Methane and nitrous oxide fluxes across an elevation gradient in the tropical Peruvian Andes

Abstract. Remote sensing and inverse modelling studies indicate that the tropics emit more CH₄ and N₂O than predicted by bottom-up emissions inventories, suggesting that terrestrial sources are stronger or more numerous than previously thought. Tropical uplands are a potentially large and important source of CH₄ and N₂O often overlooked by past empirical and modelling studies. To address this knowledge gap, we investigated spatial, temporal and environmental trends in soil CH₄ and N₂O fluxes across a long elevation gradient (600–3700 m a.s.l.) in the Kosñipata Valley, in the southern Peruvian Andes, that experiences seasonal fluctuations in rainfall. The aim of this work was to produce preliminary estimates of soil CH₄ and N₂O fluxes from representative habitats within this region, and to identify the proximate controls on soil CH₄ and N₂O dynamics. Area-weighted flux calculations indicated that ecosystems across this altitudinal gradient were both atmospheric sources and sinks of CH₄ on an annual basis. Montane grasslands (3200–3700 m a.s.l.) were strong atmospheric sources, emitting 56.94 ± 7.81 kg CH₄-C ha⁻¹ yr⁻¹. Upper montane forest (2200–3200 m a.s.l.) and lower montane forest (1200–2200 m a.s.l.) were net atmospheric sinks (−2.99 ± 0.29 and −2.34 ± 0.29 kg CH₄-C ha⁻¹ yr⁻¹, respectively); while premontane forests (600–1200 m a.s.l.) fluctuated between source or sink depending on the season (wet season: 1.86 ± 1.50 kg CH₄-C ha⁻¹ yr⁻¹; dry season: −1.17 ± 0.40 kg CH₄-C ha⁻¹ yr⁻¹). Analysis of spatial, temporal and environmental trends in soil CH₄ flux across the study site suggest that soil redox was a dominant control on net soil CH₄ flux. Soil CH₄ emissions were greatest from habitats, landforms and during times of year when soils were suboxic, and soil CH₄ efflux was inversely correlated with soil O₂ concentration (Spearman's ρ = −0.45, P < 0.0001) and positively correlated with water-filled pore space (Spearman's ρ = 0.63, P <0.0001). Ecosystems across the region were net atmospheric N₂O sources. Soil N₂O fluxes declined with increasing elevation; area-weighted flux calculations indicated that N₂O emissions from premontane forest, lower montane forest, upper montane forest and montane grasslands averaged 2.23 ± 1.31, 1.68 ± 0.44, 0.44 ± 0.47 and 0.15 ± 1.10 kg N₂O-N ha⁻¹ yr⁻¹, respectively. Soil N₂O fluxes from premontane and lower montane forests exceeded prior model predictions for the region. Comprehensive investigation of field and laboratory data collected in this study suggest that soil N₂O fluxes from this region were primarily driven by denitrification; that nitrate (NO₃⁻) availability was the principal constraint on soil N₂O fluxes; and that soil moisture and water-filled porosity played a secondary role in modulating N₂O emissions. Any current and future changes in N management or anthropogenic N deposition may cause shifts in net soil N₂O fluxes from these tropical montane ecosystems, further enhancing this emission source.