Plant, microbial and ecosystem carbon use efficiencies interact to stabilize microbial growth as a fraction of gross primary production.

The carbon use efficiency of plants (CUEa ) and microorganisms (CUEh ) determines rates of biomass turnover and soil carbon sequestration. We evaluated the hypothesis that CUEa and CUEh counterbalance at a large scale, stabilizing microbial growth (μ) as a fraction of gross primary production (GPP). Collating data from published studies, we correlated annual CUEa , estimated from satellite imagery, with locally determined soil CUEh for 100 globally distributed sites. Ecosystem CUEe , the ratio of net ecosystem production (NEP) to GPP, was estimated for each site using published models. At the ecosystem scale, CUEa and CUEh were inversely related. At the global scale, the apparent temperature sensitivity of CUEh with respect to mean annual temperature (MAT) was similar for organic and mineral soils (0.029°C-1 ). CUEa and CUEe were inversely related to MAT, with apparent sensitivities of -0.009 and -0.032°C-1 , respectively. These trends constrain the ratio μ : GPP (= (CUEa  × CUEh )/(1 - CUEe )) with respect to MAT by counterbalancing the apparent temperature sensitivities of the component processes. At the ecosystem scale, the counterbalance is effected by modulating soil organic matter stocks. The results suggest that a μ : GPP value of c. 0.13 is a homeostatic steady state for ecosystem carbon fluxes at a large scale.