SOM is essential for providing nutrients and energy for plant and microbially mediated processes and contributes to soil resistance and resilience. SOM is a main store for several key nutrients, among others N, P and S and enhances the water holding capacity of soils. At the same time SOM binds soil particles into aggregates providing a habitat for organisms living in soil and improving soil structural stability. Since SOM contents and functionality vary with depth, we are currently facing the need to improve our understanding of carbon stability and functionality change throughout the soil profile. Since a substantial part of the subsoil C pool appears to be much less recalcitrant than expected, small changes in environmental conditions could affect carbon turnover and composition in subsoils. While organic matter stabilization mechanisms and factors controlling its turnover are well understood in topsoils, the underlying mechanisms are not valid in subsoils due to depth dependent differences regarding (1) amounts and composition of C-pools and C-inputs, (2) aeration, moisture and temperature regimes, (3) relevance of specific soil organic carbon stabilisation mechanisms and (4) spatial heterogeneity of physico-chemical and biological parameters.
Due to very low C concentrations and high spatio-temporal variability of properties and processes, the investigation of subsoil phenomena and processes poses major methodological, instrumental and analytical challenges. This session will focus on studies that have overcome some of these challenges with innovative approaches and provide new insights in our understanding of the processes and controlling factors of SOM composition, functionality and turnover in sub- versus topsoils.
Solicited speaker: Jérôme Balesdent INRA,France (jerome.balesdent@aix.inra.fr)