Use of the USDA National Cooperative Soil Survey Soil Characterization Data to detect soil change: A cautionary tale

Authors: Tomer, Mark; James, David; SCHIPPER, LOUIS - University Of Waikato; Wills, Skye

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2017
Publication Date: 1/4/2018
Publication URL: https://handle.nal.usda.gov/10113/6592651
Citation: Tomer, M.D., James, D.E., Schipper, L.A., Wills, S.A. 2018. Use of the USDA National Cooperative Soil Survey Soil Characterization Data to detect soil change: A cautionary tale. Soil Science Society of America Journal. 81:1463-1474. https://doi.org/10.2136/sssaj2017.06.0198.
DOI: https://doi.org/10.2136/sssaj2017.06.0198

Interpretive Summary: Recently, the USDA 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, analytical methods were not described with TN data downloaded from the NSCD, therefore, historical changes in methods used to measure TN were not considered in that report. We used NSCD archives to correct between current and historical methods, then determined trends in time for TN and soil organic carbon (SOC) stocks among soils typical of the U.S. Corn Belt. The correction between methods nullified the reported trend of increasing TN. In contrast, after accounting for SOC as a control on TN, decreasing TN trends with time were found. Declines in C:N ratios further suggested a slow decrease in susceptibility of TN to leaching among farmed soils in the Corn Belt. However, low C:N ratios (<9.0) were prevalent, indicating some TN remains susceptible to leaching. Results suggest improvement in agricultural soils of the Corn Belt, but using the NSCD to detect soil change should be approached cautiously. These results are of interest to those in the agricultural and environmental communities who are interested in understanding how agricultural soils may be changing under current agricultural systems, and how historical soils data can be used to document soil change.

Technical Abstract: 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, due to omission of metadata from the NSCD, historical changes in methods used to measure TN were not considered 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 representative of the U.S. Corn Belt. Data were grouped by moisture regime, farming history (presence of Ap horizon), and depth (0-20, 20-60, and 60-100 cm). The correction between TN methods nullified the reported trend of increasing TN. Regressions showed geographic and textural controls on SOC, and that SOC increased with time among farmed soils at 20-60 cm. Soil TN was strongly controlled by SOC, but decreasing TN trends with time were found among farmed soils above 60-cm depth. Declines in C:N ratios further suggested a slow decrease in susceptibility of TN to leaching among Corn Belt soils. However, C:N ratios <9.0 were prevalent, indicating large TN stores remain in Corn Belt soils. An increasing trend in SOC, TN, and C:N ratios among nonfarmed, aquic Mollisols suggested C and N have accumulated in wet soils below croplands. While results suggest improvement in agricultural soils, using the NSCD to detect soil change should be approached cautiously.