Is It Possible to Achieve Goals of Water Saving, Warming Mitigation Potential without Rice Yield Loss Under Limited Irrigation in Paddy Field?.

• Rice is the major crop in southern region of China. Water consumption is largely demanded by traditional flooded irrigation in paddy field. This flooded environment made paddy fields a major source for methane emission. Nowadays, water-saving and less greenhouses gases emission have been highlighted to meet the shortage of water resources and sustain yield in a long-term run. Our studies tried to develop proper paddy irrigation regime to synergistically achieve goals of water saving, less greenhouses gasses emission without rice yield loss. The experiment was conducted in 2012 and 2013 at the experimental farm of Huazhong Agricultural University, Wuxue County, Hubei Province, China. Four water regimes were designed based on bed-ditch base at the field experiment, that is, (1) continuous flooded irrigation (CF); (2) flooded and wet intermittent irrigation (FWI); (3) flooded and dry intermittent irrigation (FDI); (4) rain-fed with limited irrigation in case of serious drought (RFL). Two types of rice varieties were used, one of which is popular hybrid rice variety of Fengyuanyou299 (FYY299) in central China; another is a hybrid rice variety of Hanyou3 (HY3) with some extent to drought tolerance. Treatments were established following a split-plot design of a randomized complete block with water regimes in the main plot and rice varieties in the sub-plots. In 2012, we constructed fixed bed-ditch no-tillage system, and practiced the four water regimes, and improved them in 2013. Compared with CF, the amount of irrigation of FWI, FDI and RFL were decreased by 3%, 24%, 64%, respectively in 2012, and 38%, 41%, 80%, respectively in 2013.  In 2012, the yield of two rice varieties had no difference among FWI, FDI and RFL, and all were significantly higher than that at CF treatment. In 2013, yield of HY3 among CF and FWI did not show statistically different, but the yield was significantly decreased by 10% at FDI and 18% at RFI successively compared with CF. Yield of FYY299 was significantly reduced by 13% at FWI, 16% at FDI and 35% at RFL compared with that of CF in 2013. More irrigation water was input in 2013 than in 2012, because less rainfall dropped through July to September and thus exerted drought impact on rice growth in 2013. Water use efficiency increased successively from CFI, FWI, FDI and RFL in both of the years.  Measurement of cumulative CH₄ emission dramatically decreased successively from CF (937kg.ha^-1), FWI (372kg.ha^-1), FDI  (162kg.ha^-1) to RF (77kg.ha^-1) for both of two varieties in 2013. While cumulative CO₂ and N₂O emission significantly increased successively from CF (8351 kgCO₂.ha^-1; 7.2 kgN₂O.ha^-1), FWI (13802 kgCO₂.ha^-1; 9.7 kgN₂O.ha^-1), FDI (17940 kgCO₂.ha^-1; 10.3 kgN₂O.ha^-1) to RF (19772 kgCO₂.ha^-1; 14.5 kgN₂O.ha^-1) in 2013. However, Global warming potentials (GWPs) of rice variety HY3 were no difference among treatments of FWI, FDI and RFI, all of which were significantly lower than that of CF treatment.  Rice variety of FYY299 had significant higher GWPs at CF treatment, and lowest GWPs at FDI treatment. Considering without rice yield loss on the base of two years results, we suggested that FDI water regime could be applied to rice variety HY3 with 35~45% water saving and about 30% warming mitigation, and FWI water regime for rice variety FYY299 with 25~35% water saving and 15% warming mitigation in the usual climatic years. However, more studies are needed to perfect water regime for different rice varieties in different climatic years.

  Keywords: paddy; water-saving; CH₄; CO₂; N₂O; GWPs