A Race against Time: Nutrient Legacies and Time Lags Provide New Insight into Improving Water Quality in Human-Impacted Landscapes

Global flows of reactive nitrogen (N) and phosphorus (P) have increased significantly over the last century in response to land-use change, agricultural intensification and elevated levels of atmospheric N deposition. Despite widespread implementation of a range of conservation measures to mitigate the impacts of intensive agriculture, N and P concentrations in surface waters are in many cases remaining steady or continuing to increase. Such lack of response has been attributed to legacy stores in subsurface reservoirs that contribute to time lags between conservation measures implemented on the landscape and water quality benefits realized in receiving water bodies. It has remained unclear, however, what the magnitudes of such stores might be, and how they are partitioned between various landscape elements (shallow soil and deeper groundwater reservoirs, reservoir and stream sediments). We have developed a unique modelling framework called the ELEMeNT (Exploration of Long Term Nutrient Trajectories)– which pairs a simulation of soil nutrient dynamics with a travel time-based approach – to reconstruct historic nutrient yields at the outlets of these watersheds and to model future nutrient loading under a range of scenarios.  The uniqueness of the model is that in addition to time lags it can also provide information on magnitudes of legacy accumulation in various landscape elements. Our results, using a 200 year N input trajectory for the Mississippi River Basin (MRB) and the Susquehanna watershed, show significant N loading above baseline levels before the widespread use of commercial N fertilizers, largely due to 19^th-century conversion of natural forest and grassland areas to row-crop agriculture.  Our modeling of future scenarios indicates that even if agricultural N use were to become 100% efficient, it would take on the order of decades to meet policy goals for improving water quality.