Dryland-to-Paddy Conversions Lead to Short-Term Decreases in Soil Organic Carbon and Carbon Pool Management Index in Karst Soil of Guizhou Province, China

To respond to China’s policies of “balancing and supplementing high-quality farmland, converting dryland to paddy fields” and of improving the quality of drylands, large-scale dryland conversion to paddy fields has been recently implemented to improve farmland quality and increase grain production capacity in China. However, the effects of the conversions on the soil carbon (C) pool remain unclear. Therefore, in a karst area of Guizhou Province, China, we selected farmland soils that had undergone dryland-to-paddy conversion and planted with rice for one year, ensuring that the historical management practices, soil parent material, and spatial proximity were consistent. We compared changes in and interactions between soil physicochemical properties, organic C components, and the C pool management index (CPMI) in pre-conversion drylands, post-conversion paddy fields, and reference unused lands. We found that dryland-to-paddy conversion suppressed most soil physicochemical properties in the short term. After dryland conversion to paddy fields, total C, total organic C, dissolved organic C, easily oxidizable organic C, inert organic C, microbial biomass C, and soil CPMI significantly decreased temporarily. With conversion, the proportion of easily oxidizable organic C decreased, whereas those of dissolved organic C, microbial biomass C, and inert organic C increased. Correlation and redundancy analyses indicated that bulk density was negatively correlated with organic C, its components, and the CPMI, whereas soil nitrogen (N), alkaline N, available phosphorus (P), and available potassium were significantly positively correlated with organic C. The carbon pool activity and CPMI were primarily influenced by easily oxidizable organic C, microbial biomass C, alkaline N, and available P. According to a partial least squares structural equation model, soil physicochemical properties and organic C and its components were the main drivers of C pool changes. Organic C and its components directly influenced C pool changes, whereas soil physicochemical properties mostly indirectly influenced C pool changes. Therefore, although dryland-to-paddy conversion can maintain a balance of arable land and ensure food security, such conversions may lead to short-term declines in organic C stability and C pool management index, indicating mitigation strategies, such as tailored N, P, and K fertilization regimes, should be developed to increase farmland C sequestration capacity.