Globally rising soil heterotrophic respiration over recent decades

Global soils store at least twice as much carbon as Earth's atmosphere^1,2. The global soil-to-atmosphere (or total soil respiration, R_S) carbon dioxide (CO₂) flux is increasing^3,4, but the degree to which climate change will stimulate carbon losses from soils as a result of heterotrophic respiration (R_H) remains highly uncertain^5-8. Here we use an updated global soil respiration database⁹ to show that the observed soil surface R_H:R_S ratio increased significantly, from 0.54 to 0.63, between 1990 and 2014 (P = 0.009). Three additional lines of evidence provide support for this finding. By analysing two separate global gross primary production datasets^10,11, we find that the ratios of both R_H and R_S to gross primary production have increased over time. Similarly, significant increases in R_H are observed against the longest available solar-induced chlorophyll fluorescence global dataset, as well as gross primary production computed by an ensemble of global land models. We also show that the ratio of night-time net ecosystem exchange to gross primary production is rising across the FLUXNET2015¹² dataset. All trends are robust to sampling variability in ecosystem type, disturbance, methodology, CO₂ fertilization effects and mean climate. Taken together, our findings provide observational evidence that global R_H is rising, probably in response to environmental changes, consistent with meta-analyses^13-16 and long-term experiments¹⁷. This suggests that climate-driven losses of soil carbon are currently occurring across many ecosystems, with a detectable and sustained trend emerging at the global scale.