Coupling side-deep fertilization with Azolla to reduce ammonia volatilization while achieving a higher net economic benefits in rice cropping system

Abstract

Ammonia (NH3) volatilization reduces nitrogen (N) use efficiency (NUE) and damages the environment, resulting in economic costs and human health threats. Side-deep fertilization decreases NH3 volatilization and the fertilizer application amount but sometimes risks reducing rice yield because of an unstable N supply. Azolla incorporation can improve NUE and rice yield by biological N fixation in paddy fields. However, the effects of coupling side-deep fertilization with Azolla on NH3 volatilization, NUE, and net economic benefits in rice cropping systems are unclear. Therefore, a two-year field trial was performed in the Taihu Lake region of China with five treatments: (i) a control with no N input (CK), (ii) conventional fertilization using the local practice with three-split broadcasting with 300 kg N ha−1 (CN), (iii) reduced N fertilization with three-split broadcasting with 255 kg N ha−1 (RN), (iv) reduced N fertilization with 60% of urea side-deep placement and with 40% broadcasting (RNS), and (v) combined RNS with Azolla (RNSA). In the two-year field observations, compared with CN, RN decreased NH3 volatilization but tended to reduce rice yield and NUE, and RNS and RNSA significantly decreased NH3 volatilization by 32.42%− 50.65% and 48.65%− 59.72% by decreasing floodwater NH4+-N and pH. In addition, RNSA significantly increased NUE by 23.73% (26.92–30.89%) by enhancing rice root growth and N uptake compared to CN. Moreover, the economic cost of NH3 volatilization in RNSA (548.07–590.67 CNY ha−1) was notably lower than that of CN (1149.99–1302.51 CNY ha−1). The RNSA achieved the maximum net economic benefits (24.72–25.75 * 103 CNY ha−1) with the minimum economic costs of NH3 volatilization. The results provide theoretical support for guiding field production and government decision making.