Long - term conservation agriculture increases nitrogen use efficiency by crops, land equivalent ratio and soil carbon stock in a subtropical rice - based cropping system

Conservation Agriculture (CA) is still a relatively new approach for intensively cultivated (3 crops yr-1) rice-based cropping systems that produce high crop yield and amounts of residues annually. With the recent development of transplanting of rice into tilled strips on non-puddled soil, CA could become feasible for rice-based cropping patterns. However, the effect of increased retention of crop residues on crop response to nitrogen (N) fertilization rate in strip tilled systems with the transplanted rice and other crops grown in the annual rotation is yet to be determined. For nine years, we have examined the effects of soil disturbance levels - strip tillage (ST) and conventional tillage (CT), two residue retention levels –15% residue by height (low residue, LR) and 30% residue (high residue, HR) and five N rates (60%, 80%, 100%, 120%, and 140% of the recommended N fertilizer doses (RFD)) for a rice-wheat-mungbean cropping sequence. The 100% RFD was 75, 100 and 20 kg N ha-1for rice, wheat, and mungbean, respectively. Rice yields were comparable between the two tillage systems for up to year-6, wheat for up to year-3 but mungbean yield markedly increased in ST from year-1; however, the land equivalent ratio increased from year-1, principally because of higher mungbean yield. Introduction of ST increased land equivalent ratio by 26% relative to CT, N use efficiency and partial factor productivity. Nitrogen fertilizer demand for maximum yield in ST was increased by about 10% for rice and 5% for mungbean but decreased by 5% for wheat. Although fertilizer N demand had increased in ST system due to higher yield than CT, the N requirement declined by50–90% when the same yield goal is considered for ST as for CT. The soil organic carbon stock (0–15 cm) after 8 years increased from 21.5 to 30.5 t ha-1 due to the effect of ST plus high crop residue retention. Annual gross margin increased by 57% in ST over CT practice and 26% in HR over LR retention. In conclusion, after 9 years practicing CA with increased residue retention under strip tillage, the crops had higher N use efficiency, grain yield, land equivalent ratio and annual gross margin in the rice-wheat-mungbean cropping system while the N fertilizer requirement increased minimally.