Different characteristics of soil CH4 emissions and methanogenic communities in paddy fields under gradually and abruptly elevated CO2 concentrations

Abstract

Studying methanogenesis in paddy fields under future climate change scenarios is important for carbon sequestration and reduction in agroecosystems. Most studies have investigated the effects of elevated CO2 concentrations (e[CO2]) on CH4 emissions in paddy fields under a constant CO2 concentration ([CO2]). However, the increase in the [CO2] in the atmosphere is a gradual process rather than an abrupt increase, and the [CO2] in the atmosphere has remained high and relatively constant. In this research, we explored the differences in methanogenesis in paddy fields under gradually and abruptly e[CO2] using open-top chambers (OTCs). CH4 flux, soil physicochemical properties, enzyme activities, methane production potential (MPP), and methanogenic (mcrA gene) characteristics were analyzed under CK (ambient [CO2]), C1 (gradually e[CO2] by 200 μmol mol−1), and C2 (abruptly e[CO2] by 200 μmol mol−1) treatments. The results showed that the C1 and C2 treatments significantly promoted the cumulative amount of CH4 emissions (CAC) by 63.3% and 86.5%, respectively, and significantly increased the CAC/yield by 30.8% and 47.3%, respectively, compared with the CK treatment. In addition, the C1 and C2 treatments increased the DOC concentration, enzyme activities (urease, invertase, and catalase), MPP, and mcrA gene abundance in paddy soils. Notably, CAC/yield, shoot biomass, and soil seasonal average urease activity under the C2 treatment were increased by 12.6%, 12.2%, and 3.0%, respectively, compared with those in C1. The methanogenic activity of the C2 treatment was significantly increased by 24.1% and 24.4% during the tillering and grain-filling stages, respectively, and the methanogenic abundance of the C2 treatment was significantly increased by 53.9% and 72.2% during the tillering and elongation stages, respectively, compared with that of C1. Therefore, it is inappropriate to ignore the fact that [CO2] increases gradually and to only examine the effects of high e[CO2] on agroecosystems.