Aeration and mineral composition of soil determine microbial CUE

Abstract. Microbial carbon use efficiency (CUE) in soils is used to estimate the balance of CO₂ respired by heterotrophs versus the accumulation of organic carbon (C). While most CUE studies assume that aerobic respiration is the predominant degradation process of organic C, anoxic microniches are common inside soil aggregates. Microorganisms in these microniches carry out fermentation and anaerobic respiration using alternative electron acceptors, e.g. NO^3-, Fe, SO₄^2-. Extracellular metabolites are also not traditionally accounted for but may represent a significant C flux. Moreover, climate change may modulate soil microbial activity by altering soil aeration status on a local level due to warming and elevated frequency of extreme precipitation events. Therefore, CUE should be measured under more realistic assumptions regarding soil aeration. This study focused on the effect of oxygen and Fe on C mineralisation in forest soils and quantified C distribution between biomass and different extracellular metabolites. Forest soils were collected from two Bohemian Forest (Czechia) sites with low and high Fe content and incubated under oxic and anoxic conditions. A solution of 13C-labelled glucose was used to track stable isotope incorporation into the biomass, respired CO₂, and extracellular metabolites. We estimated CUE based on microbial respiration, glucose consumption, biomass growth, and extracellular metabolites. RNA-SIP was used to identify the active bacteria under each treatment. As expected, the oxic incubation showed a rapid utilisation and immediate production of biomass and CO₂. Under anoxic conditions, 90 % of the added glucose was still present after 72 h, and anoxic soils showed significantly lower microbial activity. The low-Fe soil samples were more active under oxic conditions, while the high-Fe samples were more active under anoxia.

Our findings confirm that anoxia in soils enhances short-term C preservation. Accordingly, excluding exudates in mass flux calculations would underestimate apparent CUE values.