Changes in Canada's Phosphorus Cycle 1961-2018: Surpluses and Deficits

Human activities have greatly changed global phosphorus (P) cycling, posing urgent challenges related to both supply uncertainty and aquatic eutrophication. However, the long-term dynamics of P across Canada remain unquantified and under-explored. Using a material flow analysis model, we quantified temporal dynamics of P cycling in Canadian provinces from 1961 to 2018 and characterized the changes in soil P balances through the study period. We found most Canadian agricultural regions had soil P surpluses except Saskatchewan, where large P deficits (-10-0 kg ha(-1) y(-1)) were detected in almost all study years (except 1961). In 2018, Quebec and Atlantic provinces had the highest P surpluses (34 and 159 kg ha(-1) y(-1), respectively), and low P surpluses were observed in Ontario, Manitoba, Alberta and British Columbia (9, 16, 5, and 28 kg ha(-1) y(-1), respectively). P surplus was reduced in Quebec and Ontario after the nutrient management regulations were put in place in the 1980s. We demonstrated that P flows in cropland played a larger role in Canada P cycling than pasture. P use efficiency tended to be greatest in the Prairie provinces (0.74 in 2018), and least in the Atlantic provinces (0.12 in 2018). However, the rate of increase was considerably steeper in Ontario and Quebec than other provinces. Reducing inorganic fertilizer and manure application would be the most effective method to reduce remaining P surpluses.

Automated summary

Human activities have had a significant impact on global phosphorus cycling, and the long-term dynamics of phosphorus in Canada have not been well understood. This study quantified the temporal dynamics of phosphorus cycling in Canadian provinces and found that most agricultural regions had phosphorus surpluses, except for Saskatchewan which had deficits. Quebec and the Atlantic provinces had the highest phosphorus surpluses, while Ontario, Manitoba, Alberta, and British Columbia had lower surpluses. Cropland played a more important role in phosphorus cycling than pasture, and the Prairie provinces had the highest phosphorus use efficiency.