## 3 Results
Although the difference in climate zone GPP dynamics was quite similar for both sites, after reaching peak values in mid-June (12 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) for the Kursk field and  
Figure 4: Seasonal variation in daily NEE, Reco and GPP in growing season and the end of 2013 at the EC stations in Moscow (small crosses) and in Kursk (empty dots). Lines represent 7-day running mean values for station in Moscow (dotted line) and for station in Kursk (solid line). 7.5 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) for Moscow), the GPP decreased to 1 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) in the middle of August (the time of harvest) for both sites. The GPP pattern showed several local minima on the gradually declining curve, different in amplitude, but coinciding in locations, for both sites. The first minimum was related to the cold snap in mid-June and the second may be explained by the low SWC values due to low precipitation during the period before the second decade of July (Figure 2). Values of daily GPP different from zero during August and the beginning of September may be explained by the photosynthetic activity of stubble in the fields in Moscow and in Kursk, and some weeds remaining after harvesting in the field in Kursk. The daily Reco values for the Kursk site were constantly above 2.5, and above 3 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) for the Moscow site. However, the daily Reco patterns were similar and matched in dynamic temporal changes. It seems that the depressions are associated with the decreases in SWC and the Reco peaks correspond well with periods of rain. During September, October and November, values of GPP, Reco and NEE for fields were falling, with the decrease in solar radiation and temperature. Each of these two agroecosystems showed a net CO\({}_{2}\) release. During the growing season, the number of net uptake days for the Kursk and Moscow fields was 85 and 67 days, respectively. The GPP and Reco for the growing season were consistently higher at the Kursk site than at the Moscow site: the cumulative GPPs were 613 g C CO\({}_{2}\) m\({}^{-2}\) and 334 g C CO\({}_{2}\) m\({}^{-2}\) (for Kursk and Moscow, respectively) and cumulative Recos were 525 g C CO\({}_{2}\) m\({}^{-2}\) and 248 g C CO\({}_{2}\) m\({}^{-2}\) (for Kursk and Moscow, respectively). On the other hand, cumulative NEE were similar (\(-89\) g C CO\({}_{2}\) m\({}^{-2}\) and \(-86\) g C CO\({}_{2}\) m\({}^{-2}\), respectively, see Table 1.  
GPP, which is directly linked to photosynthesis, is an indicator of biomass production. We have put daily GPP curves together in one plot and have shifted them in order to synchronize the time of planting as the zero time (Figure 5). Although the difference in climate zone GPP dynamics was quite similar for both sites, after reaching the peak values in mid-June (12 g C m\({}^{-2}\) d\({}^{-1}\) for the Kursk field and 7.5 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) for Moscow), the GPP decreased to 1 g C m\({}^{-2}\) d\({}^{-1}\) at the time of harvest for the two sites. This can be explained by the fact that, in mid-June, the barley in Moscow and Kursk was at the stage of grain ripening and the CO\({}_{2}\) uptake activity decreased. Thus, the main GPP dynamic driver was the different crop development stage: the Kursk barley was sown earlier and had a longer growing period than the Moscow barley. Furthermore, higher GPP values were also due to better climate and soil parameters in Kursk: higher PPFD, temperature and soil moisture. Linear mixed modeling was performed to predict GPP dynamics in relation to daily soil temperature at a 5 cm depth, daily photosynthetic available radiation, daily mean soil water content at a 5 cm depth and stages of barley development (see Table 2). Multiple R\({}^{2}\) for the models was 0.87 for the Kursk field and 0.88 for the Moscow field. ANOVA shows that the influence of crop stage is much greater than the influence of other parameters, and in Moscow this effect is two times stronger than in Kursk.