## 4 Discussion  
The cumulative NEE, GPP and Reco values calculated throughout the vegetative season (118 and 129 days for Kursk and Moscow, respectively) show that the Kursk field has the higher GPP compared with the Moscow field (613 and 334 g CO\({}_{2}\) m\({}^{-2}\), respectively) and a higher Reco compared with Moscow (525 and 248 g CO\({}_{2}\) m\({}^{-2}\), respectively). The differences in GPP and Reco are explained by the more favorable conditions at the Kursk site, due to its mild climate and more available energy radiation. Both GPP and Reco, being higher in Kursk, balance out in terms of carbon sequestration, yielding a similar value (\(-86\) and \(-89\) g C CO\({}_{2}\) m\({}^{-2}\), respectively, for the vegetative season). Despite similar carbon sequestration values, the grain yields of the two sites differ significantly: the Kursk site yield is 2.85 t ha\({}^{-1}\), while that of the Moscow site is 4.08 t ha\({}^{-1}\). Our reported yield in Moscow is slightly above the average yield for this crop in the region [26]. This could be explained by the technological conditions applied, since the site in Moscow belongs to the university experimental station. On the other hand, according to [PERSON] et al. [27], the barley crop yield on experimental farms in the Kursk region, with a good technology protocol, should range from 7.2 to 3.5 t dry matter in relation to climate variability. Thus, the reported yield in our experiment of 2.85 t ha\({}^{-1}\) is significantly lower than the average for the region, even in unfavorable climate conditions. This result can be explained by the lack of weed control treatments on the Kursk region field. The biomass of weeds ranged from 40 to 60% of the total, contributing to a larger extent to greater GPP, Reco and low yield differences. Weed photosynthetic and respiration enhancement occurs at the expense of lower carbon efficiency on the development of grain yields.  
The slope of GPP versus PPFD is somewhat different between the two sites, showing a higher light-use efficiency in Moscow (0.86) than in Kursk (0.67). Also, this result could explain the better response of the Moscow yield, particularly at the onset of the growing season when light is more limiting.  
Thus, the investigated fields not only differ in climate conditions, but they also represent two typical agronomic models of Russia, the highly intensive model in Moscow and a lower input, less intensive, semi-organic model in Kursk. In southern taiga zone, represented by the field in Moscow, only the intensive form of farming is economically profitable; it is not possible to find other forms of farming at present because the land is simply abandoned. In the Chernozem zone there may be different forms of farming, but the semi-organic form is more common there.  
This study with two eddy covariance stations situated within two contrasting climate zones of Central Russia, on the same barley crop, showed significant differences in the behavior of the carbon balance components. In our previous investigation of the Moscow site, its carbon balance was compared to adjacent fields with vetch and oat mix being grown [28]. The maximum NEE difference between the Moscow and Kursk fields reached 2.2 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\), while for two neighboring sites in Moscow, the maximum difference was only 0.3 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\). Considering the differences in Reco and GPP between Moscow and Kursk, the observed differences for the two neighboring Moscow fields with different crops, are negligible. Thus, differences in the behavior of the carbon balance components for the two towers in Moscow and Kursk were determined to a large extent by their location in different natural zones.