# Seasonal Net Carbon Exchange in Rotation Crops in the Temperate Climate of Central Lithuania
## 3 Results and Discussion
The recorded alterations of the crops' bio-parameters might be caused by the different biological characteristics, morphological structures and physiologies of the plants. Maize belongs to the C\({}_{4}\) photosynthesis type plant [30], which has a different leaf anatomic structure, chloroplast size and photosynthesis intensity that are higher than C\({}_{3}\) type crops, i.e., ley, wheat, rapeseed and barley [27]. Differences in crop density, LAI and biomass determined the photosynthetic surface and volume of the assimilated CO\({}_{2}\) (_GPP_ and _NEP_) in agroecosystems. Hence, ley and maize assimilated and accumulated the highest CO\({}_{2}\) rate, i.e., 19.22 umol m\({}^{-2}\)s\({}^{-1}\) and 18.09 umol m\({}^{-2}\)s\({}^{-1}\) in July and August respectively, when the formation of LAI and biomass exceeded the maximum (Figure 3). The strong correlation between T\({}_{\mathrm{a}}\) and _GPP_ (\(r\) = 0.7, \(p\) = 0.001) confirmed that the seasonal temperature fluctuations resulted in the average seasonal _GPP_ alteration in the maize, ley, wheat, rapeseed and barley agroecosystems. Therefore, the highest rates of photosynthetically assimilated CO\({}_{2}\), i.e., 19.22 umol m\({}^{-2}\)s\({}^{-1}\) in ley, 18.7 umol m\({}^{-2}\)s\({}^{-1}\) in maize and 17.31 umol m\({}^{-2}\)s\({}^{-1}\) in the barley agroecosystems were determined in July when the mean temperature T\({}_{\mathrm{a}}\) (16.1-18.7 \({}^{\circ}\)C) was recorded to be the highest. Seasonal changes in the rates of assimilated CO\({}_{2}\) (_GPP_) can be attributed to the seasonal variation of FM, DM and LAI at different growth stages. This was confirmed by the strong correlation between _GPP_ and LAI (\(r\) = 0.8) in the assessed agroecosystems. Greater mean _GPP_ rates were recorded in maize (15.68 umol m\({}^{-2}\)s\({}^{-1}\)) and ley (14.28 umol m\({}^{-2}\)s\({}^{-1}\)) than those in the wheat (12.80 umol m\({}^{-2}\)s\({}^{-1}\)), rapeseed (14.25 umol m\({}^{-2}\)s\({}^{-1}\)) and barley (12.77 umol m\({}^{-2}\)s\({}^{-1}\)) agroecosystems due to the different bio-parameters and growth periods. Noteworthy is that the growth period of wheat, rapeseed and barley was significantly shorter earlier in the maturity stage than that of maize and ley with long-lasting growth. Figure 2: The variation of the soil respiration (R\({}_{\mathrm{s}}\)) in ley (L), winter wheat (W), spring rapeseed (R), maize (M) and barley (B) agroecosystems during the growing period. Mean \(\pm\) SE, \(p\) < 0.05). The numbers above indicate the plant growth stages (BBCH-scale). Even though the rates of the assimilated carbon (\(GPP\)) varied during the vegetation period and depended on biotic and abiotic parameters, the total amount of carbon sink by plants (\(NEP\)) exceeded the total respiration rate several times, thus, the agroecosystems significantly reduced the atmospheric CO\({}_{2}\) concentration (Figure 3). For the evaluation of the crops contribution to climate change, it is important to determine the net CO\({}_{2}\) exchange estimated by the \(NEP\) value in the research. \(NEP\) varied correspondingly to the environmental conditions and bio-parameters, particularly to LAI (\(r\) = 0.8) during the growth period. Such responses were reported by other researchers [8; 31; 32]. The seasonal temperate climate [33; 34] principally conditioned the seasonal character of CO\({}_{2}\) fluxes in the investigated agroecosystems. The mean \(NEP\) values responded to the seasonal climate, thus increasing from 6.22 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) to 10.09 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) in spring, and from 11.91 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) to 18.06 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) in the summer season. Nonetheless, \(NEP\) decreased from 14.24 to 6.38 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) due to the declining meteorological conditions in the autumn season.