Regulation of bacterial and fungal MCPA degradation at the soil–litter interface

Much is known about mechanisms and regulation of phenoxy acid herbicide degradation at the organism level, whereas the effects of environmental factors on the performance of the phenoxy acid degrading communities in soils are much less clear. In a microcosm experiment we investigated the small-scale effect of litter addition on the functioning of the MCPA degrading communities. 14C labelled MCPA was applied and the functional genes tfdA and tfdAα were quantified to characterise bacterial MCPA degradation. We identify the transport of litter compounds as an important process that probably regulates the activity of the MCPA degrading community at the soil–litter interface. Two possible mechanisms can explain the increased tfdA abundance and MCPA degradation below the litter layer: 1) transport of α-ketoglutarate or its metabolic precursors reduces the costs for regenerating this co-substrate and thereby improves growth conditions for the MCPA degrading community; 2) external supply of energy and nutrients changes the internal resource allocation towards enzyme production and/or improves the activity of bacterial consortia involved in MCPA degradation. In addition, the presence of litter compounds might have induced fungal production of litter-decaying enzymes that are able to degrade MCPA as well.