Carbon balance of surfaces vs. ecosystems

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract An automated open system for measurement of soil CO 2 efflux (R sc) was developed and calibrated against known fluxes and tested in the field, while measuring soil respiration also by the gradient method (R sg) at a dry sandy grassland (Bugac, Hungary). Ecosystem respiration (R eco) was measured by the eddy covariance technique. Small 5 chamber size (5 cm in diameter) of the chamber system made it possible to use the chambers also in vegetation gaps, thereby avoiding the necessity of removing shoots, the disturbance of 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. While the correlation be-10 tween ecosystem and soil CO 2 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 CO 2 m −2 s −1) range, probably due to the larger than assumed storage flux. The gradient method showed both up and downward CO 2 fluxes originating from the main rooting zone after rains. Downward fluxes within the soil profile amounted to 15% of 15 the simultaneous upward fluxes and to ∼ 7.6% of the total (upward) effluxes during the 3 months study. The upper 5 cm soil layer contributed to ∼ 50% of the total soil CO 2 efflux. The continuously operated automatic open chamber system and the gradient system makes possible the detection of situations when the eddy system underestimates R eco , gives the lower limit of underestimation (chamber system) and helps in 20 quantifying the downward flux component of soil respiration (gradient method) between the soil layers. These latter (downward) fluxes are expected to seriously affect (1) the R eco vs. temperature response functions and (2) the net ecosystem exchange of CO 2 (NEE) vs. photon flux density response functions, therefore potentially affecting also the gap filling procedures and to led to a situation (3) when the measured surface and 25 the real time ecosystem fluxes will necessarily differ in the short term. Simultaneous measurements of R eco and soil CO 2 effluxes may reveal the time and degree of the above …