Dissipation and effect of glyphosate during composting of organic wastes

The addition of organic matter (OM) containing glyphosate during compost production, through the introduction of contaminated plant residues or sewage sludge, presents a risk of hindering the proper OM breakdown carried out by microorganisms and causing the accumulation of glyphosate or aminomethylphosphonic acid (AMPA). To measure the effect of glyphosate and glyphosate-based herbicide (GBH) on OM decomposition as well as the dissipation of glyphosate to AMPA during composting, a controlled-environment experiment was conducted using mesocosm-scale vessels. Analytical-grade (AG) glyphosate (150 mg kg^-1 ) and GBH (VisionMAX) equivalent to the amounts applied in agricultural areas (300 mg kg^-1 ) were added to a mixture of green residues, which were then composted for 112 d. Sampling after 2, 7, 28, and 112 d showed a negligible effect of glyphosate and GBH on physicochemical properties of the mixture (temperature, OM%, pH, total carbon [C], total nitrogen [N], and C/N ratio), ammonification, nitrification, and phosphate content. No differences between AG glyphosate and GBH treatments were measured. Glyphosate levels decreased significantly after 2 d to reach 53.1 and 71.1% of the initial content for the AG glyphosate and GBH treatments, respectively, and glyphosate dissipation was almost complete after 112 d of composting. Aminomethylphosphonic acid could not be detected at any time during the experiment regardless of the treatment. Our results show that conditions for OM decomposition were maintained despite the addition of glyphosate and suggest that only trace amounts of glyphosate or AMPA are likely to be present in mature compost.