Soil N2O fluxes along an elevation gradient of tropical montane forests under experimental nitrogen and phosphorus addition

Abstract

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.

Complex controls on nitrous oxide flux across a large-elevation gradient in the tropical Peruvian Andes

“…Moreover, similar results from comparable research sites in the Ecuadorian Andes lend credence to our claims (Baldos et al, 2015;Müller et al, 2015). For example, Müller et al (2015) found that nitrification produced little or no N 2 O in acidic Ecuadorian soils, in agreement with findings from this study.…”

“…Moreover, similar results from comparable research sites in the Ecuadorian Andes lend credence to our claims (Baldos et al, 2015;Müller et al, 2015). For example, Müller et al (2015) found that nitrification produced little or no N 2 O in acidic Ecuadorian soils, in agreement with findings from this study. Likewise, 15 N isotope pool dilution experiments, in comparable habitats and elevations to our own, revealed that DNRA played a significant role in nitrate reduction, supporting the notion that DNRA may represent a substantial sink for NO − 3 in Peruvian soils (Baldos et al, 2015;Müller et al, 2015).…”

“…In contrast, other habitats showed little or no seasonal variation in N 2 O flux. This weak seasonality in N 2 O flux across the Kosñipata Valley probably stems from relatively modest variation in environmental variables among seasons (Table 3), in accordance with observations from elsewhere in the Andes (Baldos et al, 2015;Müller et al, 2015;Wolf et al, 2011). For example, while soil moisture (i.e.…”

“…For example, recent work in acidic soils have demonstrated that ammonia oxidizing archaea (AOA) play a more important role than AOB in ammonia oxidation, but produce significantly less N 2 O due to differences in metabolism (Hink et al, 2016;Prosser and Nicol, 2008). Likewise, under higher soil moisture conditions (> 60 % WFPS), DNRA -a process that produces substantially less N 2 O than denitrification and which also competes for NO − 3 with denitrification -can dominate nitrate reduction, depending on redox conditions and the relative availability of labile C and N (Morley and Baggs, 2010;Pett-Ridge and Firestone, 2005;Silver et al, 2001;Baldos et al, 2015;Müller et al, 2015). Thus, given the low pH of the soils in Kosñipata Valley (Table 1), it is likely that AOA dominate ammonia oxidation at lower levels of soil moisture, explaining the negligible amounts of N 2 O produced from nitrification in the 30 and 50 % WFPS treatments.…”

Performance of Seedlings of a Shade-Tolerant Tropical Tree Species after Moderate Addition of N and P

“…a.s.l., four years of N and N+P addition (but not P addition alone) resulted in increased mineral N production and decreased microbial N retention and microbial biomass C and C:N ratio (Baldos et al, 2015). N 2 O emissions were only increased during the first two years of N and NP addition (but not P addition alone), and P addition tended to reduce soil N cycling and to decrease N 2 O fluxes Müller et al, 2015). Morevover, P (but not N) additions increased the amount of water stable soil macroaggregates, which play an important role for soil stability and nutrient cycling (Camenzind et al, submitted).…”