Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2014
Publication Date: 1/8/2015
Citation: Kitchen, N.R., Blanchard, P.E., Lerch, R.N. 2015. Long-term agroecosystem research in the Central Mississippi River Basin: hydrogeologic controls and crop management influence on nitrates in loess and fractured glacial till. Journal of Environmental Quality. 44:58-70. DOI: 10.2134/jeq2014.09.0405.
Interpretive Summary: Grain crop yield improvement with nitrogen fertilization is tremendous with increases commonly two to four times greater than without fertilization, and as such a critical input to help reach global food security goals. One negative consequence of nitrogen fertilizer use is movement from farmland into lakes, rivers, and streams as well as leaching of nitrate-nitrogen into ground water aquifers. This research was conducted to assess over a 14-year period the nitrate levels for a shallow glacial-till aquifer underlying a typical U.S. Midwest claypan soil. There was no clear evidence that the farming practices used during this investigation contributed to increased nitrate concentrations in ground water. Initially, ground water from one field was notably greater in nitrate contamination than the other two fields we monitored, likely the result of concurrent applications of manure and nitrogen fertilizer prior to 1980. During this study, nitrate levels on this field decreased an average of 0.3 ppm per year. Results also showed that both physical properties and processes (e.g., glacial till material and thickness, nitrate diffusion into the till matrix, and preferential flow) and biological process (e.g., denitrification and plant uptake) were present in the experimental fields being assessed. These properties and processes were helpful in understanding changes in nitrate concentration over time. The results of this study will be used to help develop relationships explaining the long-term impacts of agricultural practices and water quality. From these, preventative and corrective best management practices can be identified and promoted. Nitrogen managed so as to minimize leaching and runoff losses represents economic savings for farmers and the general public, as well as ecological and environmental benefits.
Technical Abstract: Nitrogen (N) from agricultural activities has been suspected as a primary source of elevated ground water nitrate (NO3-N). The objective of this research was to assess the impact of common cropping systems on NO3-N levels for a glacial till aquifer underlying claypan soils in a predominantly agricultural watershed in north-east Missouri. Three cropped fields with 10 years of similar management were selected and each instrumented with 20-25 monitoring wells, 3-15 m in depth, in 1991-92. Wells were sampled and analyzed for NO3-N at least annually from 1991 to 2004. Initial NO3-N concentrations were variable, ranging from non-detectable to over 24 mg/L with an average NO3-N concentration of 7.0 mg/L. Ground water NO3-N was significantly higher on Field 3, the result of concurrent applications of manure and N fertilizer before 1980. Overall changes in NO3-N levels on Fields 1 and 2 were generally small. However NO3-N levels for Field 3 have decreased an average of 0.28 mg/L/yr. Excessive loading of N into the matrix of the glacial till has had a long-term impact on NO3-N levels for this field. Despite the presence of dissolved oxygen in the aquifer, evidence of denitrification in some upper-landscape ground water wells was found. However, the greatest decreases in NO3-N concentration occurred as ground water moved through an in-field tree line or through a riparian zone. While overall conclusions were complicated by the long-term impact of past management, capacity of the till to buffer changes in NO3-N, hydrogeologic variability found among well sites, and activity of biological processes, we conclude farming practices during this study did not increase glacial till NO3-N.