Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRT_OC). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRT_OC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional- scale cropland and grassland topsoil (0– 30 cm) SOC stock data and average site- specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRT_OC. Explanatory variables related to OC input composition and other soil- forming factors were used to explain the variability in MRT_OC by means of a machine- learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher MRT_OC than croplands (19.4 ± 10.7, n = 1635). This was explained by the higher proportion of root- derived OC inputs in grassland soils, which was the most important variable for explaining MRT_OC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRT_OC is driven by a combination of site properties and OC input composition. However, the great importance of root- derived OC inputs indicated that MRT_OC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.