The present study reports on the influence of sludge application on understory abundance and species richness in an eight year-old maritime pine forest. Four types of sludge (liquid, dewatered, limed and composted sludge) were applied at a rate equivalent to 3 tons dry weight per ha−1 per year−1. Understorey vegetation was monitored before treatment and for three successive years following initial sludge application. Species richness, the biodiversity index and naturally occurring plant community cover in maritime pine forests were measured. Results showed a significant increase in species richness two years following initial sludge application. In fact, compared to the unamended plot, eighteen additional species (Aira caryophyllea, Cerastium glomeratum, Conyza canadensis, Danthonia decumbens, Geranium robertianum, Gnaphalium sylvaticum, Hypericum humifusum, Hypericum perforatum ssp. perforatum, Jasione montana, Lonicera periclymenum ssp. periclymenum, Ornithopus compressus, Phytolacca americana, Rhamnus frangula, Teesdalia nudicaulis, Veronica arvensis, Vicia sativa angustifolia and Vulpia myuros) appeared after sludge application. Most of these species were observed with the solid limed or composted sludge treatments. The new species then declined the following year, highlighting the temporary effect of sludge treatment on species diversity. However, the cover by these eighteen new species was low and did not exceed 1%, except for Hypericum perforatum and Jasione montana for the limed sludge treatment and Vulpia myuros for the liquid sludge treatment. Additionally, sludge significantly reduces bare soil percentage, which ranged from 5 to 18% compared to the control (38%). According to sludge type, treatment also led to a significant change in species dominance of the understorey plant communities. Indeed, the cover of Molinia caerulea decreased in spite of an increase in Agrostis capillaris and/or Holcus lanatus following application of solid limed, liquid or composted sludge.