Abstract
Nutrient deposition to tropical forests is increasing, which could affect soil fluxes of nitrous oxide (N2O), a powerful greenhouse gas. We assessed the effects of 35-56 months of moderate nitrogen (N) and phosphorus (P) additions on soil N2O fluxes and net soil N-cycling rates, and quantified the relative contributions of nitrification and denitrification to N2O 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 N2O source at all elevations, but that annual N2O emissions from control plots were low, and decreased along the elevation gradient (0.57 ± 0.26 to 0.05 ± 0.04 kg N2O-N ha-1 yr-1). We attributed the low fluxes to our sites’ conservative soil N cycling as well as gaseous N losses possibly being dominated by N2. Contrary to the first 21 months of the experiment, N addition did not affect N2O fluxes during the 35-56 month period, possibly due to low soil moisture contents during this time. With P addition, N2O 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 N2O fluxes were not linear with time of exposure, highlighting the importance of long-term studies.
Highlights
Nitrous oxide (N2O) is both a potent greenhouse gas and a dominant ozone-depleting substance (Denman et al, 2007)
Net ammonification rates in control plots did not differ across the elevation gradient (P = 0.126; Table 2), which was caused by the large spatial variability, but net nitrification rates were larger at 1000 m than at 2000 and 3000 m (P < 0.001; Table 2)
Relative contributions of nitrification and denitrification to soil N2O fluxes did not differ between the two measurement campaigns for each treatment (P ≥ 0.500) and we reported the average values of these two measurements
Summary
Nitrous oxide (N2O) is both a potent greenhouse gas and a dominant ozone-depleting substance (Denman et al, 2007). Soil emissions are the largest natural (i.e., non-anthropogenic) source of N2O (Ehhalt et al, 2001), which is mainly produced and consumed by the microbial processes of nitrification and denitrification (Chapuis-Lardy et al, 2007). In a meta-analysis of N enrichment effects on greenhouse gas fluxes, Liu and Graever (2009) showed that the N2O response to elevated N input could even be stronger in tropical forests than in other ecosystems. These authors suggested that the strength of the tropical forest response to N enrichment may be due to the phosphorus (P) limitation (rather than N limitation) of many tropical soils
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