Divergent NEE balances from manual-chamber CO2 fluxes linked to different measurement and gap-filling strategies: A source for uncertainty of estimated terrestrial C sources and sinks?

Manual closed-chamber measurements are commonly used to quantify annual net CO2 ecosystem exchange (NEE) in a wide range of terrestrial ecosystems. However, differences in both the acquisition and gap filling of manual closed-chamber data are large in the existing literature, complicating inter-study comparisons and meta analyses. This data set contains data of a study which compares common approaches for quantifying CO2 exchange at three methodological levels. (i) The first level included two different CO2 flux measurement methods: one via measurements during mid-day applying net coverages (mid-day approach) and one via measurements from sunrise to noon (sunrise approach) to capture a span of light conditions for measurements of NEE with transparent chambers. (ii) The second level included three different methods of pooling measured ecosystem respiration (RECO) fluxes for empirical modeling of RECO: campaign-wise (single-measurement-day RECO models), season-wise (one RECO model for the entire study period), and cluster-wise (two RECO models representing a low and a high vegetation status). (iii) The third level included two different methods of deriving fluxes of gross primary production (GPP): by subtracting either proximately measured RECO fluxes (direct GPP modeling) or empirically modeled RECO fluxes from measured NEE fluxes (indirect GPP modeling). Measurements were made during 2013-2014 in a lucerne-clover-grass field in NE Germany. Besides modelled half-hourly RECO, NEE and GPP, measured CO2-fluxes, plant height and C content of aboveground biomass as well as weather conditions are given.