Unexpected high reduction of methane emission via short-term aerobic pre-digestion of green manured soils before flooding in rice paddy

Abstract

Soil organic matter (SOM) is used as an important indicator of soil quality and a countermeasure to mitigate global warming. To increase SOM stock, cover cropping and its biomass incorporation as green manure are strongly recommended in mono-rice paddy soils. However, green manure application significantly increased greenhouse gas emission, in particular, methane (CH4) during rice cultivation, and then its simultaneous positive and negative outcome has become a serious issue. We hypothesized that the short-term aerobic pre-digestion of green manured soil under dry soil condition before flooding might degrade labile organic C into carbon dioxide (CO2) and then reduce CH4 production during the flooded rice cultivation period. In order to evaluate the feasibility of the short-term aerobic predigestion of green manured soil on reducing CH4 emission in rice paddy, cover crop biomass was incorporated in the inner dry soil at different time intervals from 0 to 30 days before flooding, and then CH4 and CO2 emission rates were monitored. Over 10 days of aerobic pre-digestion significantly decreased CH4 flux by 88-98% over the control (flooded soil without aerobic pre-digestion) during the two month's incubation test. Similar results were observed during the field test, in which only at 10 days' aerobic predigestion under dry soil condition, total CH4 flux decreased by approximately 60% over the control. This reduction effect was slightly increased when aerobic pre-digestion period was extended. In contrast, rice productivities were not significantly different from 0 to 30 days of aerobic pre-digestion. As a result, more than 10 days of aerobic pre-digestion of green manured soil before flooding decreased CH4 flux intensity (kg CH4 kg (1) grain) by 60% over the control. In conclusion, the short-term aerobic pre-digestion of green manured soil before flooding can be a good soil management strategy to mitigate CH4 emission without productivity decrease in rice field. (C) 2019 Elsevier B.V. All rights reserved.