Microbial carbon use efficiency and biomass turnover times depending on soil depth – Implications for carbon cycling

Processing of organic carbon (C) by soil microorganisms is a key process of terrestrial C cycling. For this reason we studied (i) microbial carbon use efficiency (CUE) defined as C allocated to growth over organic C taken up by the microbial community , and (ii) the turnover time of microbial biomass in a pasture and in two forest soils. We hypothesized that microbial CUE decreases in mineral soils with depth from the topsoil to the subsoil , while the turnover time of the microbial biomass increases due to energetic constrains. We determined microbial CUE and turnover of microbial biomass C using a novel substrate-independent method based on incorporation of 18 O from labeled water into microbial DNA with concurrent measurements of basal respiration. Microorganisms showed decreasing C uptake rates with decreasing C contents in the deeper soil layers. In the forest soils, no adaptation of microbial CUE with soil depth took place, i.e., microbes in the forest topsoil used C at the same efficiency as microbes in the subsoil . However, in the pasture soil, microbial CUE decreased in the lower soil layers compared to the topsoil, indicating that microorganisms in the deeper soil layers allocated relatively more C to respiration. In the organic soil layer, microorganisms respired more per unit microbial biomass C than in the subsoil, but had a similar CUE despite the high C-to-nitrogen and C-to-phosphorus ratios of the litter layers. The turnover time of microbial biomass increased with soil depth in the two forest soils. Thus, in the forest soils, a lower microbial C uptake rate in the deeper soil layers was partially compensated by a longer turnover time of microbial biomass C. In conclusion, our findings emphasize that in addition to microbial CUE, the turnover time of the microbial biomass strongly affects soil C cycling. • Microbial carbon use efficiency was determined by a substrate-independent method. • Microbial carbon use efficiency hardly changed with soil depth in forest soils. • Turnover time of the microbial biomass increased with soil depth. • Microbial carbon uptake rates decreased with soil depth. • Both microbial carbon use efficiency and biomass turnover affect carbon cycling.