Paddy rice is a staple food for > 50% of global population, while paddy soils are substantial (> 10%) sources of methane (CH4) emitted anthropogenically. Alternative wet dry irrigation management can reduce ca. 55% of methane emission, while water saving irrigation and dry direct-seeded rice are assumed to decrease CH4 emission by ca. 80%, with increasing emission risks of nitrous oxide (N2O) and carbon dioxide (CO2) from aerobic decomposition of soil organic matter. Water management practices could affect root exudation of organic acids, the potential substrates for methanogenesis. We tested whether DDSR and water saving irrigation change greenhouse gas emission, root exudation, and soil microbial composition, compared to continuous flooding.
We compared continuous flooding (CF), water saving irrigation (WS), and dry cultivation (DC) of paddy rice systems in Tottori, Fukushima, and Akita. The fluxes of greenhouse gas emission were periodically measured using closed chamber method in each plot. Greenhouse gas emission trade-off between N2O and CH4 were assessed using global warming potential values relative to CO2 [27 (CH4) and 273 (N2O); IPCC]. Mature rice roots were rinsed with distilled water to remove soils and grown in hydroponic systems to collect root exudates. Organic acids in root exudates were measured by HPLC. Soil pH, bulk density, soil C and N concentrations, microbial biomass C and N were measured. Prokaryote (bacteria and archaea) and fungal community diversities in soils were assessed using PCR amplification of prokaryote rRNA 16S (V4) and fungal rRNA Internal Transcribed Spacer 1 (ITS1).
The CH4 emission fluxes follows order: CF > WS > DC, while higher fluxes of N2O emission were observed in WS and DC than in CF. However, overall impacts on global warming potentials were still smaller in DC and WS, compared to CF, suggesting a decrease in global warming potentials by water saving irrigation. Major organic acids released from roots were acetate, followed by malate and citrate. Root exudation rates of acetate, the substrates for methanogenesis, were greater in CF than in DC and WS. Fungal diversity was lower in CF than in DC and WS. Relative abundance of methanogenic archaea was also higher in rice root of CF, compared to DC and WS. Despite there being tradeoff between reducing greenhouse gas emission and increasing rice yields, increasing management options for farmers can provide opportunity to get C credit without increasing labor costs at the expense of crop yield.