Biochar strengthens rainfall regime resistance of soil inorganic C by enhancing pH–bicarbonate and carbonic anhydrase regulation in dryland region

Soil inorganic carbon (SIC) is a major carbon pool in drylands, yet its long-term response to agricultural management under interannual rainfall variability, and the mediating role of carbonic anhydrase (CA) as a key enzyme in SIC formation, remain poorly quantified. We conducted an eight year rainfed field experiment, comparing no fertilizer (CK), mineral fertilizer (F), fertilizer plus straw (FS), and fertilizer plus biochar (FB). Growing season precipitation was classified into dry, normal, and wet regimes by hierarchical clustering. We quantified annual changes in SIC and carbonate related variables, and evaluated CO 2 emissions and maize productivity. Across rainfall regimes and 0–60 cm soil layer, FB consistently increased the SIC content (mean ΔSIC = 0.22 g kg −1 ), whereas FS caused net SIC loss (−0.04 g kg −1 ) and F showed little difference from CK (0.12 g kg −1 ). Wet years promoted topsoil SIC loss and HCO 3 - leaching to subsoil, while dry years generally favored net SIC accumulation. FS decreased soil pH, reduced HCO 3 - , increased CO 2 emissions, and showed a stronger decline in CA activity, consistent with intensified carbonate dissolution and leaching. In contrast, FB increased pH, ΔHCO 3 - , and buffered CA declines across rainfall regimes. Biochar sustained CA related bicarbonate availability and favored bicarbonate mediated SIC formation. FB achieved the highest mean yield (10.75 × 10 3 kg ha −1 ) and smaller CO 2 increase. Overall, long-term biochar application enhanced the rainfall regime resistance of SIC accumulation, particularly under high precipitation (504 mm), while improving crop production with a lower CO 2 emission than straw in dryland region.