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 (Cmic), 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 (Vmax) 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, Cmic, 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 Vmax increased following biochar-containing amendment application, Vmax of the other enzymes decreased. Although AP showed the highest Vmax, 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.