A research team led by Professor Li Hu at the Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, has reported new progress in understanding the spatial variation and driving mechanisms of soil organic carbon (SOC) in cropland soils. The study systematically characterized the spatial distribution patterns, dominantfactors, and regulatory pathways of SOC and its labile fractions—particulate organic carbon (POC) and permanganate-oxidizable carbon (POXC)—across China's major agricultural regions, providing a scientific basis for region-specific soil carbon management in cultivated land. The findings were published in Soil & Tillage Research.

Soil organic carbon is crucial for maintaining soil quality and is a core element of carbon cycling in agroecosystems.Compared with focusing solely on total SOC, a fraction-based perspective provides deeper insights into carbon activity, stability, and ecological functions. Among these indicators, the POC/SOC ratio is commonly used to reflect carbon stability, while POXC is an important indicator of the active carbon pool and soil fertility. However, large-scale systematic studies on the spatial heterogeneity of SOC and its labile fractions in China's croplands remain limited.

To address this gap, the research team conducted systematic sampling along a representative northeast–southwest transect across China, covering three major agricultural regions: the Northeast Plain, the Huang-Huai-Hai Plain, and the Yunnan-Guizhou Plateau. A total of 230 topsoil samples werecollected and analyzed forSOC, POC, POXC, and related physicochemical properties. By integrating multi-scale environmental factors with machine learning methods and structural equation modeling, the team quantitatively examined the spatial patterns and driving mechanisms of SOC and its labile fractions.

The results showed that the average contents of SOC, POC, and POXC in China's major agricultural regions are 14.03, 4.64, and 0.67 g/kg, respectively, with POC accounting for approximately 34% of SOC. The Northeast Plain exhibits the highest POXC levels and the lowest POC/SOC ratio, indicating relatively high soil fertility and carbon stability. In contrast, the Huang-Huai-Hai Plain shows lower SOC and POXC levels but a higher POC/SOC ratio, suggesting dual constraints in improving soil fertility and carbon stability. The Yunnan-Guizhou Plateau features relatively high SOC and POC contents, with POXC levels comparable to those in the Northeast Plain, but with relatively weaker carbon stability.

Further analysis indicates that soil properties were the dominant drivers of spatial variation in SOC and its fractions. In particular, total nitrogen and clay content play key roles through stoichiometric regulation and physical protection mechanisms, respectively. Topographic factors mainly exert indirect effects by influencing regional hydrothermal conditions and soil properties.

This study advances the understanding of spatial heterogeneity and environmental controls of SOC in China's agricultural soils from the perspective of carbon fractions. It provides a theoretical basis for region-specific strategies to enhance soil carbon sequestration, fertility, and stability in cultivated land.

The research was supported by the State Key Laboratory of Efficient Utilization of Arable Land in China, the Fundamental Science Research Program of the Science and Technology Innovation Project of the Chinese Academy of Agricultural Sciences, and the National Natural Science Foundation of China.

Article link: https://doi.org/10.1016/j.still.2026.107192