CALCIUM SPECIATION AND ORGANIC CARBON RESPONSE TO ACIDIFICATION IN KARST CULTIVATED SOILS OF HUNAN

To clarify the mechanisms of acid suppression and enhancement of soil quality, this study investigated the interaction between calcium and carbon during soil acidification in karst arable land in Hunan, China.Soil samples were categorized into four acidification grades: strong (pH ≤ 4.5), weak (4.5 < pH ≤ 6.5), neutral (6.5 < pH ≤ 7.5), and alkaline (pH > 7.5).The relationship between soil calcium speciation and acidification was examined.A sequential variation in calcium content was observed across different speciation in the following order: exchangeable calcium, acid-soluble calcium, water-soluble calcium, residual calcium, and organically bound calcium.Active calcium migration in the study area reflected its bioavailability.Compared with alkaline conditions, soil calcium, carbon content, and carbon storage decreased with increasing soil acidification (p < 0.05).The average soil organic carbon (SOC) content declined from 44.4% to 34.3%, and the carbon stock decreased from 41.4% to 28.4%.A significant correlation was found between the total soil calcium and organic carbon content (p < 0.05).Under strong acidification, the calcium and carbon contents, as well as the carbon stocks, reached their lowest levels, resulting in a 215-fold decrease in calcium effectiveness.The primary factors contributing to soil acidification in this area were the soil calcium levels and morphology.Enhancing SOC content and improving the activity of Ca 2+ in karst soils serve as mechanisms for acid suppression.