Influence of co-applied biochar and enhanced basanite weathering on soil enzyme kinetics in an agricultural soil

Soil microbes are major regulators of soil ecosystem services and play a crucial role for carbon and nutrient cycling. Soil microbial activity can be altered by the application of biochar and of rock powder for enhanced weathering – two promising carbon dioxide removal (CDR) techniques. While most recent research considered both CDR methods separately, their co-application could offer additional benefits for CDR, soil health, and crop yield. Here, we compare the influence of pure wood biochar and pure basanite powder with the product of pre-pyrolytic combination of woody biomass and basanite powder (referred to as PyMiCCS). To determine the influence of joint pyrolysis, we also include a post-pyrolysis-combination (PPC) equivalent to PyMiCCS. The aim of this study was to determine the influence of co-applied biochar and basanite powder on enzyme kinetics. Therefore, we grew cabbage turnip (Brassica oleracea) in lysimeters filled with a sandy agricultural topsoil (control) and an amendment (biochar, basanite powder, PyMiCCS, PPC) over a period of nine weeks. Afterwards, the soil samples were analyzed for enzyme kinetics of ß-glucosidase, chitinase, leucine-aminopeptidase, and acid phosphatase.Preliminary results show significantly enhanced Vmax (maximum rate of soil enzyme activity) of acid phosphatase in all treatments compared to the other studied enzymes, implying a relatively high demand for P. Furthermore, we found that treatments containing biochar, PyMiCCS, and PCC had up to 50% lower Vmax values for ß-glucosidase, chitinase, and acid phosphatase relative to control and basanite treatments. In contrast to this, leucine-aminopeptidase showed an increase in Vmax of up to 40% in biochar, PyMiCCS, and PCC treatments compared to control and basanite treatments. This could be interpreted as a shift of nutrient demand towards N due to the addition of biochar, PyMiCCS, and PCC, resulting in an increased production of the N-cycle-related leucine-aminopeptidase. This increased N demand could be caused by the fixation of N-rich molecules by the amendments, or by the release of other nutrients, such as P or C. Consistently with the latter, we observed a significant increase in C content of up to 50% following the application of biochar, PyMiCCS, and PPC, whereas the N content showed little to no increase. Our results so far indicate that the co-application of biochar and basanite powder affects soil microbial activity by shifting nutrient availability. However, the interactive effect of the co-applied amendments on mineral N and microbial biomass is still subject to further analyses.