Estimating the effect of past century global warming on agricultural topsoil carbon stocks

Estimating the effect of past century global warming on agricultural soil carbon stocks

 

Christopher Poeplau, Rene Dechow

 

Climate change is likely to affect soil organic carbon (SOC) stocks across the globe. Therefore, modelling efforts are undertaken to estimate the effect of future climate change on SOC stocks at different spatial scales and for various climate change scenarios. However, global average air temperature already increased by more than 1°C, which most likely already affected and affects global SOC stocks. Agricultural soils were observed to lose SOC in many parts of the world, which is partly interpreted as climate-driven. For deconfounding management and climate change effects, the latter needs to be estimated comprehensively. In this study, an established FAO framework, including the global SOC map as well as the RothC and MIAMI models, was used to model global agricultural topsoil SOC stock dynamics from 1919 to 2018 as attributable to climate change.

On average, global agricultural topsoils lost 2.5±2.3 Mg C ha^-1 with constant net primary production (NPP) or 1.6±3.4 Mg C ha^-1 when NPP was modified by temperature and precipitation. These loss rates per °C were comparable to those observed in long-term geothermal warming experiments, which are also presented as a source of validation. Regional variability in SOC stock changes was explained by the complex patterns of alterations in temperature and moisture, as well as initial SOC stocks as major drivers of mineralisation and partly also C inputs in the models. On average, SOC losses have been a persistent feature of climate change in all climatic zones during the past century. This needs to be taken into consideration in reporting or accounting frameworks and halted in order to mitigate climate change and secure soil health. At the same time, the estimated climate-driven loss rates were partly much smaller than observed SOC losses from agricultural soils, indicating that other management-related drivers have been more important.