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Effects of Supplementary Irrigation on Soil Respiration of Millet Farmland in a Semi-Arid Region in China

Carbon dioxide (CO2) is recognized as key part of evaluating the soil environment, and the soil respiration rate is an effective indicator of CO2 emission. To explore the influence and coupling mechanism of irrigation on the soil respiration of millet farmland in the Northern Shanxi Province in China, conventional rainfed (CK) and supplementary irrigation (W1) at the late jointing stage were conducted. The soil respiration rate and carbon emission flux in millet farmland under different treatments were observed. The relationship between soil respiration rate and soil physical–chemical properties and the crop growth index was further analyzed. The result showed that the soil respiration rate and carbon emission flux of W1 were higher than those of CK treatment. The comparison of the linear regression correlation between soil respiration rate and soil physical–chemical properties revealed that the major regulating factors of the soil respiration rate were soil moisture (<10.6%) followed by soil pH, soil moisture (>10.6%), soil temperature, and finally soil organic matter content. There are uncertainties regarding the soil moisture content variation range in soil respiration. Moreover, supplementary irrigation promoted the growth indexes, yield, and irrigation water use efficiency in millet farmland. Further research with less irrigation treatment is necessary for exploring an optimization model of water use efficiency and low carbon dioxide emissions in millet fields, which would be helpful to realize agricultural water utilization and a “carbon peak” in the sense of farmland.