Formation and Fate of Bound Residues from Microbial Biomass during 2,4-D Degradation in Soil

During organic contaminant degradation in soil, bound or nonextractable residues (NER) are formed. Part of these residues may be biogenic, because degrading microorganisms assimilate carbon derived from the pollutant and mineralized CO₂ to form cellular components for example, [fatty acids (FA) and amino acids (AA)], which are subsequently stabilized within soil organic matter (SOM). We investigated the formation and fate of FA and AA from biodegradation of ¹³C₆-2,4-D in soil and the incorporation of the ¹³C-label into living biomass via ¹³CO₂ fixation. After 64 days of incubation, ¹³C-AA in SOM indicated that 44% of the initially applied ¹³C₆-2,4-D equivalents had been converted to microbial biomass and finally to biogenic residues. The intermediate maximum of ¹³C-FA in SOM indicated a 20% conversion of ¹³C₆-2,4-D to biomass, but ¹³C-FA decreased to 50% of that value whereas ¹³C-AA in the SOM remained stable. We provide the first evidence that nearly all bound residues from 2,4-D are biogenic, containing natural microbial residues stabilized in SOM. Because of biogenic residue formation, the potential risk of bound residues from readily metabolized xenobiotics in soils is highly overestimated. Hence, the formation of biogenic residues must be considered in general when performing mass balances of pollutant biodegradation in soils.