Carbon fluxes of surfaces vs. ecosystems: advantages of measuring eddy covariance and soil respiration simultaneously in dry grassland ecosystems

Abstract. An automated open system for measurement of soil CO₂ efflux (R_sc) was developed and calibrated against known fluxes. The system was tested in the field, while estimating soil respiration simultaneously by the gradient method (R_sg) at a dry, sandy grassland site (Bugac, Hungary). Ecosystem respiration (R_ego) was measured using the eddy covariance technique. The small chamber size (5 cm in diameter) made it possible to use the chambers in vegetation gaps, thereby avoiding the necessity of removing shoots and disturbing the spatial structure of vegetation and the upper soil layer. Low air flow rates associated with small chamber volume and chamber design allowed the overpressure range to stabilize between 0.05–0.12 Pa. The correlation between ecosystem and soil CO₂ efflux rates as measured by the independent methods was significant, R_eco rates were similar or even lower than R_sc in the low flux (up to 2 μmol CO₂ m⁻² s⁻¹) range but the differences were within the uncertainty limits for the two fluxes. R_sc from trenched and non-trenched plots amounted to 16 % and 44 % of R_eco, respectively. The gradient method showed both up and downward CO₂ fluxes originating from the main rooting zone after rains. Diffusive retardation played a smaller role than CO₂ production considering the soil air CO₂ concentration increase after rains in a given layer. Downward fluxes within the soil profile amounted to 15 % of the simultaneous upward fluxes and to ~7.6 % of the total (upward) effluxes during the 3-month study. The upper 5 cm soil layer contributed to ~50 % of the total soil CO₂ efflux. Downward fluxes are expected to seriously affect (1) the R_eco vs. temperature response functions and (2) the net ecosystem exchange of CO₂ (NEE) vs. photon flux density response functions, therefore potentially affecting the gap filling procedures and to lead to a situation (3) when the measured surface and the real time ecosystem fluxes will necessarily differ in the short term. Simultaneous measurements of R_eco and soil CO₂ effluxes may reveal the timing and magnitude of the decoupling, thereby contributing to decreasing uncertainty associated with eddy flux measurements over flat terrains. While the correlations between CO₂ effluxes measured by independent systems are strong, R_sg was generally larger than R_sc or R_eco, mainly due to overestimation of effective diffusivity in the soil.