Responses of biogeochemical properties of different soil aggregates to glyphosate application and degradation

Relative abundances of adsorption sites increase with the specific surface area of soil particles and aggregates. In turn, larger sorbed-phase distributions may attenuate pesticide biodegradation rates and ecotoxicological effects. Thus, soil aggregate size may be an important determinant of the fate and ecotoxicological effects of glyphosate and its metabolite aminomethylphosphonic acid (AMPA). We investigate the dynamics of glyphosate degradation, AMPA formation, AMPA degradation, and soil biogeochemical properties over 30 days, in soils composed of aggregates of various sizes: bulk soil (<10 mm), and five distinct size classes (2-10, 1-2, 0.25-1, 0.05-0.25, <0.05 mm). Results show that glyphosate and AMPA degradation rates decreased with decreasing aggregate size, but was statistically significantly different from other sizes only for glyphosate in < 0.05 mm aggregates. Soil biogeochemical properties (organic carbon, total nitrogen, total phosphorous, pH, available phosphorous (AP)), enzyme activities (β-glucosidase, N-acetylamino-β-glucosidase, alkaline phosphatase), and their dynamics significantly differed across different aggregate sizes. The biogeochemical effects of glyphosate, and their sensitivities to aggregate size, were relatively large for enzyme activities and AP. This reveals significant effects on soil microbial activity, particularly activity associated with phosphorus cycling. Crucially, the soil biogeochemical response appears faster, larger and longer lasting in small aggregates. Overall, the biodegradation rates of glyphosate and AMPA are substantially less sensitive to aggregate size than their biogeochemical effects are. Therefore, glyphosate and AMPA persistence, which are straightforward to observe and quantify under field conditions, may not be reliable indicators of the durations or effect sizes of various underlying aspects of their environmental ecotoxicities.