Nocturnal carbon efflux:: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique

Micrometeorological measurements made on single towers often underestimate nighttime respiration of terrestrial ecosystems because they cannot account for vertical and horizontal advection, thereby causing systematic errors in estimates of net ecosystem carbon exchange. We show that there is a maximum in the sum of the turbulent flux and change in storage of CO2 in the early evening, R-max, that is in close agreement with concurrent and independent estimates of net carbon exchange from soil and plant chambers. We hypothesize that the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. We propose taking advantage of this time delay to develop relationships between R-max and soil temperature and moisture. The new parameterization leads to realistic values of nighttime respiration, and therefore to improved estimates of net ecosystem exchange.