Long-term sensitivity of soil carbon turnover to warming

Abstract

The sensitivity of soil carbon to warming is a major uncertainty in projections of carbon dioxide concentration and climate¹. Experimental studies overwhelmingly indicate increased soil organic carbon (SOC) decomposition^{2,3,4,5,6,7,8} at higher temperatures, resulting in increased carbon dioxide emissions from soils. However, recent findings have been cited as evidence against increased soil carbon emissions in a warmer world^9,10. In soil warming experiments, the initially increased carbon dioxide efflux returns to pre-warming rates within one to three years^{10,11,12,13,14}, and apparent carbon pool turnover times are insensitive to temperature¹⁵. It has already been suggested that the apparent lack of temperature dependence could be an artefact due to neglecting the extreme heterogeneity of soil carbon¹⁶, but no explicit model has yet been presented that can reconcile all the above findings. Here we present a simple three-pool model that partitions SOC into components with different intrinsic turnover rates. Using this model, we show that the results of all the soil-warming experiments are compatible with long-term temperature sensitivity of SOC turnover: they can be explained by rapid depletion of labile SOC combined with the negligible response of non-labile SOC on experimental timescales. Furthermore, we present evidence that non-labile SOC is more sensitive to temperature than labile SOC, implying that the long-term positive feedback of soil decomposition in a warming world may be even stronger than predicted by global models^{1,17,18,19,20}.