The nitrogen cycle across topography: insights from natural abundance nitrate isotopes

Abstract:

Most of the wetlands in the Midwest Corn Belt were drained, primarily to allow for crop cultivation. Despite drainage, the low-lying areas of many Corn Belt fields still experience periodic flooding. Excess moisture in these areas frequently damages or destroys crops but may also reduce harmful nitrate (NO₃^-) export downstream by promoting denitrification. The extent to which drained wetlands either export nitrogen (N) downstream or remove NO₃^- through denitrification is largely unknown. To help address this uncertainty, we established 5 transects along topographic gradients in Central Iowa crop fields. The transects spanned from drained, depressional wetlands to the surrounding uplands. Agricultural management was a conventional corn-soybean rotation with typical fertilization rates during corn years (~168 kg N ha^-1 y^-1). Resin lysimeters along each transect were used to quantify NO₃^- and ammonium (NH₄⁺) infiltration in 2018. In 2017 and 2018, eight additional zero-tension lysimeters were maintained at one of the transects. Samples from the zero-tension lysimeters were collected following rainfall and subsequently at 48-hour intervals. These samples were analyzed for NO₃^- and NH₄⁺ concentration, and a subset were chosen for NO₃ ¹⁵N and ¹⁸O isotope analysis. NO₃^- isotopes collected in the depressional wetland indicated periods with no enrichment as well as enriched periods indicative of substantial denitrification. In contrast, samples collected from uplands show relatively little enrichment. These results suggest that depressions can variably remove NO₃^- via denitrification or export nitrate through infiltration without denitrification depending on antecedent conditions. Despite evidence of greater denitrification, we found no consistent effect of topography on total N infiltration. Paired with lower crop yield, our research suggests that drained wetlands could be targeted for alternative management to in increase N use efficiency and reduce downstream N export.