Invasive plants can impact nutrient cycling, potentially creating positive feedbacks for further invasion. We studied the impact of three woody nitrogen (N)-fixing invasive plant species on soil N-cycling and phosphatase activity in a montane tropical forest-grassland mosaic in the Nilgiri Biosphere Reserve in southern India, an ecosystem with > 100-year history of biological invasions. Soils were collected over a year from patches invaded by Acacia mearnsii, Cytisus scoparius, and Ulex europaeus, and from uninvaded grasslands, to assess inorganic N-availability and N-mineralization rates (using in situ open-top mineralization tubes). Phosphatase activity was measured from soils collected at the beginning of the growing season. Soils of invaded areas had higher inorganic N-availability and phosphatase activity than soils of uninvaded areas. Whilst net N-mineralization rates were unchanged between invaded and uninvaded sites, net nitrification rates were higher and net ammonification rates lower in invaded areas, particularly in the dry season. Impacts of C. scoparius and U. europaeus on these variables were similar to each other, and lower than the impacts of A. mearnsii. These results show that invasive N-fixers are significantly altering nutrient availability and cycling, and also changing the proportion of the forms of inorganic N available, in the Nilgiri grasslands. Restoration activities in these invaded grasslands should explore soil N management strategies such as soil C amendments and planting of specific native species, in conjunction with other strategies, to control invasive plants.