CHANGES IN ANTHROPOGENIC NITROGEN AND PHOSPHORUS INPUTS TO THE ST. LAWRENCE BASIN OVER THE LAST 100 YEARS: IMPACTS ON RIVERINE EXPORT

English Abstract:

Human activities have increased the flow of Nitrogen (N) and Phosphorus (P) on earth, leading to the degradation of air, soil and water quality. Here we aim to identify the hotspots of N and P inputs and the variability of major sources in the St. Lawrence basin (SLB) for 76 watersheds throughout the 20^thcentury. We also aim to quantify the fraction of those inputs that is exported to rivers. Using historical data, all known anthropogenic N and P inputs (fertilizer, biological fixation, atmospheric deposition, detergent and net imports in food and feed) were quantified using NANI/NAPI (net anthropogenic nitrogen/phosphorus input), a mass balance modeling approach. Our results show that N inputs vary greatly among regions (110 to over 10000 kgN km²yr^-1) and are strongly related to riverine fluxes, where NANI explains 85% of the variance. As expected, riverine P exports show more variable patterns relative to NAPI revealing a clear legacy effect. NANI and NAPI have increased 2- and 5-fold respectively in the SLB since 1900 with a peak in 1990 mainly due to atmospheric N deposition and P fertilizer application. Since that time, atmospheric deposition has reduced substantially, however N fertilizer inputs have increased resulting in a minimal net change. This work highlights the relevance of NANI as an excellent management tool and the potential use of historical NAPI to quantify P legacies in rivers.