Observation‐based global soil heterotrophic respiration indicates underestimated turnover and sequestration of soil carbon by terrestrial ecosystem models

Soil heterotrophic respiration (Rh) refers to the flux of CO2 released
from soil to atmosphere as a result of organic matter decomposition by
soil microbes and fauna. As one of the major fluxes in the global carbon
cycle, the estimation of global Rh still exists large uncertainties, which
further limited our current understanding of the carbon accumulation in
soils. Here, we applied a Random Forest algorithm to create a global
dataset of soil Rh, by linking 761 field observations with both abiotic
and biotic predictors. We estimated that the global Rh was 48.8 ± 0.9 Pg C
yr-1 for 1982–2018, which was 16% less than the ensemble mean (58.6 ± 9.9
Pg C yr-1) of 16 terrestrial ecosystem models. By integrating our
observational Rh with independent soil carbon stock datasets, we obtained
a global mean soil carbon turnover time of 38.3 ± 11 yr. Using
observation-based turnover times as a constraint, we found that
terrestrial ecosystem models simulated faster carbon turnovers, leading to
a 30% (74 Pg C) underestimation of terrestrial ecosystem carbon
accumulation for the past century, which was especially pronounced at high
latitudes. This underestimation is equivalent to 45% of the total carbon
emissions (164 Pg C) caused by global land use change at the same time.
Our analyses highlight the need to constrain ecosystem models using
observation-based and locally adapted Rh values to obtain reliable
predictions of the carbon sink capacity of terrestrial
ecosystems.