Vegetation cover affects soil organic matter and activity of soil microorganisms. We investigated the intercropping effect of forage cactus with tree legumes on soil microbial biomass and organic matter in the semi-arid tropic, state of Pernambuco, Brazil. We assessed the following cropping systems: (i) Gliricidia sepium intercropped with cactus cv. IPA-Sertânia; (ii) Leucaena leucocephala intercropped with cactus cv. IPA-Sertânia; and (iii) Cactus cv. IPA-Sertânia in monoculture. Samples were collected during the dry and rainy seasons in the 0- to 0.10- and 0.10- to 0.20-m soil layers at 0, 1.5, 3.0, and 4.5 m in a perpendicular gradient from tree legume rows. The following responses were determined: δ13C and δ15N, C and N, microbial activity and biomass, and metabolic, microbial, and mineralization quotient. δ13C and δ15N varied with the distance from the trees. In the dry season and beginning of the rainy season, the cropping systems showed similar values for C, N, microbial activity, carbon, and nitrogen in the microbial biomass. The presence of tree legumes at the end of the rainy season favored soil microbiota, which showed a reduced loss of C-CO2, with no indication of metabolic stress and greater microbial biomass and microbial quotient in relation to forage cactus in monoculture.