Competition within low-density bacterial populations as an unexpected factor regulating carbon decomposition in bulk soil

Abstract Bacterial decomposition of organic matter in soils is generally believed to be mainly controlled by the access bacteria have to their substrate. The influence of bacterial traits on this control has, however, received little attention. Here, we develop a bioreactive transport model to screen the interactive impacts of dispersion and bacterial traits on mineralization. We compare the model results with two sets of previously performed cm-scale soil-core experiments in which the mineralization of the pesticide 2,4-D was measured under well-controlled initial distributions and transport conditions. Bacterial dispersion away from the initial substrate location induced a significant increase in 2,4-D mineralization, revealing the existence of a regulation of mineralization by the bacterial decomposer density, in addition to the dilution of substrate. This regulation of degradation by density becomes dominant for bacteria with an efficient uptake of substrate at low substrate concentrations (a common feature of oligotrophs). The model output suggests that the distance between bacteria adapted to oligotrophic environments is a stronger regulator of degradation than the distance between these bacteria and the substrate initial location. Such oligotrophs, commonly found in soils, compete with each other for substrate even at remarkably low population densities. The ratio-dependent Contois growth model, which includes a density regulation in the expression of the uptake efficiency, provide a more versatile representation than the substrate-dependent Monod model in these conditions. In view of their strong interactions, bioreactive and transport processes cannot be handled independently but should be integrated, in particular when reactive processes of interest are carried out by oligotrophs. Highlights -The impact of spatial distributions on decomposition depends on bacterial traits -Decomposition can be reduced by competition between bacteria even at low densities -Bacterial density regulation counterbalances substrate accessibility regulation -Regulation of decomposition by bacterial density is more acute for oligotrophs