One of the methods to study the exchange of mass and energy between the atmosphere and the surface of various terrestrial ecosystems is direct measurement of the fluxes of greenhouse gases. Particular attention is focussed on carbon dioxide whose concentration in the atmosphere dramatically increases [Urbaniak 2006]. The paper presents results of the measurements of carbon dioxide exchange conducted on 29th of June 2011 on three experimental plots (alfalfa, winter wheat and potato crops) situated in Agricultural Experimental Station in Brody (Wielkopolskie Province) and modeled values of ecosystem respiration ( Reco ), net ecosystem exchange ( NEE) and gross ecosystem production ( GEP). Measurements were carried out by means of closed dynamic chamber system. Reco measured on 29th of June 2011 ranged from 3.61 to 16.62 mol CO 2.m -2.s -1 and measured NEE - from 4.22 to -24.08 mol CO 2.m -2.s -1. Modeled values of the ecosystem respiration ( Reco ) for particular crops obtained with the LLOYD and Taylor [1994] function did not differ from measured values by more than 4% on average. The results of NEE modeling indicate that selected model [Michaelis, Menten 1913] worked well for sites with alfalfa and winter wheat crops (mean absolute percentage error - MAPE was 25.0 and 7.3%, respectively). NEE predictions for the site with potato crop differed largely ( MAPE=98.5%) from measured values and hence there is a need for looking for a model that would consider, apart from PPFD, also other environmental factors driving photosynthesis.
