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 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, 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 metaanalyses(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.