Water quality: 13. Phosphorus

Abstract

Phosphorus (P) is an important nutrient for plant and animal growth. However, additions of P to the land as livestock manure and inorganic fertilizer may lead to an increased risk of soil P saturation and resulting movement of P to water bodies. Excessive amounts of P in surface water contributes to eutrophication of rivers and lakes and to Cyanobacteria blooms. These result in decreased water quality and limitations on water use. The Risk of Water Contamination by Phosphorus (IROWC-P) Indicator was developed to assess the trends over time for the risk of surface water contamination by P from Canadian agricultural land at the watershed scale. Overall risk of water contamination by P is increasing in Canada. Increases in livestock production and the use of mineral fertilizers repeatedly created regional P surpluses between 1981 and 2006. The wide range of soil types across Canada have different characteristics for retaining nutrients such as P and therefore some soils are better able than others to sustain intensive agriculture. Surface runoff, deep drainage and soil erosion by water on agricultural land contribute significantly to the risk of P contamination of surface water in eastern Canada. In western Canada, surface runoff seems to be the major factor contributing to P transport. Local implementation of nutrient management plans, regulations, conservation practices and beneficial management practices (BMPs) have considerably decreased the P surplus in some areas. However, cumulative P surpluses over time continue to enrich soil P levels. Increased efforts at controlling both P sources and transport are required to reduce the risk of P loss to water and prevent surface water eutrophication and algal blooms.