Testing the dependence of microbial growth and carbon use efficiency on nitrogen availability, pH, and organic matter quality

Microbial carbon use efficiency (CUE), or the partitioning of assimilated C into growth or respiration, is a key parameter that is central to understanding the soil C cycle and its feedback to environmental and climate change. The availability of nitrogen (N), organic matter (OM) quality and environmental factors influence CUE indirectly by affecting growth rates and respiration of the major microbial decomposers in soil, including fungi and bacteria. In the present study we set out to evaluate the effect of N-additions (mineral N fertiliser), increased pH (lime), and increased OM quality (plant litter addition) on microbial growth, respiration, and resulting CUE. We sampled beech and spruce forest stands each including two levels of soil fertility. In laboratory microcosm experiments we then manipulated the availability of mineral N, pH and OM quality during the course of 60 days and measured rates of bacterial and fungal growth, respiration, and resulting CUE. We observed that growth rates of both bacteria and fungi were stimulated by increased OM quality through litter additions, but when combined with increased pH, the ratio shifted in favour of bacteria, while a shift towards fungal dominance was observed when litter was combined with N additions. Overall bacterial growth was stimulated by increased pH and reduced by addition of mineral N, while fungal growth appeared unaffected by both factors. The ratio of fungal to bacterial growth varied between 0.02 and 0.7, suggesting that 0.4 to 50 times more detrital-C was used by bacteria than by fungi in the dataset. Consistently negative correlations between fungal and bacterial growth suggested competitive interactions during the microbial use of detrital C, with bacteria being the dominant competitor. Estimated levels of microbial CUE ranged from < 0.05 to 0.5, and higher levels of CUE were associated with higher dominance of bacteria in soils with higher pH and lower N availability. Taken together, differences in CUE were linked to the dominance of fungi or bacteria. When bacterial growth was inhibited by mineral N or low pH, a competitive release resulted in a stimulated fungal growth and detrital C-use, which yielded reduced CUEs.