Atmospheric CO2 concentration effects on rice water use and biomass production

Numerous studies have addressed effects of rising atmospheric CO2 concentration on rice biomass production and yield but effects on crop water use are less well understood. Irrigated rice evapotranspiration (ET) is composed of floodwater evaporation and canopy transpiration. Crop coefficient Kc (ET over potential ET, or ETo) is crop specific according to FAO, but may decrease as CO2 concentration rises. A sunlit growth chamber experiment was conducted in the Philippines, exposing 1.44-m2 canopies of IR72 rice to four constant CO2 levels (195, 390, 780 and 1560 ppmv). Crop geometry and management emulated field conditions. In two wet (WS) and two dry (DS) seasons, final aboveground dry weight (agdw) was measured. At 390 ppmv [CO2] (current ambient level), agdw averaged 1744 g m-2, similar to field although solar radiation was only 61% of ambient. Reduction to 195 ppmv [CO2] reduced agdw to 56±5% (SE), increase to 780 ppmv increased agdw to 128±8%, and 1560 ppmv increased agdw to 142±5%. In 2013WS, crop ET was measured by weighing the water extracted daily from the chambers by the air conditioners controlling air humidity. Chamber ETo was calculated according to FAO and empirically corrected via observed pan evaporation in chamber vs. field. For 390 ppmv [CO2], Kc was about 1 during crop establishment but increased to about 3 at flowering. 195 ppmv CO2 reduced Kc, 780 ppmv increased it, but at 1560 ppmv it declined. Whole-season crop water use was 564 mm (195 ppmv), 719 mm (390 ppmv), 928 mm (780 ppmv) and 803 mm (1560 ppmv). With increasing [CO2], crop water use efficiency (WUE) gradually increased from 1.59 g kg-1 (195 ppmv) to 2.88 g kg-1 (1560 ppmv). Transpiration efficiency (TE) measured on flag leaves responded more strongly to [CO2] than WUE. Responses of some morphological traits are also reported. In conclusion, increased CO2 promotes biomass more than water use of irrigated rice, causing increased WUE, but it does not help saving water. Comparability with field conditions is discussed. The results will be used to train crop models.

已有诸多研究探讨了大气CO₂浓度升高对水稻生物量生产与产量的影响，但针对其对作物水分利用的影响，学界的认知仍相对不足。灌溉水稻的蒸散量（evapotranspiration, ET）由田间淹水蒸发与冠层蒸腾两部分构成。作物系数Kc（实际蒸散量与潜在蒸散量（potential evapotranspiration, ETo）的比值）因作物种类而异，符合联合国粮食及农业组织（Food and Agriculture Organization, FAO）的规范，但会随CO₂浓度升高而降低。
本研究在菲律宾开展了一项光照型生长箱试验，将面积为1.44 m²的IR72水稻冠层暴露于4种恒定CO₂浓度（195、390、780及1560 ppmv）环境中。试验中作物种植构型与田间管理均模拟大田实际条件。在两个湿季（wet season, WS）与两个干季（dry season, DS）中，研究人员测定了水稻地上部总干重（aboveground dry weight, agdw）。在390 ppmv的CO₂浓度（当前大气本底浓度）下，水稻地上部总干重平均值为1744 g·m⁻²，与大田水平相近——尽管试验中的太阳辐射仅为自然环境的61%。当CO₂浓度降至195 ppmv时，地上部总干重降至对照的56±5%（标准误, SE）；浓度提升至780 ppmv时，干重增至对照的128±8%；而1560 ppmv浓度下，干重则增至对照的142±5%。
在2013年湿季中，研究人员通过称量控制箱内湿度的空调每日抽出的水量，测定了试验箱内水稻的蒸散量。试验箱内的潜在蒸散量（ETo）依据FAO标准计算，并通过对比箱内与大田的蒸发皿蒸发量进行经验校正。在390 ppmv CO₂浓度下，作物定植阶段的Kc值约为1，至开花期则升至约3。CO₂浓度为195 ppmv时，Kc值有所降低；780 ppmv时Kc值升高，但1560 ppmv时Kc值反而出现下降。全生育期作物总耗水量分别为：195 ppmv时564 mm，390 ppmv时719 mm，780 ppmv时928 mm，1560 ppmv时803 mm。
随着CO₂浓度升高，作物水分利用效率（water use efficiency, WUE）逐步提升，从195 ppmv时的1.59 g·kg⁻¹升至1560 ppmv时的2.88 g·kg⁻¹。对剑叶测定的蒸腾效率（transpiration efficiency, TE）对CO₂浓度的响应幅度较WUE更为显著。本研究同时报道了部分形态性状对CO₂浓度的响应情况。
综上，CO₂浓度升高对灌溉水稻生物量的提升作用大于其对水分利用的影响，进而提升了作物水分利用效率，但并未实现节水效果。研究还讨论了试验结果与大田实际条件的可比性。本研究所得结果将用于作物模型的训练与优化。