Abstract
Abstract. Tropical forest soils are an important source and sink of greenhouse gases (GHGs), with tropical montane forests, in particular, having been poorly studied. The understanding of this ecosystem function is of vital importance for future climate change research. In this study, we explored soil fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in four tropical forest sites located on the western flanks of the Andes in northern Ecuador. The measurements were carried out during the dry season from August to September 2018 and along an altitudinal gradient from 400 to 3010 m a.s.l. (above sea level). During this short-term campaign, our measurements showed (1) an unusual but marked increase in CO2 emissions at high altitude, possibly linked to changes in soil pH and/or root biomass, (2) a consistent atmospheric CH4 sink over all altitudes with high temporal and spatial variability, and (3) a transition from a net N2O source to sink along the altitudinal gradient. Our results provide arguments and insights for future and more detailed studies on tropical montane forests. Furthermore, they stress the relevance of using altitudinal transects as a biogeochemical open-air laboratory with a steep in situ environmental gradient over a limited spatial distance. Although short-term studies of temporal variations can improve our understanding of the mechanisms behind the production and consumption of soil GHGs, the inclusion of more rigorous sampling for forest management events, forest rotation cycles, soil type, hydrological conditions and drainage status, ground vegetation composition and cover, soil microclimate, and temporal (seasonality) and spatial (topographic positions) variability is needed in order to obtain more reliable estimates of the CO2, CH4, and N2O source/sink strength of tropical montane forests.
Highlights
To further improve our understanding of the role of tropical forest ecosystems in the global greenhouse gases (GHGs) balance, environmental gradients can offer great opportunities to study the influence of abiotic factors on biogeochemical processes under field conditions (Bauters et al, 2017; Jobbágy and Jackson, 2000; Kahmen et al, 2011; Laughlin and Abella, 2007), which complements the knowledge on short-term responses from experimental approaches
To address these knowledge gaps, we present a pilot study of the soil–atmosphere exchange of CO2, CH4, and N2O along an altitudinal gradient in a neotropical montane forest located on the western flanks of the Andes in northern Ecuador
GHG fluxes from tropical montane forests in South America are scarce with limited spatial coverage and seasonal fluctuation in rainfall but important for consideration in future field measurements and modeling research
Summary
Soils play a vital role in the global greenhouse gas (GHG) budget. Tropical forest soils, in particular, represent a net sink of carbon (C) (Pan et al, 2011), but at the same time, they are the largest natural source of N2O, with an estimated contribution of 14 %–23 % to the annual global N2O budget (Werner et al, 2007). CH4 fluxes vary from −0.7 to −30.0 kg CH4-C ha−1 yr−1, with an average consumption of −3.0 kg CH4-C ha−1 yr−1, while the mean rate of N2O emissions from tropical forest soils is 3.03 ± 0.52 kg N2ON ha−1 yr−1 (Dalal and Allen, 2008), i.e., 2–3 times higher than the mean N2O emissions from temperate forest soils (1.0 ± 0.36 kg N2O-N ha−1 yr−1; Chapui-Lardy et al, 2007; Van Groenigen et al, 2015). Teh et al (2014) and Spahni et al (2011) have argued that tropical upland soils are one potentially important source of CH4 and N2O that has been overlooked in both bottom-up and top-down emission inventories; their sink/source strength might be comparable to or greater than their lowland counterparts and, quantitatively important in regional and global GHG budgets
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