Accumulation and bioavailability of heavy metals in a soil-wheat/maize system with long-term sewage sludge amendments

YANG Guo-hang, ZHU Guang-yun, LI He-lian, HAN Xue-mei, LI Ju-mei, MA Yi-bing

1 School of Resources and Environment, University of Jinan, Jinan 250022, P.R.China

2 Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China

Abstract A long-term field experiment was carried out with a wheat-maize rotation system to investigate the accumulation and bioavailability of heavy metals in a calcareous soil at different rates of sewage sludge amendment. There are significant linear correlations between the contents of Hg, Zn, Cu, Pb, and Cd in soil and sewage sludge amendment rates. By increasing 1 ton of applied sludge per hectare per year in soil, the contents of Hg, Zn, Cu, Pb, and Cd in soil increased by 6.20, 619, 92.9,49.2, and 0.500 μg kg–1, respectively. For Hg, sewage sludge could be safely applied to the soil for 18 years at an application rate of 7.5 t ha–1 before content exceeded the soil environmental quality standards in China (1 mg kg–1). The safe application period for Zn is 51 years and is even longer for other heavy metals (112 years for Cu, 224 years for Cd, and 902 years for Pb) at an application rate of 7.5 t ha–1 sewage sludge. The contents of Zn and Ni in wheat grains and Zn, Cu, and Cr in maize grains increased linearly with increasing sewage sludge amendment rates. The contents of Zn, Cr, and Ni in wheat straws and Zn, Cu, and As in maize straws were positively correlated with sewage sludge amendment rates, while the content of Cu in wheat straws and Cr in maize straws showed the opposite trend. The bioconcentration factors of the heavy metals in wheat and maize grains were found to be in the order of Zn＞Cu＞Cd＞Hg＞Cr=Ni＞Pb＞As. Furthermore, the bioconcentration factors of heavy metals in wheat were greater than those in maize, indicating that wheat is more sensitive than maize as an indicator plant. These results will be helpful in developing the critical loads for sewage sludge amendment in calcareous soils.

Keywords: sewage sludge, agricultural use, heavy metals, calcareous soil, bioconcentration factors

1. Introduction

Sewage sludge (SS), also known as biosolids, is biological residues produced from sewage treatment processes, and is a very complicated heterogeneous substrate consisting of organic materials, bacteria, inorganic particles, and colloids. The production of SS has increased significantly in recent years due to the rapid development of industrial and economic activities as well as escalating growth of urban populations (Shaoet al.2015). The amount of SS in China increased from 30 million metric tons (at a moisture content of 80%) in 2012 to 34 million metric tons in 2015 (Fenget al.2015). The safe disposal and recycling of SS is one of the major environmental concerns worldwide. At present, the main methods of SS disposal and comprehensive utilization include incineration, landfills, and agricultural application(Harrisonet al.1999; Fytili and Zabaniotou 2008).Agricultural application of SS is the most economically viable and environmentally sustainable method (Kacprzaket al.2017), owing to its high content of organic matter(OM), nitrogen (N), phosphorus (P), potassium (K) and other plant nutrients (Herzelet al.2015; Rigbyet al.2015),which can benefit plant growth and development (Lavado 2006; Singh and Agrawal 2007, 2010; Grobelaket al.2017).In the European Union, approximately 40% of the total SS is used for agricultural purposes (CEU 1999). In Belgium,Denmark, Spain, France, Ireland, and the United Kingdom,more than 50% of SS was used for agriculture in 2010(Kacprzaket al.2017). Many studies support the positive effect of SS amendments on crops, including spinach (Bravoet al.2015) and barley (Pasqualoneet al.2016).

Sewage sludge also contains various contaminants, such as heavy metals, polycyclic aromatic hydrocarbons (PAHs),polychlorinated biphenyls (PCBs), pesticides, surfactants,hormones, and many other materials (Walteret al.2006;Oleszczuk and Hollert 2011; Siebielska 2014; Kacprzaket al.2017), which may pose a risk to the environment and human health when applied to agriculture (Richardset al.1998). Sewage sludge amendment of soil has been greatly hampered due to concern over heavy metal contamination in soils and plants (McLaughlinet al.2006). Richardet al.(1998) investigated the mobility of metals in soils from a long-term heavily loaded sludge application site. They found that after nearly 20 years of SS application, metal contents in soils were significantly increased compared with those of the control plot. Moreover, elevated Zn, Cd, Cu, and Ni levels were found in grass growing on the SS plot, while there was no significant increase for metal contents in apple fruits and leaves. Greenhouse and field experiments have demonstrated that SS amendments significantly increased the contents of heavy metals (Zn, Cu, Cr, Ni, Cd, and Pb)in soils and plants (Singh and Agrawal 2007, 2010), and the contents of several heavy metals (Cd in soil and Zn, Ni, and Cd in plants in greenhouse experiments; Cd, Pb, and Ni in seeds in field experiments) can exceed limits permissible by governments. However, most of the studies regarding soil-plant transfer of heavy metals in SS in China were carried out in laboratory or greenhouse studies (McLaughlinet al.2006), with conditions very different from those of the field. Compared with the results of a 2-year field experiment(Liet al.2012a), a field experiment that lasted 5 years (Li 2012) showed some differences in the accumulation and translocation of heavy metals in soils and the grains of wheat and maize. In the 2-year field experiment, the contents of Zn, Cu, and Cd in soils increased linearly with increasing SS application rates, and the contents of Zn and Cu in wheat grains initially increased and then reached a plateau. In the 5 year field experiment, the contents of Zn, Cu, Cd, and Hg in soils and Zn in wheat and maize grains increased linearly with increasing SS application rates. Thus, long-term field experiments are needed to reveal the accumulation and bioavailability of heavy metals derived from SS.

In this study, a long-term field experiment (10 years) with a wheat-maize rotation system was conducted to investigate the accumulation and bioavailability of heavy metals in a calcareous soil at different SS application rates. The main objectives were to 1) investigate the transfer of heavy metals in a soil-plant system through long-term field experiments; 2)evaluate the environmental and agricultural risk of SS land use, which will provide a scientific basis for the feasibility of SS land use. This study will provide a theoretical basis and data support for the safe application of SS to agricultural land and for the establishment of SS amendment standards.

2. Materials and methods

2.1. Experimental location and characteristics of field soils, sewage sludge, and chicken dung

The field experiment was carried out in a calcareous soil in the long-term experiment station of the Chinese Academy of Agricultural Sciences in Dezhou of Shandong Province, China (37°20′N, 116°38′E, 20 m elevation). The characteristics of the field soils, SS, and chicken dung (CD)used in the experiments have been reported elsewhere(Liet al.2009) and are listed in Appendix A. The SS was collected from Beijing sewage treatment plant in July 2006,and underwent an aerobic digestion process. The same SS was stored in a room for use from 2007 to 2016. The CD was collected from Hebei Province. The water contents of SS and CD were 10.0 and 9.53%, respectively. The annual plant rotation was winter wheat-summer maize.

2.2. Field experimental design and management

The 10 year field experiment began in October 2006 and finished in October 2016. Twenty-four plots of 5 m×8 m size with a 0.20-m margin were prepared after ploughing up to 0.20 m depth. There are eight treatments: three with SS-unamended soil (CK, blank control; 0.5N, 30.0 kg ha–1urea; 1N, 60.0 kg ha–1urea) as controls, four with different SS amendments rates (0.5SS+0.5N, 1SS+0.5N, 2SS+0.5N,4SS+0.5N: 4.5, 9.0, 18.0 and 36.0 t ha–1sewage sludge,respectively) and one with an addition of CD (1CD+0.5N),as shown in Appendix B. Each treatment had three replicate plots, for a total of 24 plots. The annual application rates of chemical fertilizers, SS and CD in different treatments for wheat and maize are shown in Appendix B (Liet al.2012b). Air-dried SS was applied to soils in the experimental treatments as fertilizer before wheat sowing every year.Before seeding of winter wheat and summer maize, 90 kg ha–1P (15% P2O5) and 120 kg ha–1K (50% K2O) were applied as basal fertilizers to each plot. For wheat, 60 kg ha–1N as urea was applied before seeding and the remaining nitrogen fertilizer (120 kg ha–1N as urea) was applied during the growing season. For maize, all the nitrogen fertilizer (180 kg ha–1N) was applied at small bell mouth stage (Appendix B). Wheat seeds were sown in the beginning of October and harvested at the end of June in the next year. Maize seeds were sown after wheat harvesting and harvested at the end of September.

2.3. Soil and plant sampling

Soil samples of 2 kg were collected in triplicate from the topsoil (0–20 cm) of each plot after wheat harvest in June from 2007 to 2016 (except for 2015). All topsoil samples were air-dried, crushed, passed through a 2-m plastic sieve,and then stored separately for further physicochemical analyses. The samples used for element analysis were passed through a 100-mesh plastic sieve. After 10–15 plant samples of wheat or maize were collected at maturity in triplicate from each treatment plot, the samples were divided into grain and straw. The subsamples were washed twice with deionized water and then oven-dried to constant weight at 70°C (Sukkariyahet al.2005). The grains and straws were then ground using a stainless steel grinder (FW-100,China) separately, and passed through a 100-mesh plastic sieve for elements analysis.

2.4. Chemical analyses of soil and plant samples

Soil pH was measured in a water suspension (soil:deionized water=1:5) with a pH meter (Orion pH meter, Model 420;Thermo Fisher Scientific Inc., Shanghai, China) (Lu 1998).Soil texture was analyzed according to the method described by McKenzieet al.(2002). Soil organic matter (OM)content in the samples was determined using the potassium dichromate digestion method (Liet al.2009). Total nitrogen(TN), phosphorus (TP), and potassium (TK) in soils were determined using micro-Kjeldahl digestion, colorimetric analysis, and a UV spectrophotometer, respectively (Pageet al.1982). Available N (alkali-hydrolyzable N), 0.5 mol L–1NaHCO3(pH 8.5)-extractable P (Olsen-P), and 1 mol L–1NH4OAc-extractable K were measured following the method of Pageet al.(1982).

All soil and plant samples which passed through the 100-mesh sieve were digested using US EPA Method 3052 (US EPA 1996) to measure total concentrations of Zn, Cu, Cr, Ni,Cd, and Pb. Briefly, a well-mixed air-dried soil sample (0.50 g)or plant sample (0.50 g) were weighed into a microwave digestion tube, with 9 mL concentrated nitric acid (HNO3) and 3 mL concentrated hydrofluoric acid (HF) added to the soil sample vessels and 6 mL HNO3and 3 mL H2O2to the plant sample vessels in a fume hood, stewing for overnight. The sample continued through the digestion procedure and the vessels were allowed to cool for 5 min before being removed from the microwave system. The digested solution was moved to a BFGS-20A Digestion System (Beijing Xinheda Technology Co., Ltd., China) at 165°C to volatilize acid. The digested fluid was filtered through a 0.45-μm filter and stored at 4°C before analysis. The sample was filtered into a colorimeter tube and the digest was diluted to a known volume. Because Hg and As can volatilize above 100°C, water bath digestion with aqua regia at 100°C was used to digest the samples and the contents of these two elements were determined with the Atomic Fluorescence Spectrophotometer. Blanks and standard materials of soil sample (GBW-07403) and wheat(GBW-10011) provided by the China National Center for Standard Materials were used as quality control.

2.5. Data analyses

To understand the transfer behavior of heavy metals from soils to plants, the bioconcentration factors (BCFs) of wheat and maize were calculated using the following formula:BCFs=Cp/Cs, where,Cp(mg kg–1) is the heavy metal content in maize or wheat grains andCs(mg kg–1) is the content of the corresponding heavy metal in soil.

A statistical comparison of the data was undertaken using one-way analysis of variance (ANOVA) with the Statistical Product and Service Solutions (SPSS) statistical package 18.0. Duncan’s multiple range test was performed to test the significance of difference between the treatments at the 5% probability level.

3. Results and discussion

3.1. Effects of sewage sludge amendments on soil properties

The effects of SS and CD application on the changes of soil properties are reported in Table 1. Because SS had lower pH than the soil (Appendix A), its amendment to agricultural soil reduced the soil pH to 7.78 as compared to 8.65 in unamended soil (P＜0.01), and there was also a slight decrease (pH 8.25) in CD-amended soil. The content of soil OM significantly increased with increasing SS application rates(Table 1;P＜0.01), which is due to the higher content of OM(35.5%) in SS (Appendix A). The contents of TN, available N, Olsen-P, and extractable-K also increased significantly in soils amended with SS due to higher levels of these nutrients in SS (Table 1). In CD-amended soils, the contents of OM,TN, available N, Olsen-P, and extractable-K all significantly increased (P＜0.01). However, the increases of OM, TN, and available N in SS amended-soils were significantly higher than those in the CD treatment, which means that SS is superior than CD in providing OM and N, which will enhance the growth of crops (Morenoet al.1997; Chenet al.2003; Singh and Agrawal 2008; Baiet al.2016).

3.2. Accumulation of heavy metals in soils

The contents of total Hg, Zn, Cu, Cd, and Pb in soils increased linearly with increasing SS application rates (Fig. 1 and Appendix C) and the correlation coefficients (R2) were 0.838,0.675, 0.291, 0.280, and 0.263, respectively (P＜0.01).However, there were no significant correlations between the contents of Cr, Ni, and As and the SS application rates. The contents of Hg, Zn, Cu, Pb, and Cd were increased by 6.20,619, 92.9, 49.2, and 0.500 μg kg–1, respectively, by applying 1 ton of sludge per hectare per year to the soil. Compared with the increase rates obtained from a 2-year field experiment(910, 92.6, and 0.940 μg kg–1for Zn, Cu, and Cd) (Liet al.2012a), the values for Zn and Cd were much lower in this study, while the value of Cu showed no obvious change.This indicates that the increase rates of Zn and Cd in soils could decrease with increasing SS application years.

The changes in the contents of total Hg, Zn, Cu, Pb, and Cd in the soils (ΔHg, ΔZn, ΔCd, ΔCu, and ΔPb) amended with SS were described by the following equations:

Table 1 The selected properties of soils unamended and amended with sewage sludge (SS) and chicken dung (CD)

Fig. 1 Correlations between the contents of heavy metals in soils and sewage sludge application rates from 2007 to 2016 (except for 2015). The horizontal axis indicates the cumulative dosage of applying sewage sludge for 10 years.

Based on the above equations, the contents of Hg, Zn,Cu, Pb and Cd will increase 477, 372, 390, 623 and 333 μg kg–1when 1 kg ha–1of these metals added as SS per year were applied, respectively. Cooper (2005) reported that the increased rates of soil metals per kg metals added as SS per year were 270 and 350 μg kg–1for Cu in two acidic soils in Australia, which were lower than the result obtained for calcareous soil in this study. Compared with the values obtained from a 2-year field experiment, e.g., 475, 382, and 473 μg kg–1for Zn, Cu, and Cd (Liet al.2012a), the increased rates of Zn and Cd were lower in this study, while the value of Cu showed no obvious change. This may be related to the crop uptake, leaching, and soil variation.

The contents of soil heavy metals at all SS application rates treatments did not exceed the permissible environment quality standard (GB15618-1995) limits for agricultural soil in China,except for Cd in the CK (1.2 mg kg–1) and 0.5 N (1.19 mg kg–1) treatments in 2014, As in the 2SS+0.5N treatment (97.41 mg kg–1) in 2013 and Hg in the 4SS+0.5N treatment (1.02 mg kg–1) in 2011. According to the permissible limit of Hg (1 mg kg–1) regulated in the soil environmental quality secondary standard (GB15618-1995), SS can be safely applied to the soil for 18 years at application rate of 7.5 t ha–1. SS can be safely applied for 51 years at 7.5 t ha–1for Zn, and even longer for other heavy metals (Appendix D). Thus,Hg is the priority element in SS which imposed risks on soil environmental quality. Compared with the results of a 5-year field experiment, which found 30 years for Zn, 81 years for Cu, and 32 years for Cd at SS application rate of 7.5 t ha–1(unpublished data), the 10-year field experiment found 51 years for Zn, 112 years for Cu, and 224 years for Cd and showed that the service life of farmland for SS application will be longer with increasing years of SS amendments.SS amendments increased the activity of heavy metals by decreasing soil pH, which would improve the transfer ability of heavy metals from soils to crops.

3.3. Accumulation of heavy metals in plants

The contents of heavy metals (Zn, Cu, Cr, Ni, Cd, Pb, As, and Hg) in wheat grains and straws at different SS application rates are presented in Fig. 2 (more data are listed in Appendix E). The contents of Zn and Hg in wheat grains were higher than those in wheat straws, while, the contents of Cu, Cr, Ni,Pb, and As in wheat grains were lower than those in wheat straws (Fig. 2). Moreover, the contents of Zn (31.3 mg kg–1)and Cu (4.98 mg kg–1) in wheat grains were much higher than those in maize grains (22.8 mg kg–1for Zn and 2.39 mg kg–1for Cu). This indicated wheat grains can accumulate more Cu and Zn than maize grains can, which was similar to the results reported by Liet al.(2012a).

The patterns of heavy metals accumulation in grains and straws of wheat and maize are shown in Figs. 3–6. The contents of Zn (P＜0.01) and Ni (P＜0.05) in wheat grains(Fig. 3; Appendix F) and Zn (P＜0.01), Cu, and Cr in maize grains (Fig. 4; Appendix G) increased linearly with increasing SS application rates, and by adding 1 ton of sludge per hectare per year in the soils, the contents of these heavy metals were increased by 48.5, 1.00, 99.1, 6.60, and 4.50 μg kg–1, respectively. Liet al.(2012a) reported different results that the pattern of Zn and Cu accumulation in wheat grains over a 2-year period can be described by Mitscherlich equations; the contents of the two heavy metals initially increased and then reached a plateau with increasing SS application rates; for maize grains, the contents of Zn increased linearly, and Cu showed no obvious variation when the SS application rates were increased. These different results in the accumulation of Zn and Cu in the grains of wheat and maize may be attributed to the changes of soil properties caused by SS amendments and the duration of application of SS. The 10-year field experiment results are more likely to reflect the long-term accumulation and bioavailability of heavy metals in soils and crops.

For straws, the contents of Zn (P＜0.01), Cr, and Ni in wheat straws and Zn, Cu, and As in maize straws were positively correlated with SS application rates, while the contents of Cu in wheat straws (P＜0.05) and Cr in maize straws (P＜0.05)exhibited the contrary trend (Figs. 5 and 6;AppendicesH and I). Whether for grains or straws of wheat and maize,there were no significant effects on Cd contents when the SS application rates were increased, which may be attributed to the low bioavailability of Cd in SS. When the SS was applied to the soil, Zn introduced with SS may have competitive absorption with Cd, which may decrease Cd uptake by plants(Oliveret al.1994). Moreover, the addition of co-cations(such as Ca) in SS also competitively inhibits uptake of Cd by plants. The high content of soil-dissolved organic matter after the application of SS can also reduce the availability of Cd in soil solution to plants through complexation of free Cd2+(=McLaughlinet al.2006; Chaudriet al.2007).The contents of heavy metals in wheat and maize grains in the SS-amended soil, even at the highest application rates,did not exceed the permissible limits regulated in Hygienic standard (GB2715-2005). Therefore, the allowable amounts of SS estimated by the maximum content of soil metals are probably conservative for food safety.

3.4. Bioconcentration factors

Fig. 2 The contrast of heavy metal contents in wheat grains and straws in 2016. The horizontal axis indicates the cumulative dosage of applying sewage sludge for 10 years. Bars indicate SE.

Fig. 3 Correlations between the contents of heavy metals in wheat grains and sewage sludge application rates from 2007 to 2016(except for 2012). The horizontal axis indicates the cumulative dosage of applying sewage sludge for 10 years.

Bioconcentration factors (BCFs) can be used to indicate the transfer ability of heavy metals from soils to plant grains. The BCF values of heavy metals (Zn, Cu, Cr, Ni, Cd, Pb, As, and Hg) in wheat grains and straws in 2016, and maize grains and straws in 2013, are shown in Appendix J. For wheat,the BCF values of grains for Zn and Hg were significantly higher than those of straws, while the BCF values of grains for Cu, Cr, Cd, Pb, and As were significantly lower than those of straws. Similarly, for maize, the BCF values of grains for Cr, Ni, Pb, and As were significantly lower than those of straws, which indicated lower translocation of these elements from maize straws to grains. Furthermore,the average BCFs of wheat grain were greater than those of maize grain (Fig. 7 and Appendix K), indicating that wheat is more sensitive than maize and could be used as a potential indicative plant.

Fig. 4 Correlations between the contents of heavy metals in maize grains and sewage sludge application rates from 2007 to 2013(except for 2012). The horizontal axis indicates the cumulative dosage of applying sewage sludge for seven years.

Fig. 5 The contents of heavy metals in wheat straws at different sewage sludge application rates in 2016. The horizontal axis indicates the cumulative dosage of applying sewage sludge for 10 years. Bars indicate SE.

Fig. 6 The contents of heavy metals in maize straws at different sewage sludge application rates in 2013. The horizontal axis indicates the cumulative dosage of applying sewage sludge for seven years. Bars indicate SE.

Fig. 7 Box-and-whisker plots of the average bioconcentration factors (BCFs) of heavy metals in wheat and maize grains with sewage sludge amendates from 2007 to 2016 (except for 2015). A, the average BCFs of heavy metals in wheat grains. B, the average BCFs of heavy metals in maize grains. The single asterisk points (＊) are the outliers; the small horizontal bars with whiskers represent upper and lower extremes (except outliers); the boxes (top and bottom)represent the ranges from upper and lower quartiles; the lines and points within the boxes represent the medians and averages, respectively.

The BCFs of most heavy metals showed no obvious changes in low SS application rate treatments (0.5SS+0.5N and 1SS+0.5N), while the BCFs of Zn, Cu, and Cd decreased in high SS application rate treatments (2SS+0.5N and 4SS+0.5N). For example, the BCF value for Zn in maize grains in 2013 decreased from 0.588 (0.5SS+0.5N) to 0.258(4SS+0.5N) (Appendix K). This is probably because SS amendments significantly increased the contents of Zn, Cu,and Cd in soils, but the changes of Zn, Cu, and Cd contents in crop grains were usually much less than those in the soils.

The average BCF of all heavy metals in wheat and maize grains with SS applications from 2007 to 2016 (except for 2015) was less than 0.381 at all treatments (Fig. 7). The BCFs of Zn, Cu, and Cd in wheat and maize grains were significantly higher than those of the other heavy metals and the bioaccumulation capacity of Zn was the highest with the maximum BCFs of 0.587 and 0.588, respectively.The average BCFs for heavy metals in wheat grains followed the order of Zn (0.381)＞Cu (0.238)＞Cd (0.100)＞Hg (0.0746)＞Ni (0.0162)＞Cr (0.00940)＞Pb (0.00561)＞As(0.00494). In maize grains, the average BCFs for heavy metals followed a similar order: Zn (0.313)＞Cu (0.124)＞Cd(0.0727)＞Hg (0.0148)＞Cr (0.0142)＞Ni (0.0133)＞Pb(0.00740)＞As (0.00148). Liet al.(2012a) studied the BCFs of Zn, Cu and Cd in wheat and maize grains and also found the same order (Zn＞Cu＞Cd). Jamaliet al.(2009) also reported the same order for the BCF values of wheat grains (Zn from 0.460 to 0.600, Cu from 0.207 to 0.526, and Cd from 0.179 to 0.417). However, Karamiet al.(2009) investigated heavy metals uptake by wheat from a SS-amended calcareous soil and found that BCF values of metal in grains (0.85, 0.20 and 0.09 for Cd, Zn and Cu, respectively) and straws (0.93, 0.12 and 0.10 for Cd, Zn and Cu, respectively) followed the order of Cd＞Zn＞Cu. This was probably ascribed to the differences in soil properties, the contents of heavy metals in SS and SS application strategies (Naiduet al.2003).

4. Conclusion

The contents of total Hg, Zn, Cu, Pb, and Cd in the soils increased linearly with increasing SS application rates. Based on soil environmental quality secondary standards (GB15618-1995), it was calculated that SS can be safely applied to soil at an application rate of 7.5 t ha–1for 18 years without exceeding limits for Hg, while for Zn, SS could be applied for 51 years and more than 51 years for other heavy metals.The contents of heavy metals in wheat and maize grains all meet the hygienic standard, and the contents of Zn in grains and straws of wheat and maize increased linearly with increasing SS application rates. The BCFs of the heavy metals in wheat and maize grains were found to be in a similar order (wheat grains: Zn＞Cu＞Cd＞ Hg＞Ni＞Cr＞Pb＞As; maize grains: Zn＞Cu＞Cd＞Hg＞Cr＞Ni＞Pb＞As).Furthermore, the BCFs of wheat were greater than those of maize, indicating that wheat is more sensitive than maize,and can therefore be used as an indicator plant. These results of this 10-year field experiment provide further data for developing the critical loads of SS amendments to calcareous soils.

Acknowledgements

The authors would like to thank the National Key Research and Development Program of China (2016YFD0800406)for financial support. The authors would also like to thank all staff who managed the field experiments.

Appendicesassociated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm