Biochar dominated the combined effect of silicate rock powder and biochar application on extracellular enzyme kinetics and nutrient dynamics in a sandy soil

The application of biochar or silicate rock powder as soil amendments combines carbon dioxide removal with soil improvement. However, their combined short-term effects on nutrient dynamics and microbial activity are poorly understood. Therefore, we combined wood biochar and basanite powder via co-application and via co-pyrolysis of biomass and basanite to rock-enhanced biochar in a nine-week semi-field-based lysimeter experiment with cabbage turnip ( Brassica oleracea var. gongylodes L.). We measured carbon (C), nitrogen (N), available phosphorous (P), mineral N, dissolved organic C (DOC), microbial biomass C (C mic ), soil pH, and electric conductivity (EC). We examined extracellular enzyme kinetics of ß-glucosidase (BG), chitinase (CH), leucine-aminopeptidase (LAP), and acid phosphatase (AP) related to C, N, and P cycles. From potential enzyme activity (V max ) of BG, LAP, and AP we calculated extracellular enzyme stoichiometry (EES), vector angle and length to assess nutrient limitations. In combined applications, the influence of biochar was dominant. The application of biochar-containing amendments (biochar, co-application, co-pyrolyzed rock-enhanced biochar) to our sandy topsoil significantly increased C, P, DOC, C mic , pH, and EC. Co-application even exceeded single biochar in increasing N, pH, and EC. Single biochar application resulted in the highest short-term P availability, while combined applications potentially result in a long-term P supply. While LAP’s V max increased following biochar-containing amendment application, V max of the other enzymes decreased. Although AP showed the highest V max , indicating a P limitation, the enzyme patterns and EES suggest an increased N demand and a shift from P-limited towards a more balanced microbial nutrient demand following biochar-containing amendment application. • Combined applications mainly driven by biochar, not by basanite • Biochar and combined applications increased P availability and relative N demand • Co-pyrolysis showed no advantage compared to co-application of biochar and basanite • No basanite weathering during 9-week planted lysimeter experiment visible in soil • Basanite application caused no negative effects, highlighting its CDR potential