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2625 WANG Shi-chao et al. Journal of Integrative Agriculture 2019, 18(11): 2619–2627 because soil nutrient mineralization and external nutrient input provided lots of available soil nutrients. For example, annual dry nitrogen deposition rates of up to 30.6–55.0 kg N ha –1 have been measured in the plains of northern China (Liu et al. 2006; Shen et al. 2009). Similarly, the annual deposition of TN at 41 sites in China was 18.02 kg N ha –1 in 2013 (Zhu et al. 2015), and the annual TN deposition for the paddy field was estimated as 22.34 kg N ha –1 in subtropical central China (Shen et al. 2013). 4.4. The CPISP3 levels of China’s soils should be improved The CPISP3 value for major crops in China is currently around 53.8%, which is approximately 20% lower than the values for the United States, Italy, and certain other countries (Table 2; Appendix E). Deep fertile plough horizons have been built up over time in Europe and the United States, making their soils sufficiently fertile to achieve high productionwith comparatively modest fertilizer inputs. For example, the rates of nitrogen and phosphorus fertilizer applied in Germany declined by 31 and 49%, respectively, between 1961–1979 and 1980–1995. However, the country’s grain yields increased by 80.7% over this period (Zhang et al. 1998). This phenomenon indicates that as fertilizer input decreases, increased production is strongly dependent on the inherent soil quality, which can be reflected by CPISP, as shown in our study. The stable CPISP levels achieved in double-cropped uplands after several years of no-fertilization were lower than those found for single cropping, indicating that a high multiple crop index causes greater reductions in soil fertility than single cropping. Due to the need to enhance food security and achieve a high multiple crop index in China, the soil fertility of cultivated land should be further improved, possibly through the increased application of organic fertilizer. Organic manure not only improved soil fertility and crop yield, but also reduced risks of soil pollution (Zhang et al. 2014). It is a win-win strategy for soil quality and plant production. 5. Conclusion Our analyses indicate that average CPISP3 for three crops was 53.8%, with average values for rice, wheat, and maize, of 58.4, 50.0, and 50.7%, respectively. Historically, CPISP3 in the 1990s was much higher than in the 1980s and 2000s. Long-term no-fertilization caused CPISP levels to gradually decline and then stabilize. Mono-cropping over a long period was more favourable to CPISP than double-cropping system of wheat-maize. However, the CPISP values for paddy and upland-paddy double-cropping system decreased less than that for wheat-maize under long-term no-fertilization. Overall, the results presented herein indicate that although the increased usage of chemical fertilizer raised the nutrient contents of China’s soils over the last three decades, further improvements of CPISP3 are required to achieve the desired levels of productivity and ensure food security. Acknowledgements We are grateful to the staff from long-term monitoring trials and the farmers who offered their farmland and contributed to the field management of the long-term experiments. Prof. Warren A. Dick from the Ohio State University, USA provided helpful suggestions for English editing. This work was financially supported by theAgro-scientific Research in the Public Interest of China (201503122) and theAgricultural Science and Technology Innovation Program of Chinese Academy of Agricultural Sciences (CAAS-XTCX2016008). Appendices associated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm References Aynehband A, Moezi A A, Sabet M. 2010. Agronomic assessment of grain yield and nitrogen loss and gain of old and modern wheat cultivars under warm climate. African Journal of Agricultural Research , 5 , 222–229. Barbieri P A, Echeverria H E, Rozas H R S, Andrade F H. 2008. Nitrogen use efficiency in maize as affected by nitrogen availability and row spacing. Agronomy Journal , 100 , 1094–1100. Bouman B A M, Wopereis M C S, Kropff M J, Tenberge H F M, Tuong T P. 1994. Water use efficiency of flooded rice fields. 2. Percolation and seepage losses. Agricultural Water Table 2 Major chemical properties of the soils subjected to conventional fertilizer treatments in the surface soil layer (0–20 cm) in the 1980s, 1990s and 2000s Year 1) Soil organic matter (g kg –1 ) Total nitrogen (g kg –1 ) pH Available phosphorus (mg kg –1 ) Available potassium (mg kg –1 ) 1980s 26.5±1.7 a 1.47±0.08 b 6.90±0.11 a 14.6±1.1 c 91.0±4.1 c 1990s 26.3±0.6 a 1.55±0.03 ab 6.97±0.05 a 17.7±0.5 b 98.6±2.0 b 2000s 26.6±0.3 a 1.58±0.01 a 6.67±0.03 b 21.8±0.4 a 113.2±1.3 a 1) Soil nutrient data were collected from 1 086 on-farm trails conducted during the 1980s and 2000s. Data are mean±SD. Different letters mean significant difference between treatments at P <0.05, respectively.