Abstract
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.
Highlights
Recent remote sensing and inverse modelling studies indicate that the tropics emit more methane (CH4) and nitrous oxide (N2O) than estimated from prior bottom-up emissions inventories, suggesting that tropical sources are stronger or more numerous than previously thought (Frankenberg et al, 2008; Frankenberg et al, 2005; Bergamaschi et al, 2009; Fletcher et al, 2004a, b; Hirsch et al, 2006; Huang et al, 2008; Kort et al, 2011)
Upland ecosystems account for a substantial fraction of land cover in the tropics; in South America alone, upland ecosystems (> 500 m a.s.l.) represent more than 8 % of total continental land cover (Eva et al, 2004), while in mountainous countries, such as Peru or other Andean states, upland ecosystems may account for upwards of 80 % of total land cover (Feeley and Silman, 2010)
This inference is supported by top-down remote sensing data that indicates the existence of strong local CH4 sources in the tropical Andes, leading to enhanced atmospheric CH4 concentrations
Summary
Recent remote sensing and inverse modelling studies indicate that the tropics emit more methane (CH4) and nitrous oxide (N2O) than estimated from prior bottom-up emissions inventories, suggesting that tropical sources are stronger or more numerous than previously thought (Frankenberg et al, 2008; Frankenberg et al, 2005; Bergamaschi et al, 2009; Fletcher et al, 2004a, b; Hirsch et al, 2006; Huang et al, 2008; Kort et al, 2011). One potentially important source of CH4 and N2O overlooked both by bottom-up inventories and top-down studies are fluxes from tropical upland soils (Spahni et al, 2011), because attention has historically focussed on seasonally inundated wetlands (e.g. várzea in Brazil) (Fung et al, 1991; Bergamaschi et al, 2009; Melack et al, 2004; Werner et al, 2007), lowland forests, savannas, or pastures (Hall and Matson, 1999; Silver et al, 1999; Teh et al, 2005; Werner et al, 2007; Keller et al, 1986, 1993; Verchot et al, 2000; Keller and Reiners, 1994). Soil N2O fluxes are more predictable, but are still poorly constrained (Werner et al, 2007); upland ecosystems, like their lowland counterparts, act as net N2O sources, with emission rates modulated by factors such as soil moisture, water-filled pore space, soil oxygen content, pH, redox potential, C availability, inorganic N availability (NH+4 , NO−3 ), or competition for NO−3 among different soil sinks (Hall and Matson, 1999; Silver et al, 1999, 2001; Veldkamp et al, 2008; Wolf et al, 2011; Firestone and Davidson, 1989)
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