FOR 1806:  The Forgotten Part of Carbon Cycling: Organic Matter Storage and Turnover in Subsoils (SUBSOM)

• Subject Area: Agriculture, Forestry and Veterinary Medicine
• Term: from 2013 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 207213200

This project focusses on the storage and turnover of organic matter (OM) in subsoils. 40-60 percent of the globally total carbon pool is fixed in the deeper soil from 30 cm up to 2 m depth. The organic carbon analysed in deeper soil horizons showed a high 14C-age of several thousand years. This suggests that OM was accumulated in subsoils over a very long time and, therefore, appears to be very stable. Thus subsoil OM was not considered to be relevant for global C-cycle. Nevertheless, fresh C-inputs due to dissolved OM and bioturbation reach the subsoil and consequently lead to soil organic matter (SOM) decomposition. To enhance the understanding of the turnover mechanisms the Research Unit raised the following hypotheses: (1) Subsoil OM largely consists of recalcitrant compounds that are the least degradable end-products of numerous metabolic and biochemical processes and which, thus, accumulate during the degradation of fresh biomass inputs. (2) The annual inputs of fresh OM into the subsoil are so low that their sequestration in the subsoil does not significantly contribute to pool size changes or 14C activity over centuries. (3) Old SOM in subsoils is more strongly bound to pedogenic minerals and, thus, is more effectively stabilised against biodegradation. This is because C-limited subsoil environments provide a larger sorption site density than topsoils with less competition effects among organic matter components during sorptive interactions. (4) The high radiocarbon age of bulk subsoil OM is caused by a small fraction of fossil or geogenic C. The mean residence time of OM in subsoil is thus grossly overestimated. (5) Microbial densities in subsoils are low and activities are limited by environmental factors like low partial O2 pressures and low temperatures so that SOM is mineralised at extremely low rates. (6) Microbial communities and activities are limited to very few hot spots where fresh OM is supplied with SOM, roots, exudates or burrowing animals. Large parts of the bulk soil are so sparsely populated that even potentially degradable OM can persist. Within ten subprojects the hypotheses will be studied in the field and in laboratory experiments, which follow a common experimental design to refer the elevated data to each other. Overall, the Research Unit aims to develop a conceptual model to enhance the knowledge of OM storage and turnover processes in the subsoil considering soil physical, chemical and biological parameters.

• DFG Programme: Research Units

Projects

• Assessing stabilisation and destabilisation processes - Tracing young and old carbon in subsoils (Applicant Don, Axel )
• Biological Regulation of Subsoil C-cycling under Field Conditions (Applicants Kandeler, Ellen ;  Marhan, Sven ;  Poll, Christian )
• Differences in topsoil and subsoil root morphology and root functioning (water and nitrogen uptake, exudation) of European beech on various bedrock types (Applicant Leuschner, Christoph )
• Einfluss des Wassergehalts, des Eintrags von Wurzeln und gelöster organischer Substanz und räumlicher Unzugänglichkeit auf den C-Umsatz & Bestimmung der räumlichen Variabilität von Bodeneigenschaften (Applicant Ludwig, Bernard )
• Organic matter stabilization processes in subsoils: Impact of small-scale physical heterogeneity of soil particle interfaces (Applicant Bachmann, Jörg )
• Origin and fate of dissolved organic matter in the subsoil driven by dynamic exchange and remobilization processes (Applicants Guggenberger, Georg ;  Kalbitz, Karsten ;  Mikutta, Robert )
• Project coordination: Field site management, data synthesis and modeling of subsoil C-turnover (Applicants Bachmann, Jörg ;  Marschner, Bernd ;  Reichstein, Markus ;  Schrumpf, Marion )
• Rhizosphere as driver of subsoil organic matter distribution and composition (Applicant Kögel-Knabner, Ingrid )
• Sources and degradability of organic matter revealed by 14C analysis (Applicant Rethemeyer, Janet )
• Spatial segregation, substrate and nutrient availability as limiting factors for subsoil C-turnover (Applicant Marschner, Bernd )

• Spokesperson: Professor Dr. Bernd Marschner