The Forgotten Part of Carbon Cycling: Organic Matter Storage and Turnover in Subsoils

In the global carbon cycle, the soil contains the largest active terrestrial reservoir. Total soil carbon stocks estimates for the top 1 m range between 1500 and 2000 Pg (1.5 – 2.0 x 1018 g) and are even higher if the recently corrected estimates for tundra soils (Tarnocai et al. 2009) are considered. In the past, carbon flux measurements and modelling have mostly considered the top 20 or 30 cm of the soil where carbon concentrations, root densities, and microbial activities are generally highest. However,  depending on climate zone and land use, this soil compartment contains only 30-50% of the carbon stocks of the first meter. If the deeper subsoil down to 3 m is also considered, the contribution of topsoil carbon stocks to total soil carbon pools is only 20-40% while an equivalent amount of the carbon stocks may be found below 1 m depth. If this is taken into account, global soil carbon stocks may be up to 840 Pg higher than stock estimates based on the top meter only. (Jobbagy and Jackson 2000, Janzen 2005)

Because of the underestimated importance of organic carbon in subsoils this research group funded by the German Research Foundation (DFG FOR 1806), build up by researchers of seven German Universities and two Research Institutions, deals with the analysis of the mechanisms of soil organic carbon storage and turnover in subsoils influenced by physical, chemical and biological factors. Due to the wide expertises and the common approach, the processes of organic matter decomposition in subsoils will be investigated by compiling physical, chemical and biological parameters. This leads to a better understanding and estimation of the mechanisms in subsoils and allows the development of a conceptual model predicting subsoil carbon decomposition.