On the role of advection for the net ecosystem carbon dioxide exchange of a subalpine grassland

At the majority of FLUXNET sites the net ecosystem exchanges of energy and carbon dioxide are quantified based on a simplified, one-dimensional mass balance, which neglects advective flux contributions. In particular in complex terrain and nighttime conditions, this approach is questionable and thought to cause an underestimation of the true net ecosystem exchange. For carbon dioxide, which typically is taken up during daytime and released during nighttime, this is likely to cause a systematic bias. Here we report on an experiment at a subalpine mountain grassland in Northern Italy situated in truly complex terrain characterized by slopes of different inclination and exposition. During the month-long experiment we attempted to quantify all terms of the full three-dimensional carbon dioxide mass balance and compared these with concurrent ecosystem respiration measurements by automated chambers. The main findings of this study can be summarized as follows: (i) the sum of the vertical covariance term and the storage term considerably underestimated nighttime ecosystem respiration as measured by the automated ecosystem chambers; (ii) advection measurements indicate that both horizontal and (less so) vertical advection were important terms of the full mass balance during nighttime; (iii) the net ecosystem carbon dioxide exchange calculated by taking into account all terms, i.e. including advection, closely resembled nighttime ecosystem respiration as measured with the automated ecosystem chambers; (iv) there was substantial spatial variability in the vertical covariance term during nighttime; (v) during daytime, advection appeared to make a negligible contribution to net ecosystem carbon dioxide exchange.