Differentiation of microbial activity between bulk soil and rhizosphere hot spots.
14C-labelled substrates or nutrients and combinations will be added to the moist soil samples after a 10-day pre-incubation period. The incubation will be conducted in a CarbO2Bot® instrument which allows temperature control and records CO2 evolution from the samples through changes in electric conductivity in an alkaline solution. After certain time intervals, the solution is replaced by a fresh one and the evolved 14CO2 is determined in a scintillation counter (Perkin Elmer Tri-Carb 2800 TR). In this way, the substrate-borne CO2 can be differentiated from the SOC-borne CO2 and priming effects are calculated by subtracting the CO2 evolved from an unsupplemented control sample (Hamer and Marschner 2005). Enzyme activities will be determined in soil suspensions to which fluorogenic substrates (MUF, AMC) are added in a 96 well microplate assay as described by Marx et al. (2001).