Shallow tillage in autumn reduced methane and nitrous oxide emissions from a paddy field with incorporated rice straw in a cold region of Japan

Abstract

In cold regions of Japan, rice straw residues cut and scattered in late autumn are often incorporated into paddy soils the next spring. Generally, emissions of methane (CH₄) from such paddy fields are very high owing to rapid anaerobic decomposition of the straw under flooding. We investigated the effects of incorporating straw by shallow tillage in autumn to decompose under aerobic conditions for reducing emissions of the greenhouse gases CH₄ and nitrous oxide (N₂O). Three plots were prepared: shallow tillage in autumn by plowing at 5–8 cm depth (STA), conventional tillage in autumn by plowing at 18–20 cm depth (CTA), and conventional tillage in the next spring by plowing at 18–20 cm depth (CTS). The study was conducted from 2010 to 2013. In the STA and CTA plots, the straw was incorporated in October, and the plots were plowed the next April. In the CTS plot, the straw was incorporated at plowing in April. All plots were irrigated and rice seedlings were transplanted in late May. CH4 and N2O fluxes were measured by the closed chamber method throughout the cropping period. Tiller number, grain yield, and brown rice quality were also measured. The cumulative CH4 emissions increased in the order of STA (19.9–85.6 g CH₄ m^-2) < CTA (24.8–107.6 g CH₄ m^-2) < CTS (45.6–134.1 g CH₄ m^-2). N₂O emissions in all plots were negligible. Tiller number was higher in the STA plot than in the other plots. There were no significant differences in grain yield or brown rice quality. From the time of snowmelt in March to plowing in April, the soil moisture and the concentration of ferrous iron (Fe²⁺) in soil were lower in the STA plot than in the CTA plot. Consequently, shallow tillage in autumn by plowing at 5–8 cm was the most effective technique for reducing emissions of greenhouse gases from paddy fields with incorporated rice straw in a cold region of Japan.