Storm Event Nitrogen Dynamics in Waterfowl Impoundments

摘要

Waterfowl impoundments are hydrologically managed to provide food and habitat for migratory birds and have the potential to export nitrogen during drawdown. The goal of this study was to describe how nitrate, ammonium, and dissolved organic nitrogen dynamics varied under two different management schemes during storm events. This was made possible through monitoring the three forms of nitrogen at a high frequency (30 min) in a moist-soil managed (MSM) impoundment and seasonally flooded agricultural impoundment (Ag) for 17 months. Substantial differences in nitrogen dynamics between sites were observed when the sites were not flooded, while similar dynamics were observed during the winter flooding period. When the Ag site was drained, storm events mobilized nitrate (average 0.3 mg L-1; increase 0.8 mg L-1) and ammonium (average 0.4 mg L-1; increase 0.2 mg L-1). Under drained conditions in the MSM impoundment, rainfall reduced ammonium (average 0.5 mg L-1; decrease 0.1 mg L-1). Storms stimulated what appeared to be coupled nitrification-denitrification in both impoundments when flooded and ammonium concentrations were elevated. A surprising result of this work was observed at the Ag site, where elevated nitrate (0.5 mg L-1) was measured during high water levels when anoxic conditions were expected to support denitrification. Although water management schemes were found to be important for controlling nitrogen dynamics, other factors, such as carbon quality, require further research. This study demonstrates a high variability between stormevents and the large influence of hydrologic management schemes on nitrogen dynamics during storm events.