Succession of bacterial and fungal 4-chloro-2-methylphenoxyacetic acid degraders at the soil–litter interface

Phenoxyacetic acids can be degraded by diverse soil microorganisms. Nevertheless, we miss information about the succession of 4-chloro-2-methylphenoxyacetic acid (MCPA) degraders in micro-environments of soils as well as specific functions of different microbial groups during MCPA degradation. We studied MCPA degradation at the soil–litter interface in a microcosm experiment and followed the succession of different degrader populations by quantifying the abundance of 16S rRNA genes as well as, the fungal ITS fragment and the functional genes tfdA (in total and divided into three classes) and cadA. Adjacent to the litter layer, a dynamic depletion zone of MCPA indicated that the litter effect on MCPA degradation depends on substrate availability and the affected soil volume. The increase of the tfdA class III and cadA genes was linked to MCPA mineralisation. Total abundance of tfdA genes was dominated by class I MCPA degraders and did not reflect MCPA degradation potential of the soil. Litter addition induced the development of pioneer and late-stage fungal communities, which were probably both involved in MCPA degradation. The results underline the importance of the ecological behaviour of different degrader populations for the understanding of herbicide degradation in soils.