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 ¹⁵N experiments supported these trends by showing that both ¹⁵NH₄₊ and ¹⁵NO_3- 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 ¹⁵N 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₊, NO_3-, 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.