The ratio of Rh over Rs from SRDB-V2
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Table 1. The ratio of Rh over Rs from SRDB-V2. (Note: N-Obs and N are the plots with the definite partitioning methods and plots with both Rh and Rs values, respectively. Methods used to partition Ra and Rh by studies in the SRDB-V2 database (described in  and references therein) included 'comparison' (comparing fluxes from nearby locations with and without roots), 'equation' (scaling tissue specific rates using biomass measurements), 'exclusion' (digging trenches around plots to exclude roots), 'extraction' (measuring efflux from roots and soil separate from each other), 'isotope' (stable or radioactive isotope labeling; exploiting or manipulating differences in isotopic signature of roots and bulk soil), 'regression' (calculating Ra from a regression of Rs and root biomass dynamics) and 'mass balance' (subtracting measurements of other respiration components).)
Soil microbial respiration (Rh) is a large but uncertain component of the terrestrial carbon cycle. Carbon–climate feedbacks associated with changes to Rh are likely, but Rh parameterization in Earth System Models (ESMs) has not been rigorously evaluated largely due to a lack of appropriate measurements. Here we assess, for the first time, Rh estimates from eight ESMs and their environmental drivers across several biomes against a comprehensive soil respiration database (SRDB-V2). Climatic, vegetation, and edaphic factors exert strong controls on annual Rh in ESMs, but these simple controls are not as apparent in the observations. This raises questions regarding the robustness of ESM projections of Rh in response to future climate change. Since there are many more soil respiration (Rs) observations than Rh data, two 'reality checks' for ESMs are also created using the Rs data. Guidance is also provided on the Rh improvement in ESMs.