Nutrient status of tropical rain forests influences soil N dynamics after N additions

Soil nitrogen (N) transformations and N-oxide emissions were measured following N additions in three tropical montane rain forests in the Hawaiian Islands that differed in substrate age and nutrient status. Nitrous oxide (N2O) and nitric oxide (NO) emissions were negligible following first-time N additions in a forest where N limits primary production, and they increased significantly following 11 yr of N fertilization. Furthermore, N-oxide fluxes in the N-limited forest were relatively low in response to a range of N additions, and all doses of N except the two highest (125 and 175 kg N/ha) resulted in net negative rates of mineralization and nitrification (i.e., soils showed significant N consumption). Short-term laboratory N-15 experiments supported these trends by showing that both (NH4-)-N-15 and (NO3)-N-15 were strongly consumed in soil from both the control and long-term fertilized plots in the N-limited site. Long-term N fertilization in the N-limited forest significantly increased N availability and turnover, but it did not appreciably alter the small population size of nitrifying microorganisms found in soils of this site. In contrast. N-oxide emissions were equally large after both first-time and long-term N fertilization in forests where production is limited by N and P in combination and by P alone. Furthermore. N-oxide fluxes and net rates of N mineralization and nitrification were equally large in response to both small and large doses of N. Net N transformation and N-15 assays suggested that N consumption processes in both the control and long-term N fertilized plots in the P-limited site were relatively weak (compared to the N-limited site) and not significantly different from one another. Long-term N fertilization did not significantly alter N availability or turnover in the NP-limited and P-limited forests (where pool sizes of NH4-, NO3-, and gross and net mineralization in control plots are high). but it increased rates of nitrification through an apparent increase in the number and/or activity of nitrifying microorganisms. In all forests, fluxes of N-oxides measured immediately following N additions were highly correlated with changes in the activity of nitrifying rather than denitrifying microorganisms. On average, N-oxide fluxes were predictable based on the N status of the ecosystem as estimated by pools of inorganic N and potential rates of nitrification. Combined, these results suggest that anthropogenic N inputs are processed in N-limited and P-limited tropical systems differently, such that the fate of added N in these tropical forest systems is determined in part by the relative strengths of the pathways of N retention (uptake and immobilization in plants and soil organic matter) vs. N loss (nitrification and denitrification). This work suggests that P-limited forests growing on highly weathered soils (where N cycles quickly) may respond differently to N additions than N-limited forests, with large and immediate losses of N-oxides.