Abstract:
Objective Paddy-upland rotation systems have different soil nitrogen residue, and the nitrogen brought into soil with the upland crop straw returning is different as well, leading to varied nitrogen supply for the following rice season. The precise nitrogen reduction rate was studied, considering the straw returned nitrogen amount, in order to fully use the straw nitrogen resources and advantages of rotation patterns.
Method Using the main promoted variety ‘F You 498’ in Sichuan as the material, a large field split-zone experiment was carried out in the Modernized Agricultural Research Park of Sichuan Agricultural University in 2018 and 2019. The main plots were three paddy-upland rotation patterns, rape-rice rotation (RR), wheat-rice rotation (WR), and cabbage-rice rotation (CR); the subplots were three nitrogen application rates: no N application (N0), traditional N application rate (N1), and precise N reduction (N2), and the upland crop straw were all returned to field. By calculation, the N application rate in the N2 treatment of RR, CR, and WR was 120, 145, and 140 kg/hm2, with N reduction rate over N1 (150 kg/hm2) of 20.00%, 3.33% and 6.67%, respectively. At jointing, heading and maturity stages, rice plant samples were collected to investigate the accumulation of dry matter and N, and the yield and its components at maturity stage. Soil samples at 0—20 cm layer were collected after upland crop and rice harvest for the measurement of total N and alkali-hydrolyzed N content.
Result Compared with CR and WR, RR increased the average rice yield by 3.85% and 13.06% in 2018, and by 14.01% and 2.57% in 2019, due to the higher effective panicles and 1000-grain weight; the average accumulation of dry matter increased by 1.84% and 23.50% in 2018, and by 12.87% and 4.19% in 2019; the average total N accumulation increased by 17.29% and 14.59% in 2018, and by 10.50% and 5.00% in 2019; enhanced the partial factor productivity of N by 11.43% and 17.08% in 2018, and by 25.57% and 11.42% in 2019. In 2018 and 2019, the soil total N content after rice harvest in RR was 16.67% and 9.25% higher than that in CR and WR in 2018, and 14.69% and 2.01% higher in 2019; the alkaline-hydrolyzed N contents were 13.90% and 9.80% higher in 2018, and 17.76% and 8.48% higher in 2019. Under the same rotation pattern, N2 and N1 treatment were recorded similar rice yield, dry matter accumulation, N uptake, and soil total and available N content. However, the partial factor productivity of N and physiological N use efficiency were mostly expressed as N2>N1. N2 significant increase the partial factor productivity of N by 23.50% under RR mode compared to N1 treatment in 2018, and by 20.89% in 2019. Comprehensive evaluation showed that the comprehensive ranking of rice productivity was RR>CR>WR, with the highest score in treatment RR+N1, followed by RR+N2.
Conclusion Rape–rice rotation pattern has the highest soil total and available nutrient content after harvest of upland crops, and the rape straw contains higher amount of nitrogen than other crops, so the nitrogen reduction in rice season is as high as 20%. Under the precised nitrogen reduction rate, the rice yields in all the three rotation systems are not significantly affected. Comprehensively, rape–rice rotation is the best system in precision N reduction, rice yield and nitrogen fertilizer use efficiency, and in maintaining soil fertility.