Agricultural activities incorporate in soils a range of pesticides, which interact with soil organic matter (SOM). A better knowledge of mechanisms affecting their biodegradation and stabilization is essential for predicting their environmental impacts. Questions still remain about the long-term consequences of the accumulation of bound residues. In this work, we used 13C labelling techniques coupled with fatty acid methyl ester (FAME) analysis to study the role of microorganisms in the fate of 2,4-D in soils.
Soil samples were incubated during 6 months with 13C-ring labelled 2,4-D in sealed flasks. The distribution of labelling was evaluated by measuring isotopic composition of the CO2 evolved, the microbial biomass extracted by fumigation-extraction, the water, methanol and dichloromethane soluble fractions, and the residual bulk soil. The incorporation of 13C-2,4-D into microbial fatty acids was followed by GC/IRMS.
The fate of the pesticide was similar to that found in previous studies using 14C-2,4-D. After 8 days a maximum of 7% of the initial amount of pesticide is incorporated in the biomass. After one month about 55% was mineralised and 45% remained associated with SOM as bound-residues. FAME analysis revealed specific isotopic enrichment of some fatty acids which confirmed that only some microorganisms are involved in the degradation process. After 6 months, CO2, biomass and a number of FAME were still enriched in 13C. These results suggest that a part of bound-residues is still bioavailable. 13C -labelled FAME profiles showed that this degradation is performed by a different population to that degrading initial soluble 2,4-D. This approach provides considerable scope for studying specific biodegradation processes.