Significant Acidification in Major Chinese Croplands

Transcription

1 Supporting Online Material for Significant Acidification in Major Chinese Croplands J. H. Guo, X. J. Liu, Y. Zhang, J. L. Shen, W. X. Han, W. F. Zhang, P. Christie, K. W. T. Goulding, P. M. Vitousek, F. S. Zhang* *To whom correspondence should be addressed. This PDF file includes: Published 11 February 2010 on Science Express DOI: /science Materials and Methods SOM Text Figs. S1 to S5 Tables S1 to S4 References and Notes

2 Supporting Online Materials for Significant Acidification in Major Chinese Croplands J. H. Guo 1, X. J. Liu 1, Y. Zhang 1, J. L. Shen 1, W. X. Han 1, W. F. Zhang 1, P. Christie 1,2, K. W. T. Goulding 3, P. M. Vitousek 4, F. S. Zhang 1,* 1 College of Resources and Environmental Sciences, China Agricultural University, Beijing , China; 2 Agri-Environment Branch, Agri-Food and Biosciences Institute, Belfast BT9 5PX, UK; 3 Department of Soil Science, Rothamsted Research, Harpenden, Herts, AL5 2JQ, UK; 4 Department of Biology, Stanford University, Stanford, CA 94305, USA. * To whom correspondence should be addressed. zhangfs@cau.edu.cn. This PDF file includes: Materials and Methods Text Figures S1 to S5 Tables S1 to S4 References and Notes 1

3 Significant Acidification in Major Chinese Croplands Supporting online material J. H. Guo et al. Contents 1. Materials and Methods 1.1 Soil classification and distribution 1.2 Soil ph data collection 1.3 Soil ph data statistics 1.4 Acidification potential calculation 2. Text 2.1 Soil ph decline based on unpaired comparisons 2.2 Long-term ph decreases in typical agricultural soils 2.3 Contribution of acid deposition, nitrification and phosphorus fertilizer to agricultural soil acidification 2.4 Strategies to improve nitrogen use efficiency 2.5 Lime application in Chinese agriculture 3. Figures 4. Tables 5. References and Notes 2

4 1. Materials and Methods 1.1. Soil type and distribution There are 12 soil classes and 61 soil types in China according to the Second National Soil Survey (S1), which brings formidable difficulties when attempting to make direct ph comparisons among all the soil types due to the substantial variations in their properties. Therefore arable soils were classified into 6 groups according to their development processes, ph ranges and geographical (climatic) distributions (Table S1). Taxonomic descriptions for each soil type can be found in a recent monograph (S2) and also briefly in Table S1. Soils in Group I, widely distributed in south and southwest China, were formed under humid tropical and subtropical climates. These highly weathered soils, such as red soils (Argi-Udic Ferrosols), are acidic or extremely acidic with high sensitivity to acidification. Paddy soils (Stagnic Anthrosols) were classified into an individual group (Group II) since long-term flooding has produced significant differences in their properties compared with other soils. Approximately 75% of paddy soils are in south China and others are scattered in other regions such as northeast China. Soils in Group III such as purplish soils (Purpli-Udic Cambosols) are mainly distributed in southwest China (e.g. Sichuan province, Yunnan province and Chongqing municipality). These soils were developed from purple sandstone and shale with less weathering and thus buffering capacity to chemical and physical degradation (e.g. acidification). Soils in Group IV are mainly distributed in (sub-) the humid climate of the middle latitude temperate zone (e.g. northeast and part of north China). These slightly acidic and neutral soils are sensitive to acidification since base cation leaching is the main buffering process at their ph range. Black soils (Udic Isohumosols), dark brown earth (Bori-Udic 3

5 Cambosols) and brown earth soils (Hapli-Udic Argosols) are the most typical soils in this group. Soils in Group V, formed in humid and semi-humid areas of the warm temperate zone, are mainly distributed on the North China Plain and Loess Plateau. Acidification has not been thought to be a major problem in these soils because the Second National Soil Survey showed them to contain 5-10% CaCO 3. Fluvo-aquic soils (Ochri-Aquic Cambosols), cinnamon soils (Hapli-Ustic Argosols/Cambosols) and cultivated loessial soils (Loessi-Orthic Primosols) are the most widely distributed soils in Group V. Soils in northwest China were classified into Group VI, well known as aeolian soils (Aridi-Sandic Primosols), gray-brown desert soils (Calci-Orthic Aridisols), lime chernozems (Calic-Ustic Isohumosols) and frigid calcic soils (Calci-Gelic Cambosols). These CaCO 3 (10-15%) containing soils in an arid climatic zone are poorly weathered and insensitive to acidification. In addition, each soil group was further divided into two sub-groups based on soil utilization pattern (cereal crop and cash crop systems). In cereal crop systems, soils are used for cereal/fiber crops (e.g. wheat, maize, rice, cotton). In cash crop systems, vegetables, fruit trees, and tea are planted with higher fertilizer loadings and higher economic outputs. The geographical distribution of the data is shown in Fig. S Soil ph data collection In this report, 8875 topsoil (0-30 cm) ph analyses were used to calculate the ph changes between the 1980s and 2000s. Most soil ph values (3603) in the 1980s were collected from China Soil Species (Volume 1-6) (S3). These data were measured during the Second National Soil Survey ( ) and are regarded as the best soil database in China. Other soil ph data were collected from 912 formal published articles, dissertations, and monographs (Table 4

6 S2). Of these data, 5070 and 202 were determined in the 2000s ( ) and 1980s, respectively. All these data were measured according to Chinese national or occupational standards using water as the extractant. We have also summarized 154 nationwide paired data on topsoil ph (1980s vs. 2000s) from many published papers. The data sets cover 7 major agricultural soil types (i.e. 11 red soils, 5 black soils, 89 paddy soils, 6 purplish soils, 3 yellow brown soils, 24 fluvo-aquic soils and 16 meadow soils (Litteri-Aquic Cambosols)) which are distributed over 35 sites in 7 provinces. All the 154 paired topsoil ph values were measured from the same agricultural sites in the 1980s vs. the 2000s. Therefore the changes in soil ph should reflect soil acidification or alkalization trends. We have selected ten representative long-term monitoring field (LTMF) sites which have received more than 8 years of NPK fertilizer applications (representing local farming practice) (S4-S12). There are three soils in Group I (two red soils distributed in Hunan province and one in Jiangxi province), one paddy soil in Group II (Jiangsu province), one purplish soil in Group III (Chongqing province), two black soils in Group IV (one in Jilin province and one in Heilongjiang province), and three soils in Group V (one in Shandong province and two in Shanxi province). At these LTMF sites, topsoil ph values were measured annually or every two to eight years, thus soil acidification status could be evaluated according to the changes in topsoil ph. Data on topsoil ph dynamics in Fallow (no crop and no fertilization), CK (no fertilization), NPK (conventional fertilization) plots were also collected from two LTMF experiment sites: one in Chongqing (purplish soil, Group III) and one in Jilin (black soil, Group IV) and these results are illustrated in Fig. S2. 5

7 1.3. Soil ph data statistics Soil ph data in the 2000s were used for comparison with reference ph data from the 1980s based on the above mentioned soil classification. Since the soil ph data in the two periods were not strictly geographically equal, an unpaired t-test statistical comparison was used Acidification potential calculation Soil acidification is the integrated result of series of biogeochemical processes in the soil-plant system and these processes have been well documented in the literature (S13-S16). Briefly, losses of base cations (i.e. Na +, K +, Mg 2+ and Ca 2+ ) and gains of anions (e.g. SO 2-4, H 2 PO - 4, Cl - and organic acids) will increase soil acidity, and vice versa. For nitrogen (N) with both cationic (i.e. NH + 4 ) and anionic (i.e. NO - 3 ) forms, its contribution to soil acidification is somewhat complex. Nitrification (from NH + 4 to NO - 3 ) and uptake of NH + 4 tend to acidify the soil, while uptake of NO - 3 makes soil more alkaline. In agricultural ecosystems uptake of base cations (i.e. Na +, K +, Mg 2+ and Ca 2+ ) and anions (H 2 PO 4 - and SO 4 2- ) by crops can be considered as the dominant source of acidity and alkalinity, respectively (eqs. 1, 2 and 3) (S15), assuming that uptake and removal of these ions from soil leaves equivalent H + or OH - in the soil. For N cycling, the contribution to soil acidification is calculated based on ecosystem mass balance budget as eq. 4 (S13-S14). H = BCs Yield + BCs Yield + BCs g g s s (eq. 1) OH = P Yield + P Yield p g g s s (eq. 2) OH s = 2( S g Yield g + S s Yield s ) (eq. 3) Where BCs g, P g and S g signify the dry weight based concentrations for BCs s, P and S in 6

8 crop grain/fruit, respectively; and BCs s, P s and S s denote the dry weight based concentrations for BCs s, P and S in crop residues (stalks and leaves) that are removed from the field. Yield g and Yield s are the dry biomass of crop grain/fruit and residue, respectively. + In Eq. 4, NH 4 In is the input of NH + 4 from atmospheric deposition and chemical N fertilizer. 90% of chemical N fertilizer was assumed as NH + 4 and R-NH 2 based fertilizer in all the studied systems; this assumption is consistent with the situation in China at the national + scale (S17). NH 4 Out, the output of NH + 4, is assumed to be zero since NH + 4 leaching is - negligible in Chinese agricultural systems (S18). NO 3 In is the influx of NO - 3 from deposition and irrigation. NO 3 - Out is the efflux of NO 3 - from the topsoil and mainly includes NO 3 - leaching and accumulation in deeper soil horizons. In calculations for the rice-rice system - (R-R), NO 3 Out is estimated as the net balance between total N inputs (i.e. deposition, fertilization and irrigation) and outputs other than NO - 3 leaching (i.e. NH 3 volatilization, crop uptake and denitrification losses), since NO - 3 leaching is very low in flooded paddy soils (S19). In the wheat-maize system (W-M), denitrification losses were estimated as the difference between N inputs and outputs (NH 3 volatilization, crop uptake and NO - 3 leaching). H + N + + = NH NH ) + ( NO NO ) (eq. 4) ( 4 In 4 Out 3 Out 3 In In the calculation in Fig. 3 and Tables S2 and S3, four regions were selected as examples for each agricultural system: the North China Plain for wheat-maize (W-M); central and east China for rice-wheat (W-R); south China for rice-rice (R-R) and Shouguang county in Shandong province for greenhouse vegetable production (G-V). These acidification potential calculations were based on meta-analysis because all the related parameters were summarized from 2800 published articles and 52 Ph.D. and M.S. degree dissertations. 7

9 2. Text 2.1. Soil ph decline based on unpaired comparisons To evaluate the ph changes in major Chinese crop lands, two data sets (i.e. from the 1980s and 2000s) were divided into 6 groups and 12 sub-groups (two in each group) (Table S1; Fig. S1). The results show significant (p<0.001) acidification from the 1980s to the 2000s for all topsoils, except soils in Group VI (Fig. S3). Soils in Group I, widely distributed in south China, usually have lower ph mainly due to natural soil development processes. Acidification of these soils and its effects have raised concerns due to their higher acidification sensitivities. In the past two decades, these soils were further acidified in both cereal crop and cash crop systems, in agreement with some studies in specific sites and small regions (S20-S22). Paddy soils (Group II) show ph declines of 0.13 and 0.35 units (p<0.001), respectively (Fig. S3). These soils are normally regarded as having a strong ph buffering capacity. In cereal crop systems (e.g. single rice, double rice and rice-wheat systems), only a slight ph decline (0.13 units, p<0.001) was found based on data comparison; while in cash crop systems (e.g. rice-vegetable systems) with higher N fertilization and high crop output, average soil ph decreased by 0.35 units during the past 20 years (Fig. S3). Among all 6 investigated soil groups, purplish soils (Group III) had the largest ph declines of 0.76 and 0.80 units in the cereal crop and cash crop systems, respectively (Fig. S3). These shallow layered and low organic matter soils, developed from poorly weathered purplish sand/shale rock, usually have lower buffering capacity to chemical and physical erosion. Based on systematic data comparison, Li and Wang (S23) also found similar acidification of purplish soils in Chongqing region. Soils in Group IV (e.g. black soils) are the most productive soils in 8

10 northeast China. Compared with the soil ph in the 1980s, these soils were significantly acidified (p<0.001) with net ph decrease of 0.32 and 0.72 units in the cereal crop and cash crop systems, respectively. Soils in Group V are widely distributed in north China where rainfall is low. Dissolution of CaCO 3 (5-10%) and other minerals can consume the external H +, and buffer the large decrease in soil ph. However, these soils also suffered from soil ph decrease by 0.27 and 0.58 units in the two systems. As for soil group VI, there was no significant ph decrease in either cereal crop or cash crop systems. These weakly developed soils are distributed widely in northwest and west China where agriculture is less developed than in other regions. Also soils in northwest and west China (Group VI) usually have higher CaCO 3 content (10-15%) and very high soil ph values (typically ) Long-term ph decreases in typical agricultural soils Regional soil acidification is also supported by the long-term monitoring and data comparison at specific sites in China. Long term monitoring of topsoil ph in ten typical experimental sites clearly shows trends of soil acidification (Fig. S4). All this acidification occurred under the local N fertilization system with the removal of plant residues after each harvest. Three soils in Group I were acidified with ph decreases of 1.50, 1.52 and 0.79 units after 13, 25 and 19 years of cereal crops (Fig. S4). Although a calcareous purplish soil (Group III) had an original ph of 7.90, the topsoil was clearly acidified with a ph decrease of 0.70 units after 16 years of R-W rotation under the local farming fertilization treatment (Fig. S4). For black soils (Group IV-1 and Group IV-2) in northeast China, soil acidification was also observed by the long term monitoring at Hailun (Jilin province) and Harbin (Heilongjiang province) agricultural experimental stations, with ph decreases of 0.45 and 1.02 units after 16 9

11 and 25 years of utilization (Fig. S4). Compared with these cereal crop systems, cash crop systems such as greenhouse vegetables can result in greater soil acidification. Paddy soils (Group II) in Jiangsu province, fluvo-aquic soils (Group V-1) in Shandong province, cultivated loessial soils (Loessi-Orthic Primosols, Group V-2) and calcareous cinnamon soils (Hapli-Ustic Cambosols, Group V-3) in Shanxi province, were greatly acidified with ph decreases of 1.97, 2.20, 1.01, and 0.77 units due to 15, 13, 15 and 8 years of cropping with greenhouse vegetables, respectively (Fig. S4). As mentioned earlier, we have also compared soil ph dynamics in Fallow, CK and NPK plots at two LTMF sites. The results confirm that significant soil acidification has occurred only in the NPK plots (soil ph decrease by 0.74 and 1.36 units, p<0.001), whereas soil ph values in the Fallow and CK plots were almost the same as their original values when these experiments initiated about 20 years ago (Fig. S2). NATESC (the Chinese National Agricultural Technology Extension Service Center) and IARRP (the Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences) (S24) recently compiled the changes in fertility of major agricultural soils based on long term monitoring results from about 190 monitoring stations. In the past decades (1985 to 2005), fluvo-aquic soils and cinnamon soils, the two major soil types in Group V, suffered from acidification with ph decreases of 0.49 and 0.60 units based on the analysis of data sets from 37 and 26 stations. Although these ph decreases in cereal crop systems are lower than those in cash crop systems (i.e. Group V-1, V-2 and V-3), the trends in acidification are substantial. The topsoil ph in paddy soils (Group II) decreased 0.54 units in the past 22 years ( ). Furthermore, a significant negative relationship between soil ph and annual N fertilization rates was observed in purplish soils (Group III) when all the 10

12 data from 8 experimental stations were combined. Based on long term monitoring at numerous stations, significant ph decreases were also found for the sensitive red soils (Group I) and black soils (Group IV), especially after In summary, the above results from long term monitoring showed clear ph decreases in major Chinese croplands, including both high-input cereal cropping systems and extremely high-input cash crop systems Contribution of acid deposition, nitrification and phosphorus fertilizer to agricultural soil acidification Atmospheric acid deposition, an important environmental problem in China, may also contribute to reported Chinese soil acidification (S25). The acidification potential of acid deposition is largely counteracted by neutralization with alkaline dust, especially in north and northwest China (S26-S28). In Chinese acid rain areas volume weighted average precipitation ph usually ranges from 4.0 to 5.6 (S29), and at the lower ph extreme H + accompanied by 2- major mobile anions (i.e. SO 4 and NO - 3 ) contributes only 0.4 to 2.0 kmol H ha -1 yr -1 as annual precipitation varies between 400 and 2000 mm. In the soil, nitrification is a key process that controls acidity. Ammonium-based fertilizer will be nitrified soon after application to most arable soils (except flooded paddy soil), generating 2 mol H + per mole of NH + 4 -N (S30). This temporally generated proton will be partly neutralized by the subsequent plant and soil processes (e.g., nitrate uptake, denitrification). Thus only the net nitrification will lead to permanent acidification in soils. With the wide use of urea, ammonium bicarbonate and diammonium phosphate (DAP) in China, the potential contribution of nitrification to soil acidification deserves more attention. Besides N fertilizer, phosphorus (P) fertilizer may also contribute to soil acidification 11

13 under certain conditions (S16). The contribution of P fertilizer to soil acidification based on the acidification potential of each P fertilizer (S16) and the national data on P fertilizer consumption (S17) in 2006 have been roughly estimated. We found on average the acidification potentials to Chinese arable soils (125 Mha, from Table S1) were only 0.18, and 0.74 kmol H + ha -1 yr -1 for single superphosphate, triple superphosphate and DAP, respectively. These numbers are much smaller than those from N cycling process (Table S3). In fact, the contribution of DAP to soil acidification has been included (at least partly) in the N cycling process, suggesting that the potential acidification of P fertilizer could be neglected in our calculations Major strategies to improve nitrogen use efficiency Since 1980 Chinese crop production has increased with rapidly increasing N fertilizer consumption, leading to decreased N fertilizer use efficiency or partial fertilizer productivity (PFP) (Fig. S5). Improving N use efficiency will reduce N loss (especially nitrate leaching and ammonia volatilization) and the subsequent soil acidification. There were several studies on the efficient use of N fertilizer in Chinese agriculture by optimized N fertilization (S18, S31-S33). Using soil N min tests, for example, Chen et al. (S31) found that fertilizer N rate can be largely reduced while N use efficiency can be more than doubled compared with conventional N management in the W-M systems. Wang et al. (S32) also reported substantially improved N use efficiency in R-R systems under optimal N management based on site-specific nutrient management. In fact, major strategies for improving N use efficiency in China should focus on the three R s: Right application rate, Right application time and Right application method. Deep application of N fertilizer or N placement in the root zone 12

14 was proven to be a very useful method to improve N use efficiency while reducing N loss rate (S19). Integrated N management incorporating balanced fertilization, optimized irrigation, rational rotation, tillage, and weed/pest/disease control will be crucial to improve N use efficiency in Chinese intensive agriculture (S33). In future, precision agriculture which aims to treat plants individually may play a more important role in the efficient use of N (as well as other resources) with minimal negative impact on the environment Lime application in Chinese agriculture Liming is most commonly practiced to overcome the impact of soil acidification (S16). Liming practice was common in China (especially southern and southwestern China) in the late 1970s and early 1980s. About Tg CaCO 3 was applied annually to 2 million ha of paddy soils in Hunan province before 1978 (S34), for example. National lime application may have been up to 30 Tg CaCO 3 in the early 1980s if the twelve Chinese southern provinces (Hunan, Hubei, Guangdong, Guangxi, Yunnan, Guizhou, Sichuan, Jiangxi, Fujian, Zhejiang, Anhui, and Jiangsu), the major areas of acidic soils, applied the same or similar amounts of lime to agricultural soils as in Hunan province. However, liming has declined greatly since the mid 1980s when the farmers started to run their croplands individually because liming is a labor-intensive without early and obvious economical benefits. The importance of lime application to neutralize soil acidity and improve fertilizer use efficiency was recently (again) recognized in some cash crop systems in China (S35-S36) which suffered from severe soil acidification. It is clear that the rapid decline in liming since the 1980s is likely to accelerate acidification rates in Chinese croplands, but there were no national statistical data on lime consumption in agricultural soils of China. 13

15 14 3. Figures Cereal crop systems Cash crop systems Cereal crop systems Cash crop systems Fig. S1. Geographical distribution of data collected. Gray and black dots denote cereal crop systems and cash crop systems, respectively.

16 A ph Fallow CK NPK Cropping year (yr) B ph Fallow CK NPK Cropping year (yr) Fig. S2. Dynamics of topsoil ph in Fallow, CK and NPK plots at two long-term monitoring field experiment sites (A: black soil; B: purplish soil). Data are means ± sd. (Fallow: no crop and no fertilization; CK: no fertilization with crop; NPK: conventional fertilization with crop) 15

17 s 2000s cereal crop systems 2000s cash crop systems ** ** ns ns 7.00 ph 6.00 ** ** ** ** ** ** 5.00 ** ** Group I Group II Group III Group IV Group V Group VI Soil groups Fig. S3. Changes in topsoil ph in cereal crop systems (gray columns) and cash crop systems (black columns) in major Chinese croplands in the 2000s compared with those in the 1980s. **, significant at p<0.001 level; ns not significant. 16

18 Soil ph decrease Group 1-1 Group 1-2 Group 1-3 Group 2 Group 3 Group 4-1 Group 4-2 Group 5-1 Group 5-2 Group 5-3 I-1 I-2 I-3 II III IV-1 IV-2 V-1 V-2 V-3 Soils Fig. S4. Topsoil ph decreases at ten typical long term monitoring sites receiving conventional NPK fertilizers for 8-25 years. Numbers in the columns denote number of years of monitoring. 17

19 N fertilizer consumption (Tg N yr -1 ) PFP (kg grain kg -1 applied N) N fertilizer consumption 350 PFP Grain production Year Grain production (Mt yr -1 ) Fig. S5. Changes in annual N fertilizer consumption, grain production and partial fertilizer productivity (PFP) in China during 1980 and PFP is the ratio of grain production to N fertilizer consumption. PFP denotes N use efficiency in this study. 18

20 4. Tables Table S1. Soil group classification in Chinese croplands Soil group I Chinese soil geographic-genetic classification Red earth soils*, yellow earth soils, humid-thermo ferralitic soils, torrid red soils, lateritic red earths Chinese soil taxonomic classification Udic Ferralosols/Ferrosols Perudic Argosols II Paddy soils Stagnic Anthrosols III IV V VI Purplish soils, skeletal soils, Litho soils Black soils, dark-brown earths, brown earths, yellow-brown earths, albic soils, meadow soils, bog soils, felty soils, dark felty soils Fluvo-aquic soils, cumulated irrigated soils, alluvial soils, cinnamon soils, dark loessial soils, cultivated loessial soils, castano-cinnamon soils, lime concretion black soils, yellow-cinnamon soils, loessial soils, cumulated irrigated soils, meadow soils Aeolian soils, lime chernozems, brown calcic soils, gray desert soils, gray-brown desert soils, grey-cinnamon soils, castanozems, brown desert soils, frigid calcic soils, irrigated desert soils Based on reference S2; * Denoting representative soils in the each group. Purpli-Udic Cambosols Udic-Orthic Primosols Haplic-Udic Isohumosols Bori-Udic Cambosols Haplic-Udic Argosols Aquic Cambosols Ustic Argosols/Cambosols Orthic Primosols Calci-Aquic Verstosols Aridi-Sandic Primosols Calic-Orthic Aridosols Calic-Ustic Isohumosols (Calic-) Gelic Cambosols Distribution region Area Proportion (M ha) (%) South, southwest Central, south, southwest Southwest Northeast, north North, northwest Northwest

21 Table S2. Complete data source list (totally 912 references, including 868 Journal articles, 39 Degree dissertations, and 5 Monographs) on soil ph data in Chinese agricultural soils in the 1980s and the 2000s. No. Language * Type Author list Journal, Dissertation, Monograph Year Published, Volume and Pages 1 C [J] Ai SL, Ma YM, Wei CC. Acta Agriculturae Boreali-occidentalis Sinica : C [J] Ai YW. Chinese Journal of Soil Science : C [J] An SS, Huang YM, Li BC, Yang JG. Chinese Journal of Soil Science : C [J] An SS, Guo M, Yang JG, Chang QR. Chinese Journal of Soil Science : C [J] An SS, Liu MY, Li BC, Jiao JY. Acta Botanica Boreali-Occidentalia Sinica : C [J] An TT, Wang JK, Li SY, Yu S, Zhu P. Chinese Journal of Applied Ecology : C [J] Ao HJ, Zou YB, Shen JB, Peng SB, Plant Nutrition and Fertilizer Science : Tang QY, Feng YH. 8 C [J] Bai SQ, Lu GS. Chinese Journal of Soil Science : C [J] Bai YF, Zhang H, Zhang LX. Chinese Journal of Eco-Agriculture : C [J] Bao JF, Xia RX, Peng SA, Li GH. Soils :

22 11 C [D] Bao YX. Soil Physical and Chemical Properties of Dam Land and Terrace in the Loess Hilly Region. M.S. dissertation, Northwest A&F University. 12 C [J] Cai H, Yang H, Sun B. Zang B. Soils : C [J] Cai HY, Fan GN, Xiong DZ. Fujian Science & Technology of Tropical : C [J] Cai XB, Dong GZ. Acta Ecologica Sinica : Crops. 15 C [J] Cai YF, Liao ZW, Zhang JE, Kong Chinese Journal of Applied Ecology : WD, He CX. 16 C [J] Cai ZC. Acta Pedologica Sinica : C [J] Cao B, Jin XX, Cai GX, Fan XH, Sun Plant Nutrition and Fertilizer Science : HX. 18 C [J] Cao H, Yang H, Sun B, Zhao QG. Soils : C [J] Cao N, Chen XP, Zhang FS, Qu D. Acta Pedologica Sinica :

23 20 C [J] Cao N, Qu D. Chinese Journal of Soil Science : C [J] Cen J, Wang XJ, Zhu LJ. Soils : C [J] Chang QR, An SS, Liu J, Wen ZG. Acta Pedologica Sinica : C [J] Chang QR, Lei M, Feng LX, Yan X. Acta Pedologica Sinica : C [J] Chang QS, Cai ZG, Yang WJ, Wang Research of Soil and Water Conservation : XB. 25 C [J] Chang XB, Liu J, Han JL, Su H. Journal of Anhui Agricultural Science : C [J] Chang XB, Liu J, Han JL. Journal of Anhui Agricultural Science : 11143, C [J] Chang XG, Huang GK, Xiong KM, Cultivation and Planting : Cao KW, Zhou PJ, Liu LW. 28 C [J] Chen AL, Hong W. Acta Agricuturae Universitatis Jiangxiensis : C [J] Chen AL, Wang KR, Xie XL, Su YT. Plant Nutrition and Fertilizer Science : C [J] Chen CL, Yao SW, Yuan TY, Zhao Journal of Anhui Agricultural Sciences : AL. 31 C [J] Chen CP, Xu XJ, Zhang Q, Yang ZP, Plant Nutrition and Fertilizer Science :

24 Cheng B, Liu P, Li L. 32 C [J] Chen CQ, He YQ, Bian XM, Yu DG. Plant Nutrition and Fertilizer Science : C [J] Chen GC, He ZL, Huang CY. Acta Pedologica Sinica : C [J] Chen HL, Zhou JM, Jin YB, Du AF, Journal of Soil and Water Conservation : Yu WF, Yang XD. 35 C [J] Chen HS, Liu GS, Ye XF, Wei YW. Journal of Henan Agricultural University : C [J] Chen J, Pan GX, Wang YL. Scientia Agricultura Sinica : C [J] Chen JG, Liu XJ, Yi H. Research of Soil and Water Conservation : C [J] Chen JZ, Wang S, Zhang LL, Lv GA. Scientia Agricultura Sinica : C [J] Chen L, Hao DM, Zhang SM, Fan HL. Plant Nutrition and Fertilizer Science : C [J] Chen L, Hao MD, Qi LH. Plant Nutrition and Fertilizer Science : C [J] Chen LH, Lu SW, Zhang XP, Yu XX, Research of Soil and Water Conservation : Sun ZF. 42 C [J] Chen M, Chen YH, Shen ZG, Shen Plant Nutrition and Fertilizer Science : QR. 43 C [J] Chen Q, Zhang HY, Zhang XS, Wu JF, Plant Nutrition and Fertilizer Science :

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