Farm-Scale Variability of Soil Organic Carbon Pools in Tilled and No-Tilled Chernozems

Understanding the spatial variability of soil organic carbon (SOC) content and its functional pools under current farming practices is crucial for developing targeted C management. This study quantified and predicted the farm-scale variability of SOC pools across conventional tillage (CT) and no-tillage (NT; 8–14 years) practices at two sites (Rostov and Krasnodar) in Russia. The soil types at Rostov and Krasnodar farms were Calcic Chernozem (sunflower–wheat rotation) and Stagnic Chernozem (maize–wheat rotation), respectively. The average SOC content at the Rostov site was higher than the Krasnodar site by 41% and 28% in 0–10 and 10–30 cm, respectively. For both sites, there was no clear trend in SOC variability between NT and CT practices. However, topsoil microbial-available C pool (mineralized for 180 days) was most sensitive to tillage systems, unlike unchanged particle-size C pools. Specifically, it increased from CT to NT at the Rostov site (by 7–16%), but it showed a decreased trend at the Krasnodar site (by 11–29%). Gradient boosting machines statistical models with remote sensing data based explanatory variables (spectral, topography) accurately predicted the spatial distributions of topsoil C content (R2 = 0.99) and its microbial-available pool (R2 = 0.78) across the farmland areas. The main explanatory variables included topography, vegetation distribution, moisture and thermal regimes. For both sites, DNDC modeling showed that applying NT versus CT for 30 years could potentially increase SOC in the topsoil by 27–28% and decrease it in the subsoil by 6–9% (sunflower–wheat–maize–wheat rotation; annual N and P rates of 41–80 and 0–52 kg ha−1). This study provides insights into current agricultural challenges and the developing site-specific strategies for managing soil C accrual in the Chernozem region.