Interactive Effects of Soil Acidification and Moisture on Carbon Mineralization in Karst Grassland Soils

Understanding how soil acidification and moisture jointly regulate carbon mineralization is particularly important in karst grasslands, where high carbonate content can interfere with CO2-based measurements. In this study, a controlled incubation experiment was conducted using soils collected from a typical karst grassland in Guizhou Province, China. Two pH levels (4.5 and 6.5) and three moisture levels (30%, 40%, and 60% of field water-holding capacity, WHC) were applied in a full-factorial design following a pre-incubation step to minimize carbonate-derived CO2 interference. Soil CO2 efflux, emission rate, and cumulative mineralization were monitored over a 60-day incubation period. Both soil moisture and pH significantly affected carbon mineralization, with a clear interaction between the two factors (p < 0.05). CO2 efflux peaked during the early incubation stage and declined thereafter, indicating rapid depletion of labile carbon substrates. Across both pH levels, increasing moisture consistently enhanced CO2 efflux and cumulative mineralization. Under comparable moisture conditions, near-neutral soils (pH 6.5) exhibited higher mineralization rates than acidic soils (pH 4.5). The highest carbon mineralization was observed at 60% WHC under pH 6.5, whereas the lowest occurred at 30% WHC under pH 4.5. These results suggest that moisture availability regulates substrate diffusion and microbial activity, while soil acidification constrains microbial metabolism and enzyme function. Notably, the effect of pH became less pronounced under low moisture conditions, indicating that water limitation can override pH regulation. This study offers a methodological framework for quantifying carbon mineralization in carbonate-rich soils and underscores the necessity of accounting for both physical and chemical limiting factors, as well as the confounding influence of inherent carbonates. Nevertheless, given the exclusive use of a single soil type and controlled laboratory conditions, the findings constitute preliminary evidence and require validation under field conditions and across diverse soil types before broader generalization.