Shrub encroachment of grassland is a global phenomenon that can cause substantial and rapid changes in soil nutrient levels and distribution. If the woody plants in question also have the capacity to fix nitrogen (N), the effects on soil nutrients may potentially be large. Cytisus scoparius L. Link (Scotch broom) is an introduced leguminous shrub from the Mediterranean region that colonizes open grasslands. We measured several properties of the surface soil (0-10 cm depth) across advancing fronts of Cytisus to assess changes in soil fertility associated with invasion in western Washington state (USA). Both total soil C and N increased (1.8% and 6.5%, respectively), resulting in a decrease in C to N ratio of 22.5 in uninvaded soils to 21.5 inside Cytisus patches. δ15N signature of surface soil did not clearly demonstrate a signal of N2-fixation across the Cytisus gradient. On the other hand, seven-day laboratory aerobic incubations demonstrated 3 and 2.4 times greater mineralization and nitrification rate, respectively, in Cytisus soils compared to uninvaded soils. Despite this increase in N availability, bioassay plants (Achillea millefolium) grown in uninvaded soils in the greenhouse were ∼ 30% larger than those grown in Cytisus-invaded soils, suggesting that Cytisus may have inhibitory effects on some plants growing in invaded soils. These results suggest that the impact of Cytisus invasion on grassland plant communities may be influenced or tempered by chemical or microbial effects on the soil other than simply increased labile N.