Reducing Ammonia Volatilization and Increasing Nitrogen Use Efficiency in Machine-Transplanted Rice with Side-Deep Fertilization in a Double-Cropping Rice System in Southern China

Ammonia (NH3) volatilization contributes to major nitrogen (N) fertilizer loss in rice fields. Deep fertilization can effectively mitigate NH3 volatilization and increase N use efficiency (NUE) in agroecosystems. However, the effects of machine-transplanted rice with side-deep fertilization (MRSF) on NUE and NH3 volatilization from paddy fields in double-cropping systems are still unknown. In this study, a field experiment was conducted with double-cropping rice fields in two regions in Southern China (Miluo and Yiyang). NH3 volatilization was measured during the rice growing seasons using the continuous airflow enclosed chamber method under different N application rates with MRSF treatments. The results show that NH3 fluxes in rice soil subjected to different N application rates varied. The ratio of total NH3 loss to applied N by conventional fertilization (CF) ranged from 41.2 to 45.3% in the early-season rice (ESR) and from 44.9 to 45.2% in the late-season rice (LSR). Unlike CF treatment, MRSF treatments significantly reduced the cumulative NH3 loss by approximately 24.8-26.9 % in Miluo and 20.1-20.8 % in Yiyang for ESR and by 18.6-23.0 % in Miluo and 20.2-21.8 % in Yiyang for LSR. MRSF treatments increased the content of available N in the soil by 0.2-8.8 % and 0.7-6.6 % in Miluo and Yiyang, respectively. Moreover, this mitigation effect is expected to be more effective with lower N application rates. An NH3 accumulation model described by the Elovich equation was established over the study period; notably, the field observations fitted the calculated values well. MRSF with a 20-30 % reduction in N application rate yielded a higher NUE in terms of N recovery efficiency (NRE), by 11.5-27.9 % and 11.7-23.7 % in Miluo and 8.2-17.0 % and 7.0-25.8 % in Yiyang relative to that of the CF treatment in ESR and LSR, respectively. Overall, MRSF can reduce the N loss through NH3 volatilization while improving content of available N in the soil and NUE; therefore, further maximizing the yield and simplifying fertilization practices for a cleaner rice production in Southern China.