Nutrient deposition to tropical forests is increasing, which could affect soil fluxes of nitrous oxide (N 2 O), a powerful greenhouse gas. We assessed the effects of 35-56 months of moderate nitrogen (N) and phosphorus (P) additions on soil N 2 O fluxes and net soil N-cycling rates, and quantified the relative contributions of nitrification and denitrification to N 2 O fluxes. In 2008, a nutrient manipulation experiment was established along an elevation gradient (1000, 2000, and 3000 m) of montane forests in southern Ecuador. Treatments included control, N, P, and N+P addition (with additions of 50 kg N ha −1 yr −1 and 10 kg P ha −1 yr −1 ). Nitrous oxide fluxes were measured using static, vented chambers and N cycling was determined using the buried bag method. Measurements showed that denitrification was the main N 2 O source at all elevations, but that annual N 2 O emissions from control plots were low, and decreased along the elevation gradient (0.57 ± 0.26-0.05 1 1 ±0.04 kg N 2 O-N ha − yr − ). We attributed the low fluxes to our sites' conservative soil N cycling as well as gaseous N losses possibly being dominated by N 2 . Contrary to the first 21 months of the experiment, N addition did not affect N 2 O fluxes during the 35-56 month period, possibly due to low soil moisture contents during this time. With P addition, N 2 O fluxes and mineral N concentrations decreased during Months 35-56, presumably because plant P limitations were alleviated, increasing plant N uptake. Nitrogen plus phosphorus addition showed similar trends to N addition, but less pronounced given the counteracting effects of P addition. The combined results from this study (Months 1-21 and 35-56) showed that effects of N and P addition on soil N 2 O fluxes were not linear with time of exposure, highlighting the importance of long-term studies.

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Table 1. 95 88 [a] 91 [a] 87 [a1 81 [a] 203 186 207 95 29 (3a), bis(diethy1-

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