Stability of soil organic carbon in the subsoil

Abstract

Soils contain the largest carbon (C) pool of the global terrestrial carbon cycle and can act as sources or sinks for CO2. Although, more than 50 % of the global soil organic carbon (SOC) stocks are stored in subsoils (> 30 cm deep) and the high mean residence time of subsoil organic carbon (OC) indicates that SOC in subsoils is more stable than in topsoils (< 30 cm deep), there is a lack of knowledge on the mechanisms controlling the turnover of SOC in subsoils. In addition, the decreasing SOC content with soil depth also indicates that subsoils may have the potential to sequester additional C and therefore contribute to climate mitigation. Thus, understanding the C dynamics in subsoils are essential to predict the vulnerability of SOC stocks to land-use or climate change and to assess the C sequestration potential of the world soils. The objectives of this thesis were to quantify in situ CO2 production and to identify the sources for CO2 production in the subsoil, in a two-year field monitoring (Article 1). Further, the temperature sensitivity of organic matter decomposition in the subsoil and the influence of substrate limitation on SOC mineralization were investigated in a laboratory incubation experiment (Article 2) and the stability of additional C inputs into the subsoil was examined in a laboratory and a field incubation (Article 2 and 3). Lastly, the influence of different environmental conditions along a soil profile on the organic carbon decomposition were examined during a field incubation (Article 3). Field monitoring in a Dystric Cambisol in a Northern German beech forest showed that the annual CO2 production in the subsoil accounted for 10 % of total soil respiration. Further, isotopic data suggest that CO2 in the subsoil mainly originated from root respiration and the mineralization in the rhizosphere. Hence, the subsoil contains a large labile C pool, which contributes to the annual soil respiration, despite the high 14C age of the bulk SOC. The laboratory incubation pointed out that the temperature sensitivity of SOC decomposition decreases with soil depth, which implies that SOC recalcitrance is not the main stabilization mechanisms in the subsoil. In addition, the decreasing temperature response of soil respiration with depth indicates that losses of subsoil SOC due to climate change might be even lower than previously assumed. The addition of root litter into the topsoil and the subsoil did not enhanced the mineralization of native SOC. Moreover, root litter was more stable in the subsoil environment as in the topsoil environment, which can be explained by the low and the heterogeneous C inputs into the subsoil. The higher C stability in the subsoil underlines the large C-sequestration potential of the subsoil and climate change mitigation research should also include the deeper soil horizons.