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 km2), 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.
Bibliographical noteFunding Information:
We are grateful to Kathryn Andrews for assisting with census data collection for this project. We thank Joann Whalen, Garry Peterson, Bronwyn Keatley, Navin Ramankutty, Jim Fyles, Tom Bruulsema, Kathleen Arrington, Douglas Beegle, Maurice Korol, and Georges Thériault for comments or feedback on various aspects of this research and two anonymous reviewers for suggestions on improvements to an earlier draft of this manuscript. Byron Modlofsky provided historical GIS data from the GEORIA project that aided this analysis. This research was funded by the National Science and Engineering Research Council of Canada and through Le Fonds québécois de la recherche sur la nature et les technologies.
- Agricultural pollution
- Historical ecology
- Historical legacy
- Land use
- Nutrient budgets
- Soil P