Effects of water saving irrigation and rice variety on greenhouse gas emissions and water use efficiency in a paddy field fertilized with anaerobically digested pig slurry

Proper floodwater management during the cultivation period of paddy rice is essential for regulating the trade-off between methane and nitrous oxide emissions, as well as for saving irrigation water resource and maintaining crop productivity. To elucidate the effects of water saving irrigation (WS) and rice variety on different agro-environmental parameters such as CH4 and N2O emissions, global warming potential (GWP), greenhouse gases intensity (GHGI), carbon efficiency ratio, water use efficiency (WUE), rice growth and yield, a field experiment was conducted with two factors of split-plot design consisting of (1) main factor; WS (WS in which irrigation was withdrawn at soil Eh < -150 mV and kept 0-1 cm floodwater afterward) and continuous flooding (CF), and (2) sub-factor; a forage rice variety Leaf Star (LS) and a normal rice variety Kinuhikari (KN). Results showed that WS irrigation significantly (P < 0.05) mitigated CH4 emission by 65 % and by 60 %, and enhanced WUE significantly (P < 0.01) by 180 % from that of CF. Carbon use efficiency of the grain and aboveground biomass carbon increased by 2.4 and 2.5 times, respectively, and GHGI was reduced by 3.1 times in WS. No significant differences were observed in the plant height, tiller number, rice biomass, N uptake and C accumulation between water treatments but the rice varieties: LS showed significantly (P < 0.05) higher values in these parameters than KN. This study highlighted that the WS is an effective measure for mitigating GWP, saving water while sustaining crop productivity.