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With limited phosphorus (P) supplies, increasing P demand, and issues with P runoff and pollution, developing an ability to reuse the large amounts of residual P stored in agricultural soils is increasingly important. In this study, we investigated the potential for residual soil P to maintain crop yields while reducing P applications and losses in Canada. Using a P cycling model coupled with a soil P dynamics model, we analyzed soil P dynamics over 110 years across Canada's provinces. We found that using soil residual P may reduce mineral P demand as large as 132 Gg P year−1 (29%) in Canada, with the highest potential for reducing P applications in the Atlantic provinces, Quebec, Ontario, and British Columbia. Using residual soil P would result in a 21% increase in Canada's cropland P use efficiency. We expected that the Atlantic provinces and Quebec would have the greatest runoff P loss reduction with use of residual soil P, with the average P loss rate decreasing from 4.24 and 1.69 kg ha−1 to 3.45 and 1.38 kg ha−1, respectively. Ontario, Manitoba, and British Columbia would experience relatively lower reductions in P loss through use of residual soil P, with the average runoff P loss rate decreasing from 0.44, 0.36, and 4.33 kg ha−1 to 0.19, 0.26, and 4.14 kg ha−1, respectively. Our study highlights the importance of considering residual soil P as a valuable resource and its potential for reducing P pollution. [ABSTRACT FROM AUTHOR]

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. Key Points: Phosphorus (P) flows within cropland played a larger role than those in pasture in Canada's P balances from 1961 to 2018In 2018, most Canadian agricultural areas had soil P surplus except Saskatchewan, where large P deficits were observedP use efficiency tended to be greatest in the Prairie Provinces, and the rate of increase was considerably steeper in Ontario and Quebec [ABSTRACT FROM AUTHOR]