Abiotic and biotic influences on acetochlor fate in pristine soils and subsoils

Soil exists as an intricate matrix in which a wide variety of biotic (e.g. enzymes, macro-, micro- fauna) and abiotic (e.g. clay minerals, oxides, humic substances, organo-mineral composites) factors interact, forming a highly dynamic and heterogeneous environment. Upon release into this complex environment, pesticides are subject to a number of processes that result in sorption to soil surfaces, biodegradation/transformation, or leaching. Pesticides leaching through a soil profile to groundwater will be exposed to changing environmental conditions as different horizons with distinct physical and chemical properties are encountered. The way these divergent soil properties influence pesticide degradation and retention needs to be assessed to allow accurate predictions of environmental fate and more efficient management practices.

To address this issue, soil cores were taken from two soil profiles (surface textures: silty clay loam and loamy sand), and samples taken from 0-30 cm (surface), 1.0-1.3 m (mid) and 2.7-3.0 m (deep; clay) and 3.9-4.2 m (deep; sand). A variety of soil biotic (microbial numbers, microbial biomass and enzyme activities) and abiotic (pH, organic matter content, texture, CEC) properties were measured for each soil. Microbial numbers and enzyme activities were found to decrease significantly with soil depth and were positively correlated to the organic matter content. An exception was urease activity in the clay soil, under buffered conditions, where a 2.9-fold greater activity was exhibited in the mid soil compared to the surface soil. Although microbial numbers did decrease with soil depth substantial numbers of bacteria were still isolated from the deep soils (direct counts: 5.6 x 108 sand, 4.5 x 108 clay) despite only representing 4.7 and 1.7 % of those in the respective surface soils.

Equilibrium sorption and desorption isotherms of 14C-ring-labelled acetochlor revealed that the sorptive behaviour of this pesticide varied with soil depth. The difference in retention capacity with soil depth was strongly correlated to soil organic carbon content. Differential desorption characteristics were also apparent between different particle size fractions, highlighting the influence of microsite variation on pesticide fate in soil, and this was also related to the soil organic carbon content of the fractions.

Degradation and sorption processes were coupled in a long-term (100 d) fate study of acetochlor, under laboratory conditions. Acetochlor was shown to dissipate under biotic and sterile conditions, with the formation of a number of environmentally stable metabolites including ethanesulphonic acid and oxanilic acid derivatives. Mineralization was not a major fate process with less than 5 % of the initially applied acetochlor recovered as 14C02. Nonextractable residue formation occurred instantly and rapidly progressed over the initial 21 d of incubation. Nonextractable residues were unevenly distributed between soil size factions, concentrated in the macroaggregate fractions. Nonextractable residue formation was enhanced under biotic conditions for those fractions. Under biotic conditions, DT50 values of 9.32, 12.32 and 12.56 d were determined for acetochlor in clay surface, mid and deep soil, respectively. Further experiments are needed to generate more data, to enable accurate modelling of pesticide fate.