slopingfarmland induces obvious losses of SOC and N, and alsoencourages great spatial variations.

The distribution patterns of SOC and total N were consistentacross the 241 sloping farmland, that is, their contents graduallyincreased along the down-slope transect (Fig. 4). A significantlypositive correlation (r¼0.99, po0.001) between the farmlandswas observed. Compared to those in the control fields (SOC,5.72 kg m�2; total N, 0.54 kg m�2), decreases in SOC and total Ninventories were most serious in the top-slope positions (0 m) ofthe farmlands, with a decrease of 47% in SOC and 48% in total N.The SOC and total N inventories in the bottom-slope position(30 m) were highest within the sloping farmland, but decreases of7% and 8% were still found in the SOC and total N inventories,respectively, compared to those in the control field.

3.3. Relationship of SOC and N to soil erosion

In the seven sloping farmlands, SOC and total N inventoriesdecreased with decreasing 137Cs inventory (Fig. 5). A significantand positive correlation was also found between SOC and 137Cs(r¼0.88, p¼0.01) and total N and 137Cs (r¼0.91, p¼0.004). Theserelationships show that storage of SOC and N decreases withincreasing soil erosion in sloping farmlands.

In the 241 sloping farmland, SOC and total N showed adistribution pattern similar to 137Cs across the entire slope,resulting in significant and positive correlations between them(r¼0.96, p¼0.001 between SOC and 137Cs; r¼0.95, p¼0.001between total N and 137Cs). The results are consistent withprevious studies by Ritchie and McCarty (2003), Zhang et al.(2006b), and Wei et al. (2008), which suggest that erosion-induced soil redistribution exerts remarkable impact on thespatial dynamics of SOC and N.

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Fig. 4. Distribution of SOC and total N across the 241 sloping farmland.

4. Discussion

Land-use change from grassland to farmland in sloping areasintensified soil erosion, which is mainly attributed to two reasons.One, after the conversion from original grassland to slopingfarmland, tillage can destroy the initial soil structure and exposeorganic-rich soils to open air, thus accelerating soil organic mattermineralization and reducing SOC content. Consequently, reduc-tions in SOC result in increased susceptibility of soil to erosion(Cambardella and Elliott, 1993; Six et al., 2000; Mostafa et al.,2008). Another is the impact brought about by tillage erosion. The137Cs inventory of 0 Bq m�2 in the top-slope position (0 m) of the241 sloping farmland confirms severe soil loss induced by tillageerosion. In this type of erosion, soils located upslope aretransported entirely downward and then deposited in bottomslope positions. Accordingly, water erosion risks increase, espe-cially when slope runoff gathers at deposited sites.

Conversion from grassland to sloping farmland in the studyarea significantly reduced the contents of SOC and total N. Similarresearch with consistent results have also been reported in otherclimate regions (Guo and Gifford, 2002; Wu and Tiessen, 2002; Suet al., 2004; Li et al., 2007; Yang et al., 2008; Mostafa et al., 2008).Li et al. (2007) reported that SOC decreased by 29–41% 28 yearsafter the conversion of an alpine grassland to a cropland in thenortheastern fringe of the Tibetan plateau (an altitude of 2960 m).In this study, about 50 years of cultivation resulted in a decreaseof 27% in SOC. The low decrease in SOC compared to that observedby Li et al. (2007) could be attributed to a lower SOMmineralization ratio under high altitudes (43400 m) and weaktillage intensity in the study area.

Spatial variability observed for soil erosion, SOC, and total N insloping farmlands is thought to be attributed to the combinedeffects of water erosion and tillage erosion. From the controlfields, CV values of 137Cs (10%), SOC (7%), and total N (10%) weresmall under the influence of only water erosion. After land-useconversion, however, the combination of tillage erosion and watererosion increased the difference in soil erosion (CV, 38%) betweenthe different sloping farmlands, also causing SOC and total Ninventories to decrease with increasing soil erosion. Theseresulted in large spatial variations in SOC (CV, 23%) and total N(CV, 20%). Schumacher et al. (1999) also reported that thecombination of tillage erosion and water erosion induces largervariability in soil productivity than a single erosion can inagricultural sloping landscapes.

In the 241 sloping farmland, 137Cs inventory graduallyincreased along the down-slope transect, with the lowest valueat 0 Bq m�2 in the top-slope position (0 m). Soil clay(o0.002 mm) content increased with decreasing elevation.

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The distributions of 137Cs and clay across the whole slope indicatethat both tillage erosion and water erosion determined soilredistribution pattern in these short and steep sloping farmlands.Furthermore, according to the clay distribution, water erosioncould be mainly responsible for soil redistribution. This inference,however, does not agree with previous studies conducted in thehilly areas of the Sichuan Basin in southwestern China (Zhanget al., 2006b; Ge et al., 2007; Zhang et al., 2008). In these studies,tillage erosion played a primary role in the soil redistributionpattern of steep slopes (slope lengths r30 m), and the entiretransport of tillage erosion does not induce an increase in claycontent with decreasing elevation. This inconsistency can beexplained by the fact that there are hardly any boundaries acrossthe slope. Even if there had been any, these boundaries wouldhave been quite loose and present as dwarf heaps of gravel in thestudy area. Without the effective protection of boundaries acrossthe slope, plus the fact that sloping farmlands lie in lower regionsof the long hillslope, runoff from up the slope would maintain asingle path and proceed straight downward during the watererosion process. Hence, the entire slope of the farmland suffersobvious water erosion. On the other hand, in hilly areas of theSichuan Basin, the slope runoff across steep hillslopes isdiscontinuous because of the set-up of boundaries between fields.Thus, runoff within a field finds difficulty in forming in the upperregions of the slope, gathering only in the lower regions. In fact,soil redistribution in steep sloping farmlands in hilly areas of theSichuan Basin is not induced by water erosion but by tillageerosion. As such, building boundaries between fields may helpreduce soil erosion in the agricultural sloping landscapes of theTibetan plateau.

Significant and positive correlations between SOC, total N, and137Cs were found in the 241 sloping farmland. This result is in linewith the previous studies (Schumacher et al., 1999; Thapa et al.,2001; Ritchie and McCarty, 2003; Li and Lindstrom, 2001;Heckrath et al., 2005; Zhang et al., 2006a, 2006b; Ge et al., 2007;Ritchie et al., 2007; Zhang et al., 2008; Wei et al., 2008; Mabitet al., 2008), suggesting the same pathway during soil redistribu-tion. As there exist strong correlations between SOC, N, and 137Cs,the 137Cs technique can provide a useful tool for extensiveresearch on the dynamics of soil erosion, SOC, and N stocks in theTibetan plateau.

5. Conclusions

In the alpine gorge regions of eastern Tibet, the conversionfrom alpine grassland to sloping farmland was found to haveaccelerated soil erosion and resulted in significant losses of SOCand N. The combined impacts of tillage and water erosion insloping farmlands induced increase in spatial variability in soilerosion, SOC, and total N, and significantly decreased SOC andtotal N storages with increasing soil erosion. Particularly insteeply sloping farmlands, tillage erosion contributed to severesoil losses. However, the soil redistribution pattern was domi-nated by water erosion, not tillage erosion, due to lack ofboundaries across the field patches. Moreover, 137Cs, SOC, andtotal N moved along the same pathway within these slopingfarmlands, resulting in net C and N losses during soil redistribu-tion. Building boundaries could be an effective solution inminimizing soil erosion, organic carbon, and N losses in suchareas. Future research on the relationships between land-usechanges, soil erosion, and SOC and N contents in the plateau couldbe conducted using the 137Cs technique to better understandalpine soil dynamics.

Acknowledgements

We acknowledge the Science and Technology Project of China(2007BAC06B06, 2007BAC06B08) and the Program for Scienceand Technology Innovation Talents at the Universities of HenanProvince (2008HAST1T004) for the research funds they providedour team. We would also like to thank Dr. Zhang Junyan for herassistance in editing this paper and two local farmers for theirassistance with soil sample collection.

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