Spatial and temporal variation of CO₂ efflux along a disturbance gradient in a <i>miombo</i> woodland in Western Zambia

Abstract

Carbon dioxide efflux from the soil surface was measured over a period of several weeks within a heterogeneous Brachystegia spp. dominated miombo woodland in Western Zambia. The objectives were to examine spatial and temporal variation of soil respiration along a disturbance gradient from a protected forest reserve to a cut, burned, and grazed area outside, and to relate the flux to various abiotic and biotic drivers. The highest daily mean fluxes (around 12 μmol CO2 m−2 s−1) were measured in the protected forest in the wet season and lowest daily mean fluxes (around 1 μmol CO2 m−2 s−1) in the most disturbed area during the dry season. Diurnal variation of soil respiration was closely correlated with soil temperature. The combination of soil water content and soil temperature was found to be the main driving factor at seasonal time scale. There was a 75% decrease in soil CO2 efflux during the dry season and a 20% difference in peak soil respiratory flux measured in 2008 and 2009. Spatial variation of CO2 efflux was positively related to total soil carbon content in the undisturbed area but not at the disturbed site. Coefficients of variation of efflux rates between plots decreased towards the core zone of the protected forest reserve. Normalized soil respiration values did not vary significantly along the disturbance gradient. Spatial variation of respiration did not show a clear distinction between the disturbed and undisturbed sites and could not be explained by variables such as leaf area index. In contrast, within plot variability of soil respiration was explained by soil organic carbon content. Three different approaches to calculate total ecosystem respiration (Reco) from eddy covariance measurements were compared to two bottom-up estimates of Reco obtained from chambers measurements of soil- and leaf respiration which differed in the consideration of spatial heterogeneity. The consideration of spatial variability resulted only in small changes of Reco when compared to simple averaging. Total ecosystem respiration at the plot scale, obtained by eddy covariance differed by up to 25% in relation to values calculated from the soil- and leaf chamber efflux measurements but without showing a clear trend.