Phosphorus Accumulation in Saint Lawrence River Watershed Soils: A Century-Long Perspective

Understanding historical patterns of soil phosphorus (P) accumulation is critical to management of water quality across agricultural landscapes. To address the effects of long-term agricultural P management on soil P accumulation in the Saint Lawrence River sub-basin (574,000 km(2)), we calculated cropland P budgets at decadal intervals from 1901 to 2001 for the sub-basin and its tributary watersheds. Agricultural census data were used to estimate P inputs in the form of fertilizer and manure, and outputs (P removed in harvested crops). The resulting balances indicate the potential magnitude of P accumulation in cropland soils. Cropland P surpluses occurred in the sub-basin in each decade of the past century, with the rate of accumulation increasing after 1951 due to more widespread use of P fertilizers and manure. The largest annual P surplus occurred in 1981 (42,000 Mg y(-1)), followed by a decline in the rate of accumulation to almost half that level by 2001 (24,850 Mg y(-1)) as a result of improved management of agricultural P. Comparison of the cumulative P surpluses estimated for the entire 20th century with measured soil P data indicates a strong linear relationship between these watershed P budgets and the average soil P content across the sub-basin (R (2) = 0.712, P < 0.0001). These results support the view that historical land management can have important ecological legacies.