Spatial segregation, substrate and nutrient availability as limiting factors for subsoil C-turnover

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The spatial heterogeneity of subsoil C distribution and fluxes is much higher than in topsoils due to the greater relevance of preferential flow paths, roots, and animal burrows for C-inputs. The spatial segregation of consumers and substrate therefore is more a rule than an exception in subsoils. Taken as an explanation for the observed homogenization effects and for slow turnover rates in subsoils, this implies that hot spots are rather static and "cold regions" remain uninhabited despite substrate availability for extended periods of time. The rhizosphere soil is such a hot spot where C-inputs alter SOC composition and stimulate microbial and enzyme activities. On the other hand, microbial activity in the subsoil appears to be limited by nutrient availability, as indicated by enzyme activity patterns and incubation experiments performed during the first phase. Fresh substrate inputs in the rhizosphere may thus provide energy for microorganisms to produce enzymes for acquiring these limiting nutrients, or they may aggravate N- and P-limitations and thus lead to the accumulation of root-borne organic matter in the rhizosphere. The main objective of the activities outlined in this proposal is to investigate the factors limiting the degradation of subsoil SOC and thus contribute to resolving the contradictory findings of relatively high mineralisation rates in incubation experiments despite the high apparent 14C age of subsoil SOC. The explanatory mechanism of spatial separation between consumers and substrates will be addressed by applying new imaging techniques for analysing spatial patterns of enzyme activities and substrate mineralization in undisturbed soil samples from different depths at various sites. Another focus of this project will be on the characterization of hotspots of microbial activity, mainly the rhizosphere, regarding enzyme patterns and factors stimulating or limiting mineralization processes at these micro-sites relative to the bulk soil. By determining these rhizosphere effects and their spatial extension, important input parameters for the rhizosphere module of the COMISSION model (PC) will be made available. The analysis of 14CO2 in long-term incubation experiments will reveal, if only recent SOC inputs are mineralized or also "old" SOC is labile under these conditions. We will analyze enzyme activities in the soil solutions that are collected at the Grinderwald observatories to determine their translocation into the subsoil. Finally, this project will contribute with its specific analytical tools to several other joint experiments of the research unit.