Use of the USDA National Cooperative Soil Survey Soil Characterization Data to Detect Soil Change: A Cautionary Tale

Abstract

Core Ideas Long‐term databases are important for assessing soil change. The USDA Soil Survey Characterization Database provides long‐term soil data. Methods of analysis used have changed and soils were sampled only once. Changes in methods must be accounted for in soil change studies and trends. Recently, the USDA–NRCS National Cooperative Soil Survey Soil Characterization Database (NSCD) was reported to provide evidence that total nitrogen (TN) stocks of agricultural soils have increased across the Mississippi basin since 1985. Unfortunately, historical changes in methods used to measure TN were not fully accounted for in that report. We used NSCD archives to calibrate between wet (pre‐1995) and dry (post‐1995) digestion methods used in measuring TN and soil organic carbon (SOC), then evaluated temporal trends in SOC and TN stocks with data from 423 Alfisol and 900 Mollisol profiles representing the US Corn Belt. Data were grouped by moisture regime, farming history (presence of Ap horizon), and depth (0–20, 20–60, and 60–100 cm). Regressions showed geographic and textural influences on SOC, and that SOC increased with time among farmed soils, particularly at 20 to 60 cm. Soil TN was dependent on SOC, especially at the surface (R2 > 0.71), and decreasing TN trends with time were found among farmed soils above 60‐cm depth. Increases in C to N ratios further suggested TN has been slowly stabilizing within soils of the US Corn Belt. However, C to N ratios <9.0 were prevalent at 60–100 cm depth (>60% frequency), indicating large TN stores remain in these soils. An increasing trend in SOC, TN, and C to N ratios among non‐farmed, aquic Mollisols suggested SOC and TN accumulations in wet soils typically located below croplands. Results suggest slow improvement in agricultural soils. However, resampling has not been broadly undertaken for NSCD soil profiles, hence use of this database to detect soil change should be approached cautiously.