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Improvement of fallow vegetation can have a positive impact on the productivity of slash-and-mulch systems in eastern Amazonia. Phosphorus fertilization can increase biomass and nutrient stocks in the fallow phase, thereby improving nutrient cycling and crop productivity. Here, we compared biomass and nutrient stocks under three fallow management strategies: (1) natural fallow (regrowth vegetation) - NF; (2) NF vegetation improved with leguminous trees (Sclerolobium paniculatum Vogel and Inga edulis Mart.) - IF; and (3) NF vegetation improved with leguminous trees plus phosphorus fertilization - IF_+P. We quantified above- and belowground biomass and N, P, K, Ca, and Mg stocks after 23 months of fallow. The IF_+P increased aboveground (leaf + branch + stem + liana) biomass and N, P, Ca, and Mg stocks, compared to NF. Similarly, total (aboveground + belowground) biomass and N and P stocks were higher for IF_+P than for NF. The differences in aboveground biomass between NF and improved fallow managements were attributed exclusively to the contribution of the tree species enriching fallow vegetation. Phosphorus application increased the aboveground biomass accumulation of the species for fallow improvement. Improving the fallow vegetation with P-fertilized, fast-growing, N-fixing species represents an efficient management strategy to accelerate the reestablishment of biomass and nutrient stocks in slash-and-mulch systems in Amazonia.