Stream phosphorus and nitrogen export from agricultural and forested watersheds on the Boreal Plain

We evaluated phosphorus (P) and inorganic nitrogen (IN) export from two agr icultural and two forested watersheds on the nutrient-rich but relief-poor Boreal Plain. One agricultural stream was in a watershed that contained cro pland, while the second consisted of mixed agricultural activities. Over th e 2-year study, total dissolved phosphorus (TDP) concentrations were propor tionately high, particularly in the agricultural streams. Flow-weighted TDP averaged 82% of the total phosphorus (TP) in the agricultural streams and 43% in the forested streams. In all watersheds, TDP was almost exclusively dissolved reactive phosphorus and most of the annual P export was in summer . The type of agricultural activity in the watershed influenced IN speciati on; in the mixed agricultural watershed, 94% of IN export was ammonium, whe reas 98% of IN load was nitrate, from the cropland watershed. Disproportion ately high TDP to TP export from agricultural watersheds suggests that, in areas of low relief and relatively high soil water P content, land clearing may influence dissolved more than particulate phosphorus export.