Mark D. Tomer, USDA-ARS National Laboratory for Agriculture and the Environment, Ames, IA, David James, USDA/ARS, Ames, IA, Louis Schipper, Earth and Ocean Sciences, University of Waikato, Hamilton, NEW ZEALAND and Skye A. Wills, Soil Science Division, USDA-NRCS, Lincoln, NE
A recent analysis of soil total nitrogen (TN) data from the USDA-NRCS National Cooperative Soil Survey Soil Characterization Database (NSCD) suggested agricultural soils have increased in TN stocks across the Mississippi basin since 1985. However, key metadata on TN methods have been omitted from the NSCD, and a change in TN methodology was not considered in that recent analysis. We calibrated between wet (pre-1995) and dry (post-1995) soil digestion methods used in measuring TN and soil organic carbon (SOC), then re-assessed temporal trends in SOC and TN stocks. Data from profiles representative of the U.S. Corn Belt, comprising 423 Alfisols and 900 Mollisols, were grouped by Soil Order, moisture regime, cropping history (presence of Ap horizon), and depth (0-20, 20-60, and 60-100 cm). Correcting between TN methods reversed the prior-reported trend of increasing soil N stocks. After accounting for SOC as a control on TN, TN decreased with time among plowed soils at <60-cm depth. Ratios of C:N increased with time, suggesting that the susceptibility of TN to leaching has declined among farmed Corn Belt soils. At the same time, C:N ratios <9.0 were prevalent; therefore, large TN stores remain a soil management challenge. Among non-farmed, aquic Mollisols, increasing trends in SOC, TN, and C:N ratios suggested SOC and TN accumulated in wet soils below croplands. Although results suggest agricultural soils of the Corn Belt may be improving, the NSCD was never intended to monitor soil change and using it for that purpose may not be valid.