## 3 Results
The NEE moved from a positive value (release) to a negative value (uptake), and CO\({}_{2}\) uptake increased from 7:00 h gradually, until peak values were achieved between 11:00 and 14:00 h. CO\({}_{2}\) uptake then declined through the afternoon and turned to a release of CO\({}_{2}\) after 19:00 h. The duration of positive and negative values changed clearly across different months, because of their differences in photoperiod (the time between sunrise and sunset). Average NEE was negative in June and July for both fields. Low uptake in August can be clearly explained by harvesting. There was no significant uptake or diurnal dynamics of CO\({}_{2}\) in autumn and winter. Mean values of CO\({}_{2}\) release during this period were about 1 \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) for the Moscow field and about 2.5 \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) for the Kursk field. Significant differences in diurnal variations of NEE between the two eddy covariance stations were found in May, June, July and September. Significant differences in August were observed only for daytime. During the growing season, daily NEE, Reco (ecosystem respiration) and GPP showed distinct seasonal patterns at both fields (Figure 4), which indicates their responses to the combined effects of climate and zonal field locations. During early May, the daily GPP and Reco values for the Moscow crop site were low; and the daily NEE values at the site were about 1 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) (released CO\({}_{2}\) to the atmosphere) (Figure 4). From the middle of May, the NEE started to grow to the value of 2 g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\). The increase driver was Reco, while GPP was still about zero. The dynamics of all three variables  
Figure 3: Diurnal patterns of monthly averaged net ecosystem CO\({}_{2}\) exchange (NEE) at the eddy covariate station in Moscow (triangles) and station in Kursk (empty dots) during the growing season and end of year, 2013. Black bars denote 95% confidence intervals of the hour averages. were quite different for the Kursk field because the development of barley, in this case, began earlier: at the beginning of May, the barley was already at the milky-wax ripeness phase, so the daily NEE, Reco and GPP show higher values. With biomass development and temperature increase, the GPP and Reco at these two sites increased gradually and reached their peak values in mid-June for the Moscow field, and in early June for the Kursk field. Simultaneously, the NEE decrease continued until the same dates when it reached its negative peak: \(-4.0\) g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) and \(-6.2\) g C CO\({}_{2}\) m\({}^{-2}\) d\({}^{-1}\) for the Moscow and Kursk fields, respectively. The NEE peaks coincided with high PPFD and still high, but decreasing, SWC (Figure 2) for both agroecosystems. The NEE, at the Moscow site, retained approximately the same values until the end of June and started to grow in the beginning of July, reaching its maximal summer values at the beginning of August and then slowly decreasing until the end of the year. At the Kursk site, the NEE started to grow immediately after it had reached its negative peak at the beginning of July; it crossed the zero point at the end of June, after which point only the CO\({}_{2}\) release to atmosphere was observed. The NEE showed several positive peaks coinciding with periods of relative warming during the second part of the summer and the beginning of the autumn. The differences in moving average values were significant. The 7-day running average of daily NEE showed the transition from CO\({}_{2}\) adsorption to CO\({}_{2}\) release to the atmosphere on 30 June for the tower at Kursk and 26 July for the tower in Moscow. From the end of July to the beginning of September, the field in Moscow is characterized by much lower values of daily NEE compared with the Kursk field.