Effects of soil phosphorus status on environmental risk assessment of glyphosate and glufosinate-ammonium

The increased use of herbicides poses a risk to the aquatic environment. Easy and economical methods are needed to identify the fields where specific environment protection measures are needed. Phosphorus (P) and organophosphorus herbicides compete for the same adsorption sites in soil. In this study the relationship between P obtained in routine Finnish agronomic tests (acid ammonium acetate [P-AC]) and adsorption of glyphosate and glufosinate-ammonium was investigated to determine whether P-AC values could be used in the risk assessment. The adsorption of glyphosate ((N-(phosphonomethyl)glycine) and glufosinate-ammonium (2-amino-4-(hydroxymethyphosphinyl) butanoic acid) was studied in a clay and a sandy loam soil enriched with increasing amounts of P added as potassium dihydrogen phosphate. Desorption was also determined for some P-enriched soil samples. The adsorption of both herbicides diminished with increasing P-AC value. The correlations between Freundlich adsorption coefficients obtained in the adsorption tests and P-AC were nonlinear but significant (r > 0.98) in both soils. The exponential models of the relationship between soil P-AC values and glyphosate adsorption were found to fit well to an independent Finnish soil data set (P < 0.1 for glyphosate and P < 0.01 for glufosinate-ammonium). The desorption results showed that glufosinate-ammonium sorption is not inversely related to soil P status, and the high correlation coefficients obtained in the test of the model were thus artifacts caused by an abnormal concentration of exchangeable potassium in soil. The solved equations are a useful tool in assessing the leaching risks of glyphosate, but their use for glufosinate-ammonium is questionable.