Methane emission under straw return is mitigated by tillage types depending on crop growth stages in a wheat-rotated rice farming system

Straw return (SR) is a recommended management practice to improve soil fertility, but often increases methane (CH4) emissions from rice paddies. Tillage type (before or after irrigation) will change the contact surface between soil and straw, as well as soil structure, but little is known about how tillage type affects CH4 emission under SR from rice fields. A field experiment using split-plot design (straw as main plot and tillage type as subplot) was conducted to quantify the interactive effects of SR and tillage type on CH4 emission from transplanting to jointing, jointing to booting, and booting to maturity stages of rice in 2018 and 2019 and its underlying mechanisms. The results showed that SR significantly increased CH4 emission throughout the rice growing season by 58.8% due to enhanced methanogenic activity compared with straw removal (S0). Cumulative CH4 emission from transplanting to booting stage accounts for 95% of that from the whole rice growth season. Dry tillage (tillage before irrigation, -W) mitigates CH4 emission under SR from transplanting to jointing stage due to lower mcrA abundance compared with wet tillage (tillage after irrigation, +W). However, +W mitigates CH4 emission from jointing to booting stage in the absence of significant interaction between straw and tillage type through improved soil aggregation and higher carbon accumulation compared with -W. Overall, S0 +W had lower CH4 emission by 18.3%− 36.5%, 42.9%− 52.7%, and 44.0%− 48.6% than S0-W, SR-W, and SR+W, respectively (P < 0.0.5). Our results emphasize the importance of adjusting paddy tillage method in mitigating CH4 emission boosted by straw incorporation.