Soil–atmosphere exchange of N2O, CO2 and CH4 along a slope of an evergreen broad-leaved forest in southern China

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At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N2O), carbon dioxide (CO2) and methane (CH4) was estimated based on a few chambers located in a limited area. Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N2O and CO2, but take up less atmospheric CH4 than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N2O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N2O ranged from 2.0 to 4.4 kg N ha−1 year−1, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N2O emissions in the laboratory incubated soils varied with slope positions. Soil CO2 release and CH4 uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N2O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling.

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