Optimizing grain yields reduces CH4 emissions from rice paddy fields

Abstract

Microbial production in anoxic wetland rice soils is a major source of atmospheric CH4 the most important non-CO2 greenhouse gas. Much higher CH4 emissions from well managed irrigated rice fields in the wet than in the dry season could not be explained by seasonal differences in temperature. We hypothesized that high CH4 emissions in the wet season are caused by low grain to biomass ratios. In a screenhouse experiment, removing spikelets to reduce the plants' capacity to store photosynthetically fixed C in grains increased CH4 emissions, presumably via extra C inputs to the soil. Unfavorable conditions for spikelet formation in the wet season may similarly explain high methane emissions. The observed relationship between reduced grain filling and CH4 emission provides opportunities to mitigate CH4 emissions by optimizing rice productivity.

Fig 1.

Seasonal methane emission as a function of grain yield at Maligaya, Philippines, 1994–1998 (data from ref. , treatment T1).

Fig 2.

Methane emission before and after spikelet removal expressed relative to the emissions at 58 days after transplanting (DAT).

Fig 3.

Ratio of CH₄-C emission before and after spikelet removal at three rates of N-fertilization (n = 4). DAT, days after transplanting.

Fig 4.

Dry weight of filled grain and new tillers as a function of spikelet removal.

Fig 5.

Hypothetical relationships between grain yield and seasonal methane emission for areas with specific sets of environmental conditions causing differences in overall agronomic productivity (lines 1–4) and across such areas, for “average production” (bold broken arrow).