Spatial Differences in Topsoil Organic Matter After Grain-for-Green Conversion to Forestland and Grassland: Evidence from the Loess Hilly Region of Western Shanxi, China

Quantifying current topsoil organic matter (SOM) differences associated with cropland retirement is challenging because soil observations are spatially uneven and historical SOM maps are scarce. We integrated 10,262 measurements of 0–20 cm SOM collected in 2023 with remote sensing and environmental covariates and annual land-cover histories from 1999 to 2023 in western Shanxi, China. Random forest (RF), support vector regression, and multiple linear regression were compared using five-fold cross-validation. Ten bootstrap RF prediction runs were used to derive a mean SOM map and a pixel-wise standard deviation layer representing between-run prediction variability. RF achieved the best performance (R2 = 0.52; RMSE = 5.43 g kg−1; MAE = 3.41 g kg−1). Strict one-to-one matching showed that predicted SOM was 3.084 g kg−1 higher in converted croplands than in cropland reference areas. The mean difference was larger for forestland-covered converted croplands (4.960 g kg−1) than for grassland-covered converted croplands (3.019 g kg−1). Earlier conversion cohorts generally showed larger spatial contrasts. A complementary local comparison yielded a mean difference of 3.004 g kg−1 and revealed substantial spatial heterogeneity. These results represent current predicted spatial differences associated with reconstructed land-cover histories rather than directly measured temporal SOM gains or causal effects of the Grain-for-Green Program.