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

Abstract

Abstract. Remote sensing and inverse modelling studies indicate that the tropics emit more CH 4 and N 2 O 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 CH 4 and N 2 O often overlooked by past empirical and modelling studies. To address this knowledge gap, we investigated spatial, temporal and environmental trends in soil CH 4 and N 2 O fluxes across a long elevation gradient (600-3700 m a.s.l.) in the Kosñi-pata Valley, in the southern Peruvian Andes, that experiences seasonal fluctuations in rainfall. The aim of this work was to produce preliminary estimates of soil CH 4 and N 2 O fluxes from representative habitats within this region, and to identify the proximate controls on soil CH 4 and N 2 O dynamics. Area-weighted flux calculations indicated that ecosystems across this altitudinal gradient were both atmospheric sources and sinks of CH 4 on an annual basis. Montane grasslands (3200-3700 m a.s.l.) were strong atmospheric sources, emitting 56.94 ± 7.81 kg CH 4 -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 CH 4 -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 CH 4 -C ha −1 yr −1 ; dry season: −1.17 ± 0.40 kg CH 4 -C ha −1 yr −1 ). Analysis of spatial, temporal and environmental trends in soil CH 4 flux across the study site suggest that soil redox was a dominant control on net soil CH 4 flux. Soil CH 4 emissions were greatest from habitats, landforms and during times of year when soils were suboxic, and soil CH 4 efflux was inversely correlated with soil O 2 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 N 2 O sources. Soil N 2 O fluxes declined with increasing elevation; area-weighted flux calculations indicated that N 2 O 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 N 2 O-N ha −1 yr −1 , respectively. Soil N 2 O 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 N 2 O fluxes from this region were primarily driven by denitrification; that nitrate (NO

Seasonal variability in methane and nitrous oxide fluxes from tropical peatlands in the western Amazon basin

“…One potential source of concern are the negative N 2 O fluxes that we documented here. While some investigators have attributed negative fluxes to instrumental error (Cowan et al, 2014;Chapuis-Lardy et al, 2007), others have demonstrated that N 2 O consumption -particularly in wetland soils -is not an experimental artifact but occurs due to the complex effects of redox, organic carbon content, nitrate availability, and soil transport processes on denitrification (Ye and Horwath, 2016;Yang et al, 2011;Wen et al, 2016;Schlesinger, 2013;Teh et al, 2014;Chapuis-Lardy et al, 3680 Y. A. Teh et al: Seasonal variability in methane and nitrous oxide fluxes from tropical peatlands 2007).…”

“…A. Teh et al: Seasonal variability in methane and nitrous oxide fluxes from tropical peatlands 2007). Given the low redox potential and high carbon content of these soils, it is plausible that microbial N 2 O consumption is occurring, because these types of conditions have been found to be conducive for N 2 O uptake elsewhere (Ye and Horwath, 2016;Teh et al, 2014;Yang et al, 2011).…”

Landscape analysis of soil methane flux across complex terrain

“…Although many studies have quantified the magnitude and variability of CH 4 fluxes, they often covered large spatial extents (from transects that are tens of meters to hundreds of kilometers long) which captured significant environmental gradients at those scales, but sampling locations were generally sparse (Del Grosso et al, 2000;Yu et al, 2008;Teh et al, 2014;Tian et al, 2014). The smaller-scale patterns of CH 4 fluxes within these landscapes has not been investigated as thoroughly as ecosystem scale gradients, which could be problematic if those patterns are important for estimating CH 4 fluxes Ullah and Moore, 2011;Nicolini et al, 2013).…”

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

“…However, these models are only as reliable as the data used to parameterize them; as a consequence, ecosystems that are under-represented in the empirical literature or which are poorly understood may be modelled less accurately, with knock-on effects for larger-scale emissions estimates (Saikawa et al, 2013;Teh et al, 2014;Werner et al, 2007). Nitrous oxide dynamics in montane tropical ecosystems are particularly poorly understood, because past research has concentrated on N 2 O flux from lowland tierra firme forests (Saikawa et al, 2013;Teh et al, 2014;Werner et al, 2007). Montane ecosystems, however, are important components of many tropical landscapes, and account for a sizeable land area.…”

“…For example, in continental South America, montane ecosystems (> 500 m a.s.l.) cover more than 8 % of the land surface (Eva et al, 2004), and play key roles in regional carbon (C), nitrogen (N), and greenhouse gas (GHG) dynamics (Girardin et al, 2010;Moser et al, 2011;Teh et al, 2014;Wolf et al, 2012Wolf et al, , 2011. Processbased models predict that N 2 O fluxes from these montane environments are lower than those from the lowland tropics (i.e.…”