Does reducing agricultural phosphorus surplus consistently improve river water quality?

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Phosphorus (P) fertilizer has contributed to the eutrophication of freshwater ecosystems. Watershed-based conservation programs intended to reduce external P loading to surface waters have not produced significant water-quality improvements. One possible explanation is that accumulated legacy, or historical, P is remobilized along the terrestrial–aquatic continuum, potentially obscuring the benefits of current conservation efforts.

The researchers examined how contemporary river P trends between 1992 and 2012 responded to estimated changes in contemporary agricultural P balances across 143 watersheds in the conterminous United States. They defined the agricultural balance as fertilizer and manure inputs minus crop uptake and harvest removal, and developed a proxy estimate of the contribution of legacy P, meaning human-caused P inputs before 1992.

They concluded that legacy sources contributed to river export in 49 watersheds because mean contemporary river P export exceeded mean contemporary agricultural P balances. In the other 94 watersheds, agricultural P balances exceeded river P export, and the proxy estimate of legacy P was inconclusive. If legacy contributions occurred in those watersheds, the researchers considered them likely to be small and dwarfed by contemporary P sources.

The continental-scale P mass-balance results indicated a need for better incentives and strategies to encourage nutrient-conserving practices and reduce widespread contemporary P surpluses. But reducing a P surplus is only one component of an effective nutrient plan: agricultural balances decreased in 91 watersheds without consistent water-quality improvements, and increased in 52 watersheds without consistent water-quality degradation.