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.