Microbial community composition affects soil organic carbon turnover in mineral soils

Soil organic carbon (SOC) turnover is the most ubiquitous and ecologically fundamental process in soils. It is generally assumed that SOC is utilised by functionally redundant soil-specific microbial communities which do not differ in their capability to mineralise soil organic matter. To challenge this assumption, incubation experiments were conducted to analyse the community-specific effects on SOC turnover for six mineral soils under different land-use. Comparisons of respiration by a native soil community and an alien community both inoculated to sterilised soils revealed 29 ~c 18% higher respiration by the native community (‘homefield advantage’). Increased soil microbial community diversity, as generated by mixing several microbial inoculants, did not result in increased mineralisation rates. Even under impaired conditions, in the presence of aged engine oil as a less decomposable substance, communities with higher diversity did not show higher respiration rates. Also, in non-sterilised soils, we detected the influence of the microbial community composition on respiration rates: Investigations on the effect of mixing two communities in a 50:50 untreated soil mixture showed declining respiration in three out of six cases (by 23.9 ~c 5.9%) and increased respiration in one case (by 57%) compared to the mean respiration of the two unmixed soils. These effects were highly related to the microbial community capability, with only communities with low capability profiting from mixing with a second community. Our results question the assumption of redundancy of microbial community’s functionality for SOC mineralisation in soils.