Azolla incorporation and dual cropping influences CH4 and N2O emissions from flooded paddy ecosystems

Abstract

To investigate the influence of Azolla (A. filiculoides Lam.) incorporated as a green manure and its subsequent growth as a dual crop with rice on simultaneous methane (CH4) and nitrous oxide (N2O) emissions from a flooded alluvial soil planted with rice, a pot experiment with three treatments, chemical fertilizers (NPK) as the control, incorporation of Azolla as green manure (AGM), and AGM plus basal chemical fertilizers (NPK+AGM) was conducted in Tsuruoka, Yamagata, Japan in 2017. AGM and NPK+AGM treatments significantly increased CH4 emissions at early rice growth stages before 63 days after transplanting (DAT) by 123.0% and 176.7% compared to NPK, respectively. At late rice growth stages (after 63 DAT), only the NPK+AGM treatment significantly increased CH4 emission by 22.1% compared to NPK. However, percentage of CH4 emitted after 63 DAT relative to the seasonal CH4 emission followed the order of NPK (86.2%) > AGM (76.5%) > NPK+AGM (73.3%). Higher CH4 emissions from AGM and NPK+AGM before 63 DAT were attributed to the incorporated Azolla, while the higher emissions after 63 DAT in all treatment groups were ascribed to rice photosynthesis. AGM and NPK+AGM treatments significantly decreased N2O emissions by 71.6% and 81.1% compared to NPK, respectively, at early rice growth stages. Azolla incorporation may have restricted N2O emission from initial soil nitrate before 63 DAT and not have contributed to N2O emissions after 63 DAT. Significantly higher grain yields were observed under the AGM (32.5%) and NPK+AGM (36.3%) compared to NPK. Together, AGM and NPK+AGM treatments significantly increased seasonal CH4 emissions by 31.5% and 43.5%, and decreased seasonal N2O emissions 3.4- and 4.6- fold compared to NPK, respectively. There were no significant differences in the CH4 emissions per grain yield among the treatments; however compared to NPK, AGM and NPK+AGM treatments significantly reduced N2O emissions per grain yield by 78.7% and 84.1%, respectively.