A long-standing challenge in studying the global carbon cycle has been understanding the factors controlling inter–annual variation (IAV) of carbon fluxes related to vegetation photosynthesis and respiration, and improving their representations in existing biogeochemical models. Here, we compared an optimality-based mechanistic model and a semi-empirical light use efficiency model to understand how current models can be improved to simulate IAV of gross primary production (GPP). Both models simulated hourly GPP and were parameterized for (1) each site–year, (2) each site with an additional constraint on IAV (CostIAV), (3) each site, (4) each plant–functional type, and (5) globally. This was followed by forward runs using calibrated parameters, and model evaluations at different temporal scales across 198 eddy covariance sites. Both models performed better on hourly scale than annual scale for most sites. Specifically, the mechanistic model substantially improved when drought stress was explicitly included. Most of the variability in model performances was due to model types and parameterization strategies. The semi-empirical model produced statistically better hourly simulations than the mechanistic model, and site–year parameterization yielded better annual performance for both models. Annual model performance did not improve even when parameterized using CostIAV. Furthermore, both models underestimated the peaks of diurnal GPP in each site–year, suggesting that improving predictions of peaks could produce a comparatively better annual model performance. GPP of forests were better simulated than grassland or savanna sites by both models. Our findings reveal current model deficiencies in representing IAV of carbon fluxes and guide improvements in further model development.

not-yet-known not-yet-known not-yet-known unknown Carbonyl sulfide (COS), an atmospheric gas used as a tracer in carbon cycle studies, has an inferred missing sink in high Northern latitudes. Boreal COS budgets typically account for the contribution by forests and ignore any uptake that wetland ecosystems, widespread in Northern latitudes, may contribute. The first direct measurements of the ecosystem-atmosphere COS exchange of a boreal wetland, presented here, demonstrate their likely importance in that Northern latitude COS budgets. The investigated wetland (Siikaneva, Finland) took up on average 11 pmol m−2s−1 COS, which was ~72 % of the nearby boreal forest COS uptake. During nighttime, the COS uptake rates were similar at both sites. Upscaling our measurements to the boreal region using the ORCHIDEE model revealed in a Northern wetland sink of ~13 Gg S/y, changing the budget model output from a small source to a COS sink impacting Northern latitudes carbon uptake estimates based on COS.

Lakes have a controversial climate footprint. In fact, they are a sink of CO2 but at the same time they can be an important source of CH4. Indeed, a global synthesis of methane emission data (Bastviken et al., 2011) suggests that freshwater ecosystems - in particular lakes - may be much larger sources of methane than previously thought, questioning current methane budgets and the general role of freshwater ecosystems in the greenhouse gas balance. The main objective of this study is to improve data availability and quality regarding methane emissions from lakes in the Alpine region - a region that presently is heavily under-represented in global data sets - in order to allow a robust assessment of their role in the global greenhouse gas balance. This is of fundamental importance for the assessment of CH4 emissions from regions particularly sensitive to an increasing climatic variability. Aiming at spatial and temporal representativeness of flux measurements, we made use of an innovative mobile eddy covariance system. We installed the instruments on a small boat, and we performed measurements while cruising. Meteorological and bio-physical data got recorded simultaneously to investigate drivers of gas fluxes by means of empirical modelling. Additionally, we made use of classical chamber measurements for validating our approach. In this fashion, we investigated a number of natural and man-made lakes across a transect of two degrees of latitude across the Alps. In fact, the alpine region provides a unique opportunity to assess the role of environmental drivers on GHG emissions over a limited latitudinal range in an altitude-for-time substitution manner. We repeatedly visited target lakes across the ice-free season during the years 2018 and 2019. We demonstrated that our method is valid for capturing methane emissions from different pathways (diffusion but also ebullition and transport through vegetation). We found that most of the lakes are supersaturated and the highest emissions were measured in shallow and eutrophic lakes at low altitude. In conclusion, with this study we were able to develop new insights on the role of freshwater ecosystems in the global methane budget. References: Bastviken D, Tranvik LJ, Downing JA, Crill PM, Enrich‐Prast A (2011). Freshwater methane emissions offset the continental carbon sink. Science 331, 50.

As lakes receive and transform significant amounts of terrestrial carbon, they often act as source of atmospheric CO2. Yet, long-term measurements of lake-atmosphere CO2 exchange with high temporal resolution are sparse. In this study, we measured the CO2 exchange of a small lake situated in complex mountainous topography in the Austrian Alps continuously for one year. We used the eddy covariance (EC) and the boundary layer model (BLM) approaches to estimate the lake’s CO2 source or sink strength and to analyze differences between these methods. Overall, CO2 fluxes were small and EC measurements indicated influence of low-frequency contributions. Results from both the EC and the BLM methods indicated the lake to be a small source of atmospheric CO2 with highest emissions in fall. During night-time, the CO2 concentration gradient at the air-water interface decreased due to an increase in atmospheric CO2 above the lake, likely caused by cold and CO2-rich air draining from the surrounding land. Consequently, BLM fluxes were lower during night-time than during daytime. This diel pattern was lacking in the EC flux measurements because the EC instruments deployed at the shore of the lake did not capture low nocturnal lake CO2 fluxes due to the local wind regime. Overall, this study exemplifies the relevance of the surrounding landscape for lake-atmosphere flux measurements. We conclude that estimating CO2 evasion from lakes situated in complex topography needs to explicitly account for biases in EC flux measurements caused by low-frequency contributions and local wind systems.