The conversion of HCO3– in irrigation water into soil inorganic C under winter wheat and summer millet cultivation in a semiarid cropland

Abstract

Irrigation water in arid and semi-arid croplands often contains high concentrations of bicarbonate, which may react with soil available cations under alkaline conditions to form pedogenic inorganic carbon, thereby promoting soil inorganic carbon (SIC) accumulation. However, direct evidence and quantitative assessment of the contribution of HCO₃^– in irrigation water into SIC formation remain limited. In this study, a one-year pot experiment was conducted under winter wheat and summer millet cultivation in a temperate region. Crops were irrigated with H¹³CO₃^- labeled water, and the conversion rate of HCO₃^– in irrigation water into SIC was quantified and the underlying mechanisms were analyzed. The results showed that δ¹³C-SIC values significantly increased in all soil layers (0–80) after H¹³CO₃^- irrigation, directly confirming that HCO₃^– in irrigation water can be converted into SIC and retained in soil at a short-term. The conversion rates of HCO₃^– into SIC were 27.98% and 12.44% during the winter wheat and summer millet growing periods, respectively, with an annual average conversion rate of 20.21%, indicating clear seasonal differences in the conversion of HCO₃^– in irrigation water into SIC. The ¹³C stored in SIC decreased with soil depth, was positively correlated with soil water content, and exhibited a quadratic relationship with soil temperature. These results demonstrated that irrigation bicarbonate can contribute to short-term incorporation of carbon into SIC pools, with potential implications for longer-term carbonate formation, and highlighted the important role of hydrothermal conditions in regulating SIC formation.