Land-use intensification threatens biodiversity conservation of grasslands worldwide. One example of intensification is overseeding legumes in natural grasslands coupled with phosphorus fertilization, a management practice oriented to increase forage production and quality. Several studies show increases in forage due to this practice, at least in the short term, but less is known about impacts on diversity. We analyzed the effects of overseeding Lotus suaveolens cv. “El Rincón” coupled with phosphorus fertilization on plant diversity and composition of natural grasslands of Uruguay. We used floristic data from 29 intensified versus natural paired paddocks from a geographical wide net of observations distributed across two separated areas with distinct plant communities. Besides, a set of land-use intensification indicators were used to explore its relationship with intensification impact on plant composition and diversity. Overall, the effects of overseeding with legumes and fertilization on species richness (total, native and exotic richness) and beta diversity depended on community type. Our data evidenced that the Eryngium horridum–Juncus capillaceus community lost native species with intensification, affecting total richness that showed a 15.3% decrease. The Steinchisma hians–Piptochaetium stipoides community, in turn, was enriched by exotic species. It increased 96.4% but without affecting total species richness. Beta diversity, for its part, showed a reduction associated with intensification only in the S. hians-P. stipoides community. Our results show that neither amount of applied fertilizer nor time since overseeding and fertilizing were directly related to intensification impacts. These findings highlight the importance of accounting for various dimensions of diversity to assess community responses to land use changes. Our results point out that even slight intensification affects plant diversity of Campos Grasslands. The lack of predictability of impact magnitude by simple land-use intensity indicators challenges prevention and mitigation of this technology.