On the role of advection for the net carbon dioxide exchange of ecosystems

Land ecosystems are presently absorbing around 25% of the carbon dioxide (CO2 ) emitted into the atmosphere through human activities and thus buffer global warming. Understanding whether land ecosystems will continue to provide this ecosystem service in the future, requires accurate estimates of their carbon dioxide source/sink strength. During the past 20 years, the eddy covariance (EC) method has become a widely used technique to quantify the net ecosystem CO 2 exchange (NEE) over time scales from hours to decades. The EC method in the way it is generally implemented, however, is prone to underestimate the true NEE during nighttime conditions, when advection of CO 2 is likely to represent a significant component of the CO 2 mass balance. Previous attempts to quantify the advection of CO 2 during nighttime and thus the full CO 2 mass balance focussed on forest ecosystems and were overall not very successful. In this project we propose to quantify advection and thus the full mass balance of CO 2 for a range of mountain grassland ecosystems in complex terrain. The contribution of advection to the carbon balance of short-statured canopies is unknown at present. The proposed project will be among the first to provide advection estimates for short-statured ecosystems and thereby contribute towards reducing the uncertainty in their NEE and assessing whether the limitations of previous advection experiments at forested sites apply to short canopies as well. Constrained by limitations in the experimental setup, previous advection experiments in forest ecosystems could not systematically test for the spatial scale at which advection can be quantified best and we hypothesise this to be a major reason for some of the earlier discouraging results. The present project will provide for the first time a systematic assessment of the effects of spatial scale on advection experiments. Previous advection experiments in forest ecosystems were also conceptually hampered by a lack of a quantitative reference or "ground-truth", i.e. a quantitative estimate of ecosystem-scale NEE that the full mass balance could be compared against. The proposed project overcomes this shortcoming by using continuous ecosystem-scale chamber measurements which, within their limits, will be used as a reference for the flux measurements.

This project studied the contribution of advection to the ecosystem- atmosphere exchange of CO2 over short-statured grasslands in the Alps. The advections terms are usually ignored in most CO2 flux estimations in the literature and in routine observations of monitoring networks, which may cause an underestimation of the true ecosystem-atmosphere CO2 exchange.We designed an automatic advection measurement system and applied it during five field campaigns across the alpine regions in Austria, Italy, and Germany. Significant contributions of advection to the total ecosystem-atmosphere CO2 exchange were found during both daytime and night-time. Advection contributed more significantly to the CO2 flux at night time than at daytime at most sites, and the above-canopy advection contributed more than within- canopy advection due to short canopy heights.This work improves the estimation of ecosystem-atmosphere CO2 exchange, help to better understand the global carbon cycle, and provide supporting information for the global warming and climate change communities.