Soil trace gas fluxes along orthogonal precipitation and soil fertility gradients in tropical lowland forests of Panama

Abstract. Tropical lowland forest soils are significant sources and sinks of trace gases. In order to model soil trace gas flux for future climate scenarios, it is necessary to be able to predict changes in soil trace gas fluxes along natural gradients of soil fertility and climatic characteristics. We quantified trace gas fluxes in lowland forest soils at five locations in Panama, which encompassed orthogonal precipitation and soil fertility gradients. Soil trace gas fluxes were measured monthly for 1 (NO) or 2 (CO₂, CH₄, N₂O) years (2010–2012) using vented dynamic (for NO only) or static chambers with permanent bases. Across the five sites, annual fluxes ranged from 8.0 to 10.2 Mg CO₂-C, −2.0 to −0.3 kg CH₄-C, 0.4 to 1.3 kg N₂O-N and −0.82 to −0.03 kg NO-N ha⁻¹ yr⁻¹. Soil CO₂ emissions did not differ across sites, but they did exhibit clear seasonal differences and a parabolic pattern with soil moisture across sites. All sites were CH₄ sinks; within-site fluxes were largely controlled by soil moisture, whereas fluxes across sites were positively correlated with an integrated index of soil fertility. Soil N₂O fluxes were low throughout the measurement years, but the highest emissions occurred at a mid-precipitation site with high soil N availability. Net negative NO fluxes at the soil surface occurred at all sites, with the most negative fluxes at the low-precipitation site closest to Panama City; this was likely due to high ambient NO concentrations from anthropogenic sources. Our study highlights the importance of both short-term (climatic) and long-term (soil and site characteristics) factors in predicting soil trace gas fluxes.