Rice-crayfish integrated system enhances global warming potential via increasing methane emission mainly driven by continuous deep flooding

Abstract

The conversion from rice monoculture (RM) to rice-crayfish integrated system (RCIS) could change soil physicochemical properties and microorganisms related to greenhouse gas (GHG) emissions. Nevertheless, it is still unclear the responses of GHG emissions and global warming potential (GWP) from paddy fields to RCIS. Here, we conducted a field experiment to investigate the changes in the emissions of methane (CH4) and nitrous oxide (N2O), GWP, soil physicochemical properties and the associated microbial abundances (methanogen, methanotrophs, denitrifier and nitrifier) under RCIS in Gaoyou City, Jiangsu Province, China from 2022 to 2023. There were three treatments, including RM, rice monoculture with continuous deep flooding (RMF) and RCIS. Compared with RM, RCIS and RMF all significantly increased CH4 emission and decreased N2O emission. But the effectiveness of increasing CH4 emission and decreasing N2O emission was larger under RCIS than RMF. The increased CH4 emission was due to the significantly higher mcrA gene abundance and ratio of mcrA/pmoA under RCIS. While the significantly higher nosZ gene abundance and ratio of nosZ/(amoA+nirK+nirS) under RCIS were responsible for its reduced N2O emission. Furthermore, the increases in gene abundances of mcrA and nosZ under RCIS were closely correlated with the significantly lower redox potential and pH as well as the significantly higher contents of dissolved organic carbon, ammonium and nitrate in soil. Averaged across two years, GWP under RCIS and RMF were 4.6-fold and 3.4-fold that under RM, respectively. This revealed that continuous deep flooding had a greater effectiveness in increasing GWP than crayfish culture under RCIS. Notably, the contribution rates of CH4 emission to the total GWP decreased in the order of RCIS (98.6%) > RMF (97.5%) > RM (86.2%). Besides, greenhouse gas intensity under RCIS was 5.1-fold that of RM due to its enhanced GWP and reduced rice grain yield. In summary, RCIS could enhance GWP from paddy fields through increasing CH4 emission mainly caused by the continuous deep flooding.