The overlooked impact of rising glyphosate use on phosphorus loading in agricultural watersheds

Glyphosate is the most extensively used pesticide worldwide. In addition to raising ecotoxicological concerns, the use of glyphosate adds phosphorus (P) to agricultural landscapes, influencing the accumulation and cycling of P in soil and nearby surface waters. Yet pesticides have been largely ignored when monitoring anthropogenic sources of P in agricultural watersheds. Estimating the supply of P derived from glyphosate use, both globally and in the US alone, we show that trends have markedly increased over the past two decades. Across the US, mean inputs of glyphosate-derived P increased from 1.6 kg P km−2 in 1993 to 9.4 kg P km−2 in 2014, with values frequently exceeding 20 kg P km−2 in areas planted with glyphosate-resistant crops. Although still a minor source of P relative to fertilizers, P inputs from glyphosate use have now reached levels comparable to those from sources for which P regulations were initiated in the past. We thus argue for greater recognition of glyphosate’s influence on P flow in watershed research and management.