Carbon dioxide and temperature elevation effects on crop evapotranspiration and water use efficiency in soybean as affected by different nitrogen levels

Rising concentration of atmospheric carbon dioxide (CO2) is reported to cause fertilization effect resulting in enhanced crop biomass and yields and may alter the water use of plants. However, factors like air temperature and nitrogen (N) management could modify the feedbacks of CO2 on crop water use. Hence, this field study was carried out in open top field chambers (OTC) for two crop seasons to investigate the interactive effects of climate and N on evapotranspiration, yield and water use efficiency in soybean (Glycine max L.). Soybean crop was grown under three climate conditions (ambient : AC, elevated CO2 : eC, and co-elevation of both CO2 and temperature : eCeT) and four N treatments during July to October 2016 and 2018. Elevation of CO2 was done to the level of 535-540 mu mol mol(-1), and temperature was elevated by about 2 degrees C above ambient. The four N levels were N-0, N-50, N-100 and N-150 referring to 0, 50, 100 and 150% of recommended N dose (30 kg N ha(-1)). Crop evapotranspiration (ET) was computed by soil water balance method. The two years' field study indicated eC and eCeT showed significant yield advantage to the extent of 32-47% over ambient. Significant effect (P < 0.05) of climate was observed on crop ET, profile water storage and water use efficiency (WUE) during both the study years. The effect of N application on these three parameters was significant only in 2016 crop year. Averaged across N treatments, profile soil water storage at harvest was higher by about 5% in 2016 and by 9% in 2018 crop season under eC and eCeT treatments as compared to AC. The crop ET was significantly lower under eC and eCeT in both the years, with significantly higher WUE. The WUE varied from 2.99 to 4.48 kg ha(-1) mm(-1) in 2016 and from 4.62 to 6.42 kg ha(-1) mm(-1) in 2018 crop year. Stomatal conductance during major growth period reduced by 21-42% under eC and by 19-31% under eCeT, though it did not reflect in reduced transpiration. The study indicated significantly higher leaf area contributing to reduced soil water evaporation is the major mechanism explaining higher soil water profile under eC and eCeT. Over ambient, the WUE was higher by 48-50% in 2016 and by 37-39% in 2018 under eC and eCeT treatments, which was mostly attributed to higher grain yield.