JIA-2019-11

2624 WANG Shi-chao et al. Journal of Integrative Agriculture 2019, 18(11): 2619–2627 4.2. Why do paddy fields have higher CPISP values than upland sites? Our study showed changes in the CPISP values of double- cropping systems during the 33-year period. Paddies can maintain higher CPISP than upland areas over long periods (Fig. 4), probably because more nutrients are sequestered in paddies (Gao et al. 2008; Tong et al. 2009; Li et al. 2010). The higher CPISP was probably due to the relatively lower organic matter decomposition rate in the aerobic environment in paddies compared to upland sites, resulting in more organic matter accumulation. Previous studies have also shown that paddy soils have higher nitrogen accumulation capacities, more active soil organic nitrogen components, more nitrogen-fixing bacteria, and lower rates of decomposition and mineralization (Bouman et al. 1994; Kukal and Aggarwal 2002; Zhou 2011; Jin et al. 2014; Prabhat et al. 2014). Furthermore, additional nutrients were input into the paddy fields because of eutrophication of irrigation water. For example, the annual nitrogen input due to irrigation in the Poyang Lake area of Jiangxi Province was 11.02 kg N ha –1 (Han et al. 2014). Since the paddy fields had greater capacity for nutrient accumulation, they had higher CPISP values than their upland counterparts. 4.3. Stable CPISP levels after long-term no-fertiliza- tion in China The CPISP levels in all tested fields decreased over time but eventually stabilized after prolonged no-fertilization (Appendices C and D), being consistent with previous findings (Drury and Tan 1995; Zhan et al. 2015). The long-term effects of fertilization on the sustainability of corn yields has been investigated for over 35 years in Canada. Drury and Tan (1995) revealed that yields in unfertilized plots decreased gradually over the first eight years but then fluctuated around a mean level of approximately 1 t ha –1 thereafter. Under long-term no-fertilization, the stable CPISP values for double-cropped plots in the upland regions of northern, Northwest and southern China were roughly 20%. The corresponding values for mono-cropped plots in northern and Northeast China were 30–45%, while those for paddy fields in southern China were approximately 40–50%. Long-term no-fertilization treatment can maintain crop yield Experimental duration (yr) 0 5 10 15 20 25 30 35 CPISP (%) 0 20 40 60 80 y =–0.07 x +51.2, R 2 =0.02, P =0.58 0 5 10 15 20 25 30 35 CPISP (%) 0 20 40 60 80 100 y =59.1+27.8e –0.57 x R 2 =0.15, P <0.05 0 5 10 15 20 25 30 35 CPISP (%) 0 20 40 60 80 y =16.2+35.5e –0.03 x R 2 =0.53, P <0.01 A B C Fig. 4 Contribution percentage of inherent soil productivity (CPISP) in double-cropping systems of wheat-maize rotation in northern China (A) and wheat-rice (B) and rice-rice (C) rotation in southern China. Bar indicates SD. CPISP (%) 0 20 40 60 80 100 Mono-cropping Double cropping bc c a c b a Wheat/Maize (Northwest) Continnious maize (Northeast) Wheat/Maize/Soybean (Northeast) Wheat-maize (northem China) Wheat-rice (northem China) Early-late rice (northem China) Fig. 5 Contribution percentage of inherent soil productivity (CPISP) for different cropping systems. Different letters indicate significant difference at P≤ 0.05. Bar indicates SD.