Effects of straw and straw-derived biochar applications with equivalent straw input on ammonia volatilization and N leaching in an alternate wetting and drying paddy ecosystem

Abstract

Contrasting effects of straw and biochar with equivalent straw input on ammonia volatilization and N leaching under alternate wetting and drying irrigation (I_AWD) in paddy ecosystems are limited. A 2-yr paddy buried pot experiment was conducted with annual rice straw and single biochar applications with equivalent straw input under continuously flooded irrigation (I_CF) and I_AWD. Four treatments comprising no amendment under I_CF (I_CFA₀), no amendment under I_AWD (I_AWDA₀), rice straw (29.41 t ha⁻¹ yr⁻¹) under I_AWD (I_AWDA_S) and biochar (20 t ha⁻¹ applied once) under I_AWD (I_AWDA_B) were arranged in a randomized complete block design. I_AWD did not significantly alter ammonia volatilization, but increased N leaching. Compared with I_CFA₀ and I_AWDA₀, I_AWDA_S significantly increased ammonia volatilization by 7.92–52.71% and decreased yield by 22.98–31.87% in both years, but increased N leaching by 31.18% in 2021 only. I_AWDA_B increased ammonia volatilization, N leaching and reactive N losses in 2021 but significantly decreased reactive N losses in 2022 compared with no amendment. I_AWDA_B reduced grain yield due to the increased reactive N losses in 2021. Both I_AWDA_S and I_AWDA_B improved soil total N. I_AWDA_B had higher yield and N uptake, and lower reactive N losses than I_AWDA_S. Overall, biochar is a more effective strategy for reducing reactive N losses in I_AWD paddy systems over time. Direct straw return could take longer for its decomposition or be annually applied at a lower rate to address its higher reactive N losses and lower grain yield after a 2-yr consecutive annual application.