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

The carbon use efficiency of plants (CUE_a ) and microorganisms (CUE_h ) determines rates of biomass turnover and soil carbon sequestration. We evaluated the hypothesis that CUE_a and CUE_h 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 CUE_a , estimated from satellite imagery, with locally determined soil CUE_h for 100 globally distributed sites. Ecosystem CUE_e , the ratio of net ecosystem production (NEP) to GPP, was estimated for each site using published models. At the ecosystem scale, CUE_a and CUE_h were inversely related. At the global scale, the apparent temperature sensitivity of CUE_h with respect to mean annual temperature (MAT) was similar for organic and mineral soils (0.029°C^-1 ). CUE_a and CUE_e were inversely related to MAT, with apparent sensitivities of -0.009 and -0.032°C^-1 , respectively. These trends constrain the ratio μ : GPP (= (CUE_a  × CUE_h )/(1 - CUE_e )) 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.