Rapidly growing tropical trees mobilize remarkable amounts of nitrogen, in ways that differ surprisingly among species

Fast-growing forests such as tropical secondary forests can accumulate large amounts of carbon (C), and thereby play an important role in the atmospheric CO₂ balance. Because nitrogen (N) cycling is inextricably linked with C cycling, the question becomes: Where does the N come from to match high rates of C accumulation? In unique experimental 16-y-old plantations established in abandoned pasture in lowland Costa Rica, we used a mass-balance approach to quantify N accumulation in vegetation, identify sources of N, and evaluate differences among tree species in N cycling. The replicated design contained four broad-leaved evergreen tree species growing under similar environmental conditions. Nitrogen uptake was rapid, reaching 409 (±30) kg⋅ha⁻¹⋅y⁻¹, double the rate reported from a Puerto Rican forest and greater than four times that observed at Hubbard Brook Forest (New Hampshire, USA). Nitrogen amassed in vegetation was 874 (±176) kg⋅ha⁻¹, whereas net losses of soil N (0–100 cm) varied from 217 (±146) to 3,354 (±915) kg⋅ha⁻¹ (P = 0.018) over 16 y. Soil C:N, δ¹³C values, and N budgets indicated that soil was the main source of biomass N. In Vochysia guatemalensis, however, N fixation contributed >60 kg⋅ha⁻¹⋅y⁻¹. All species apparently promoted soil N turnover, such that the soil N mean residence time was 32–54 y, an order of magnitude lower than the global mean. High rates of N uptake were associated with substantial N losses in three of the species, in which an average of 1.6 g N was lost for every gram of N accumulated in biomass.