Litter decomposition and nutrient release for two tropical N-fixing species in Rio de Janeiro, Brazil

Litter production and decomposition are critical to forest productivity, nutrient cycling, and carbon sequestration in tropical woody ecosystems. However, nutrient release and leaf litter stoichiometry in tropical legume tree plantations over the long term after outplanting are poorly understood or even unknown. Toward improving our understanding of the pattern of changes in the decomposition of N-fixing leaf litters and their possible impact on carbon storage, we measured litter production, mass loss and nutrient release for 240 d during litter decomposition for two tropical legume tree species (Plathymenia reticulata and Hymenaea courbaril), in Rio de Janeiro, Brazil. Litter production for P. reticulata was 5.689 kg ha−1 a−1 and 3.231 kg ha−1 a−1 for H. courbaril. The patterns of mass loss rates were similar; however, nutrient release was greater for P. reticulata, while H. courbaril showed immobilization of nutrients, especially for N, which increased by almost 20% in the early phase of decomposition followed by gradual release. Litter from the N-fixing species did differ in nutrient chemistries over time, which was not surprising given that initial nutrient concentrations varied broadly, except for C and P. Most of the nutrient concentrations increased as the remaining litter mass decreased in both species, except for C and K. The C:N and N:P ratios differed between the species, but N:P did not correlate to mass loss. Both species had N-rich leaves, but P. reticulata decomposition was very likely P-limited, while H. courbaril seemed to be co-limited by N and P. The results showed different patterns in nutrient release and the stoichiometry involved in the decomposition dynamics of the two tropical N-fixing species, even though they have similar litter decay rates. Both species, but especially P. reticulata, may help re-establish nutrient cycling in disturbed ecosystems.