Agronomic and environmental phosphorus decline in coastal plain soils after cessation of manure application

Manure applications exceeding crop phosphorus (P) requirements have historically occurred in intensive animal agriculture areas because of a lack of options to sustainably utilize manure. We investigated the legacy impacts of manure application on agronomic (Mehlich 3-P, M3-P) and environmental (water-extractable P, WEP; M3-P saturation ratio, M3-PSR) soil P pools after manure applications were ceased. Three field sites located in the coastal plain soils of northeastern United States received manure applications for four years, equivalent to 0, 100, 200, 300, and 400 kg total P ha⁻¹ yr^-1. Then, no manure or fertilizer P was applied for 15 years. The soil WEP (1:10 soil to water ratio) in manure treatments declined significantly (0.7–2.5 mg kg^-1 yr^-1) for the first 9 years, but did not significantly decline (0.2–0.4 mg kg^-1 yr^-1) between 9 and 15 years after cessation of manure application. Over the 15 years, the M3-P across manure treatments declined steadily at 7.7–15.3 mg kg^-1 yr^-1. Despite the decline, P pools in soils remained above agronomic (92 mg kg^-1 M3-P) and environmental (8.6 mg kg^-1 WEP, M3-PSR >0.15) thresholds in the highest manure treatments. Percent decline in soil P pools (60–71 % for WEP; 41–53 % for M3-P) was similar across all manure treatments despite a larger absolute WEP and M3-P decline for higher manure treatments. We conclude that in fields with a legacy of manure application, M3-P may decline 5.4–6.3% per year after cessation of P applications, while sufficient P will persist for decades as indicated by the abundance of agronomic and environmental P pools.