Spatial Discrepancies Between Annual Cropland Extent and Sampling-Period Agricultural-Use Extent: Consequences for Soil Organic Carbon Aggregation

Abstract

Annual cropland data are widely used to delimit agricultural land for soil organic carbon (SOC) mapping and spatial aggregation, yet mapped cropland may not coincide with land under agricultural use at soil sampling. Such a mismatch changes the land entering aggregation and may alter both the aggregated mean of predicted SOC and the agricultural land it represents. This difference has rarely been quantified with the SOC prediction surface held fixed. At Xinglongzhuang Coal Mine in eastern China, an agricultural sampling frame established before fieldwork was verified during March 2022 field sampling and finalized as the sampling-period agricultural-use extent (Full) for comparison with Esri 2022 Crops. Within Full, Esri-covered land was Covered and the remainder Uncovered. A random forest fitted to 155 model-development sites generated the SOC prediction surface; it was independently evaluated using 60 probability-sampled validation sites, while area of applicability (AOA) analysis assessed predictor-space support. Uncovered comprised 25.25% of Full; 86.47% occurred >10 m from the Full boundary and 97.84% was within the AOA. Independent validation yielded a root mean square error (RMSE) of 2.272 g kg−1. Mean predicted SOC was 14.338 g kg−1 in Covered and 8.732 g kg−1 in Uncovered; Full averaged 12.922 g kg−1, 1.415 g kg−1 (9.9%) lower than Covered. A design-based residual correction gave a Full–Covered contrast of −1.592 g kg−1 (95% confidence interval: −2.036 to −1.147 g kg−1). The Full–Covered difference equals the Uncovered area proportion multiplied by the mean predicted SOC difference between Uncovered and Covered. Across the three reporting zones, these two components ranked differently, so Uncovered area proportion alone did not track the magnitude of the aggregation difference. With the prediction surface fixed, including Uncovered land with lower predicted SOC reduced the mean for Full relative to Covered. The reported mean is therefore tied to the agricultural extent used for aggregation and, in changing landscapes, to the period represented by that extent.