4. Discussion
4.1. Carbon mineralization and microbial biomass
For each treatment, the CO2 emission rates were significantly higher at 25 C than at 5 C, which indicates that the decomposition intensity generally increases with increasing temperature. The
this difference (CO2 emission at 25 C minus CO2
magnitude of
emission at 5 C) decreases in the sequence manure > slurry and biochar/slurry > biochar and control (differences are significant between the treatments separated by “>”). This suggests that the temperature effect on the decomposition intensity is substrate- specific and largest for manure while lowest for biochar. The overestimated CO2 fluxes by the two-pool-model for manure at 5 C probably result from this high temperature sensitivity of manure decomposition, leading to disproportionally higher CO2 fluxes at 25 C compared to 5 C.
The amount of CO2 cumulatively emitted within the first four weeks is in most cases larger than the amount emitted in weeks five to eight. This effect can be observed for all treatments
incubated at 25 C where the CO2 cumulatively respired after the first four weeks accounted for 59% (control) to 79% (manure) of the total amount of respired CO2 (cumulative after eight weeks). This effect is less pronounced for the CO2 amounts emitted within the first four weeks at 15 C and 5 C incubation temperature. The emission rates account for 48% (slurry) to 71% (manure) of the total amount of the respired C at 15 C and for 47% (control) to 66% (slurry) at 5 C. Only for the treatments with organic soil additives at 25 C incubation temperature, did the higher emissions rates measured within the first four weeks compared to those of weeks five to eight correspond with higher concentrations of the microbial biomass C measured after four rather than after eight weeks. For the samples incubated at 15 C (except slurry) and at 5 C (except control and biochar), the CO2 emission rates were higher within the first four weeks but the microbial biomass C concentration was higher after eight weeks than after four weeks (3% to 68%). The data imply that the microbial biomass seems to become less active or less efficient over time in terms of decomposition processes and CO2 production at incubation temperatures of 5 C and 15 C compared with 25 C.