General drivers of microbial carbon use efficiency in soils

The efficiency by which soil microbes direct metabolised carbon to their growth, i.e. the microbial carbon use efficiency (CUE), is hypothesised to be driven by soil pH, nutrient stoichiometry and the microbial community composition (e.g. Fungi-to-Bacteria ratio). Despite extensive research it remains difficult to identify general trends in how these drivers affect CUE and results of individual studies often point in different directions. To unravel general trends, we aggregated a unique data set of samples analysed using the ¹⁸O-labelling technique - all derived from the same laboratory - to gain deeper insights into the relationship between CUE and pH, CN ratio and the relative abundance of domains (based on 16S and ITS gene copy numbers by qPCR). To date, the growing data set comprises 685 observations of ¹⁸O-CUE, including samples from 41 individual sites under three different land use types (forest, managed grassland, cropland) from tropical to subarctic climate. A Random Forest model and a linear mixed-effects model approach were used to analyse the data. Preliminary results on a filtered and aggregated subset (n= 221; aggregated to reduce the heterogeneity of the data set structure) suggest that the CUE is strongly dependent on soil pH, following a U-shaped curve. The relationship between CUE and pH was found negative for pH < 5.5 and positive for pH ≥ 6.5, while overall the CUE was found to be negatively correlated to soil C:N ratio. Additional data on climate (MAT, MAP), soil texture, and soil microbial community will complement the analysis.