The de-inking process produces a waste by-product, called de-inking paper sludge (DS), that contains paper fibers, clay particles and inks and high carbon (C) concentrations combined with low nitrogen (N) and phosphorus (P) concentrations. The use of high rates of DS to increase the soil organic matter thus requires provision of high rates of N and P for adequate plant growth. Using dinitrogen (N2)-fixing forage legumes is an alternative to N fertilization under such circumstances. In a greenhouse study, DS rates of 0, 50 or 100 Mg ha(-1) and five rates of P (40, 80, 120, 160, or 200 kg P2O5 ha(-1)) were applied on two soil types, a clay loam (Pintendre) and a silty clay loam (St-Augustin). Nitrogen uptake and symbiotic N2 fixation (SNF) were estimated in alfalfa (Medicago sativa L.), sweetclover (Melilotus officinalis L.) and red clover (Trifolium pratense L.); Bromegrass (Bromus inermis L.) and alfalfa ineffective for N2 fixation were used as the reference (non-N2 fixing) crops. Atmospheric N2 fixation was estimated by natural abundance of 15N (815N). Under controlled conditions, high rates of DS substantially reduced delta15N values, particularly with high rates of P. In addition, N uptake of legumes generally increased with increased P concentrations and it peaked with 120 or 160 kg P2O5 ha(-1). Correlated with the trends observed with delta15N values and it peaked with 120 or 160 kg P2O5 ha(-1). Present results showed that under high rates of application of DS and adequate P supply, forage legumes fixed more atmospheric N2. delta15N can be a good indicator of SNF under the above-mentioned conditions.