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 (CO 2 ) 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 database 9 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 FLUXNET2015 12 dataset. All trends are robust to sampling variability in ecosystem type, disturbance, methodology, CO 2 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 experiments 17 . 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. Global soil respiration is rising, probably in response to environmental changes, suggesting that climate-driven losses of soil carbon are occurring worldwide.