Phosphorus budgets for the Saint Lawrence River sub-basin (1871-2001): Long-term threats to water quality

Land management that causes phosphorus (P) to accumulate in soils can pose risks to water quality through non-point source pollution. Rectification of heightened soil P concentrations can take hundreds of years, threatening water quality in the long-term. We calculated cropland P balances at ten-year intervals from 1871-2001 for the 76 watersheds of the Saint Lawrence River sub-basin (600,000 km²) to estimate the spatial and temporal pattern of P accumulation in soils. Most of the sub-basin falls within Eastern Canada, but includes areas of New York State and Vermont. Major inputs in the balance are P in inorganic fertilizer and manure, as well as atmospheric deposition.  Outputs include P in harvested crops. We mapped the spatial pattern of P surpluses through time at the Canadian Census Division and US County-level, aggregating these maps to the extent of each watershed. The period from 1971 to 2001 shows the largest surpluses, peaking in the 1980s, with an average P surplus of 13.5 kg ha^-1 in 1971 and 15.2 kg ha^-1 in 2001. The largest overall P surpluses in this period occur in the watersheds of southern Quebec, corresponding with areas of intensive livestock production. Preliminary results indicate relatively low intensity of P use in many of the watersheds pre-1950, but display a general trend in agricultural intensification and increasing P use starting around 1920. This study provides a perspective unique to most budget studies by showing spatially-explicit changes in P status within the landscape over more than a century.