Thursday, August 9, 2007 - 9:20 AMCOS 109-5: Phosphorus budgets for the Saint Lawrence River sub-basin (1871-2001): Long-term threats to water quality
Graham K. MacDonald, Kathryn Andrews, and Elena M. Bennett. McGill University
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.