Assessing stabilisation and destabilisation processes - Tracing young and old carbon in subsoils

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Subsoils store more than 50% of the global soil carbon, but little is known on stabilisation and destabilisation processes in subsoils. Subsoil organic carbon (OC) is derived from three major sources: OC leached into subsoils as dissolved organic carbon (DOC), OC derived from root litter and root exudates (together the two biogenic OC sources) and OC that was deposited with the formation of sedimentary bedrocks, so called geogenic OC or fossil OC. There are few studies on the contribution of these young and old OC sources to subsoil OC stocks. The aim of this subproject within SUBSOM is to trace the sources of subsoil OC, to differentiate between geogenic and biogenic OC and to assess the stability of young DOC derived subsoil OC and old geogenic OC in subsoils. In three workpackages (WPs) field experiments will be combined with laboratory analysis and incubation experiments on the vulnerability of different OC fractions in different soil types. Isotopic labelling (13C) will be used to trace the young DOC derived OC fraction and 14C will be used to trace the very old, geogenic OC fraction.At the SUBSOM observatories monitoring of soil CO2 fluxes will be continued (WP1) and the CO2 gradient based model will be validated using the Switch off joint experiment. Newly available data such as root turnover will be used to constrain the drivers of CO2 fluxes in subsoils based in a multifactorial analysis (machine learning algorithms) of this large observatory data set.The fate and stability of DOC in subsoil will be assessed in a joint experiment (DO13C inject, field campaigns led by our subproject) by applying a labelled DOC solution at three sites to two differed soil depths (WP2). Similarly, at the observatories the fate of the labeled litter applied onto the soil surface 20 months ago will be explored (Switch off experiment, sampling campaigns led by our subproject). Incubation experiments with addition of nutrients and easily available OC substrate will reveal the degree and mechanisms of stabilisation of such young DOC derived OC input. The role of geogenic OC in subsoils will be explored in a joint experiment (led by our subproject) at four sites with different sedimentary bedrock (loess, tertiary sand, sand stone, lime stone)(WP3). From deep cores (10 m) bedrock samples with almost 100% geogenic OC will be derived to quantify and characterised the geogenic OC fraction. Fractionation methods will be developed to separate geogenic from biogenic OC. The stability of geogenic OC entering subsoils (encounter with nutrients and easily available OC) will be assessed in incubation experiments.Data on the turnover and in-situ transport of DOC in different soil types and the contribution of geogenic OC to subsoil OC will be directly used as input variables in the numerical model developed within SUBSOM.