Does Denitrification Control Agricultural Nitrate Export in the Mississippi Delta?

Abstract

Nitrate management in agricultural systems is becoming increasingly important for farmers in the United States as they wrestle with the challenges of trying to increase yields per acre and decreasing their environmental impact. In the Mississippi River, the Lower Basin contributes a fraction of the nitrogen flux of that of the Upper and Central Basins. While this disparity in nitrogen flux can be attributed partly to agricultural practices such as tile drainage and the fertilizer requirements of the dominant crops in the regions, denitrification is the most likely mechanism functioning to remove nitrate (NO3 -) from the system. However, in the Lower Mississippi River Basin, denitrification has not been extensively studied, and previous research has only focused on denitrification in deep groundwater. In this study, we examine the shallow surface soils in the Mississippi Delta to identify denitrification fingerprints in shallow surface soil. Using a quantitative polymerase chain reaction (qPCR), nitrate concentration measurements, and isotopic fractionation, we found strong evidence for denitrification in the shallow surface soils. High concentrations of nitrous oxide reductase (nosZ) fragments, low [NO3 -] suggesting NO3 - removal, isotope signatures of δ15N and δ18O indicative of denitrification, and trace N2O emissions all support the hypothesis that denitrification occurs in shallow surface soils in the Mississippi Delta.