Stream Nitrogen and Phosphorus Loads Are Differentially Affected by Storm Events and the Difference May Be Exacerbated by Conservation Tillage

Storm events disproportionately mobilize dissolved phosphorus (P) compared to nitrogen (N), contributing to reduction in load N:P. In agricultural watersheds, conservation tillage may lead to even further declines in load N:P due to dissolved P accumulation in the top soil layers. Due to an increase in this management activity, we were interested in the impacts of conservation tillage on N and P loads during storm events. Using a 20 year data set of nutrient loads to a hypereutrophic reservoir, we observed disproportionately increasing P loads relative to base flow during storm events, whereas N loads were proportional to discharge. We also observed a change in that relationship, i.e., even greater P load relative to base flow with more conservation tillage in the watershed. This suggests conservation tillage may contribute to significantly reduced N:P loads during storms with potential implications for the water quality of receiving water bodies.