Carbon use efficiency (CUE) is the proportion of carbon resources that a microorganism commits towards cellular growth (i.e., a change in overall microbial biomass) relative to non-growth maintenance demands. As such, CUE is the rate-controlling step directing carbon between relatively persistent soil organic matter pools and mineralized carbon dioxide. Recent evidence of reduced CUE resulting in the loss of soil organic matter and accelerated CO2 efflux from soil has supported the claim that altered efficiency can have ecosystem-scale consequences and may substantially underlie soil-climate feedbacks. Here we review our results from recent experimental work aimed at understanding the sensitivity of CUE to changing climate drivers (e.g., warming, nitrogen and substrate availability) using both traditional and emerging techniques. Synthesizing this evidence allows us to address two primary objectives: 1) discerning the effects of environmental drivers on the metabolic activity (rates of growth and respiration, CUE) of soil microorganisms and 2) begin forecasting the long-term consequences of these changes for soil organic matter stocks.
Soils were collected from ongoing experiments at the Harvard Forest Long Term Ecological Research (LTER) site in Petersham, Massachusetts, USA. CUE was measured on soil samples collected from 1) the 10-year Soil Warming and Nitrogen Addition Study using both 13C and 18O tracer techniques, 2) the 28-year Chronic Nitrogen Addition Study using the 18O technique, and 3) unmanipulated soils adjacent to experimental plots using 18O and 13C approaches to specifically measure the effects of substrate availability on CUE. From these experiments, we examined the long-term, interactive effects between soil warming, N availability, and substrate levels on microbial activity and soil carbon stocks. Results indicate that substrate availability depresses CUE, likely through a shift in community composition towards more inefficient membership. Soil warming has a significant dampening effect on CUE that is, in part, mitigated by enhanced soil N concentrations. Nitrogen enrichment alone does not appear to have strong effects on CUE of the soil community, although growth rates are on average positively affected. Both traditional and current methods of measuring CUE agree on the direction of CUE responses to climate variability; however the magnitude of change is strongly method-dependent and cause for further attention. Lastly, we show evidence for significant loss of soil C stocks from Harvard Forest experiments where reductions in total community CUE occur due to warming.