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.

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Tropical forest soils are an important contributor to the global greenhouse (GHG) budget and understanding this ecosystem function is of vital importance for future global change and climate research. In this study, we quantified soil fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) of four tropical forest sites located along an altitudinal gradient from 400 to 3010 m a.s.l. on the western flanks of the Andes in northern Ecuador. We assessed the physicochemical soil properties 20 influencing these fluxes during the dry season, as well as the bulk isotopic signature of N2O. The CO2 fluxes ranged between 55.3±12.1 and 137.6±32.8 mg C m -2 h -1 , with the highest and lowest emissions at the highest strata, at 3010 and 2200 m a.s.l., respectively. CH4 fluxes at all sites exhibited a net consumption of atmospheric CH4 and ranged between -74.4±25.0 µg C m -2 h -1 at 2200 m a.s.l. to -46.7±14.7 µg C m -2 h -1 at 3010 m a.s.l. Net fluxes of N2O ranged between -5.1±1.9 and 13.2±31.3 µg N m -2 h -1 , with a marked net sink at 2200 and 3010 m a.s.l., whereas a net source at 400 m. pHwater and nitrate (NO3 -) content 25 at 5 cm depth were able to explain 83% of the observed temporal (daily measurements) and spatial (four forest sites) variability of the CO2 fluxes; indicating that an increase in pHwater and NO3contents lead to an increase in CO2 emissions. For CH4 fluxes, it was not possible to obtain a statistically significant model to identify the physicochemical soil drivers responsible for the CH4 consumption. For N2O, bulk density and pHwater at 5 cm depth were negatively correlated to the N2O fluxes, but able to explain only 36% of the temporal and spatial variability. In addition, the bulk isotope N2O data confirmed that N2O reduction 30 was at the basis of the observed net soil sink at higher altitudes. Finally, the soil GHG budget showed that all studied soils were net sources of GHG's. CO2 emissions represented the largest component of the total soil GHG budget, CH4 consumption was quite consistent along the elevation gradient, whereas N2O was highly variable, and the transition from sources to net