Behaviour of S-metolachlor and its oxanilic and ethanesulfonic acids metabolites under fresh vs. partially decomposed cover crop mulches: A laboratory study.

At the time of spring pre-emergent herbicide application, the soil surface in conservation agriculture is most of the time covered by cover crops (CC) mulches. The state of these mulches depends on their destruction date and on the selected species. Sorption and degradation of 14C-S-metolachlor on and within 8 decaying CC-covered (2 species × 4 initial decomposition state) soils corresponding to conservation agriculture were compared to its fate in bare soil (BS) corresponding to conventional agriculture. 14C-S-metolachlor and its metabolites distribution between mineralized, extractable and non-extractable (NER) fractions was determined at 5 dates during a 20 °C/84-d period. Herbicide mineralization was weak (<2%) for both CC and BS. Extractability of 14C in BS was intermediate between CC that were decomposed 28 or 56 days and 0 or 6 days before application. Degradates consisted in up to 43% of total radioactivity, with specificities according to the CC or soil compartment. NER formation was equivalent in BS and in the much decomposed CC-amended microcosms, and was stronger in less decomposed CC. S-metolachlor DT50 was 23-d in BS, and 9, 15, 39 and 25-d for CC ordered by increased decomposition state at the time of application. These results were attributed to the proportion of 14C intercepted by CC, and to higher levels of organic matter and microbial activity in less decomposed CC as compared with more decomposed ones. Then the state of decomposition level of CC residues determines the behaviour of SMOC (S-metolachlor) sprayed on the mulch in the conditions of conservation agriculture.

Fate of glyphosate and degradates in cover crop residues and underlying soil: A laboratory study.

The increasing use of cover crops (CC) may lead to an increase in glyphosate application for their destruction. Sorption and degradation of (14)C-glyphosate on and within 4 decaying CC-amended soils were compared to its fate in a bare soil. (14)C-Glyphosate and its metabolites distribution between mineralized, water-soluble, NH4OH-soluble and non-extractable fractions was determined at 5 dates during a 20 °C/84-d period. The presence of CC extends (14)C-glyphosate degradation half-life from 7 to 28 days depending on the CC. (14)C-Glyphosate dissipation occurred mainly through mineralization in soils and through mineralization and bound residue formation in decaying CC. Differences in sorption and degradation levels were attributed to differences in composition and availability to microorganisms. CC- and soil-specific dissipation patterns were established with the help of explicit relationships between extractability and microbial activity.

Nature and decomposition degree of cover crops influence pesticide sorption: quantification and modelling.

This study quantifies and models the influence of the type and the degree of decomposition of cover crops (CC) on three pesticides sorption: epoxiconazole (EPX), S-metolachlor (SMOC) and glyphosate (GLY). Residues of four cover crop species were incubated for 0, 6, 28 or 56 d in controlled conditions. For each incubation time, adsorption of pesticides on CC residues was measured in batch experiments. Additionally, the biochemical and elemental composition (Van Soest fractionation, C:N, (13)C NMR spectroscopy) of CC was characterized. Mineralization of CC residues was monitored at all incubation times using CO2 trapping. Results showed that the adsorption of pesticides differed significantly according to (i) the type of molecule, (ii) the type of CC, (iii) the degree of CC decomposition and the interaction CC×decomposition time. EPX and GLY were the most (Kd ranging from 188 to 267 L kg(-1)) and the least (Kd ranging from 18 to 28 L kg(-1)) sorbed pesticides respectively. With increasing decomposition of the CC residue, sorption increased by 1.6- to 4.7-fold according to the type of pesticide and cover crop. It was significantly correlated with the net cumulative mineralization (ρ>0.7) and other indicators of biochemical composition such as C:N ratio (ρ<-0.7), the Van Soest neutral detergent soluble fraction (ρ>0.5) and the alkyl/O-alkyl C ratio determined by NMR. An innovative model based on net cumulative mineralization of CC residues is proposed to describe the pesticide sorption and appears to be a promising approach to account for the effects of decaying plant residues on the environmental fate of pesticides.