# Seasonal Net Carbon Exchange in Rotation Crops in the Temperate Climate of Central Lithuania
## 3 Results and Discussion  
The variations of C exchange and plant growth are closely related with the meteorological conditions o the f growth period and soil chemical properties. Average air temperatures ranged between 2.7 and 19.8 \({}^{\circ}\)C, depending on the season and month (Table 1). A strong positive correlation (\(r\) = 0.6 and \(r\) = 0.8; \(p\) = 0.02, respectively) between respiration (R\({}_{\text{a}}\) and R\({}_{\text{s}}\)) and air temperature in agroecosystems was found. The vegetation period plant respiration (R\({}_{\text{a}}\)) varied from 0.202 in March to 2.384 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) in August (Figure 1). Soil respiration (R\({}_{\text{s}}\)) ranged between 0.225 and 2.526 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) and composed an insignificant part in the total carbon exchange (Figure 2); however, it was 18% higher than the plant respiration R\({}_{\text{a}}\). The maximum mean R\({}_{\text{s}}\) (1.453 and 1.405 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\)) and R\({}_{\text{a}}\) (1.389 and 1.345 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\)) were determined in maize and ley agroecosystems, respectively. The maximum respiration emissions were 4.637\(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) of ley in July and 4.232 \(\upmu\)mol m\({}^{-2}\)s\({}^{-1}\) of maize in August at the maturity stage (71-89 BBCH). A negative correlation was determined between precipitation and R\({}_{\text{a}}\) (\(r\) = \(-\)0.6) or R\({}_{\text{s}}\) (\(r\) = \(-\)0.7, \(p\) = 0.003). The related descriptions [20; 27; 28] revealed respiration and plant physiological responses to abiotic stress, such as drought or heat. Though the climate change leads to soil moisture loss [12; 28], soil moisture surplus in autumn and spring determined unfavourable anaerobic conditions for soil biota and thus decreased the soil respiration R\({}_{\text{s}}\) in our temperate climate. However, the summer precipitation deficiency reduced the soil respiration R\({}_{\text{s}}\) rates even though the air temperature T\({}_{\text{a}}\) increased. Figure 1: The variation of the plant respiration (R\({}_{\text{a}}\)) 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). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Year/Month 03** & **04** & **05** & **06** & **07** & **08** & **09** & **10** & **11** & **Mean** \\ \hline
2013 & & & & & & & 2.81 & 1.62 & 4.15 & 2.86 \\
2014 & 1.83 & 0.78 & 2.03 & 1.13 & 0.82 & 2.02 & 1.98 & 1.61 & 4.15 & 1.81 \\
2015 & 3.21 & 2.15 & 1.30 & 0.35 & 1.34 & 0.11 & 1.32 & 1.96 & 3.46 & 1.68 \\
2016 & 2.94 & 1.85 & 0.77 & 1.62 & 2.93 & 2.18 & 0.6 & 2.69 & & 1.94 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The hydrothermal coefficients (HTK) of 2013–2016. The plant growth and their bio-parameters vary seasonally, dependently on the meteorological and agrotechnical conditions. The crop density exposed a significant parameter in light energy transfer, forming the initial carbon fixation and exchange [11; 21]. The mean crop density formed the optimal and ranged from 12 un. m\({}^{-2}\) in maize and rapeseed to 106 un. m\({}^{-2}\) in the key, wheat and barley agroecosystems. The mean LAI values (which forms carbon assimilation surface) were 0.663, 0.798, 1.115, 0.883, 0.478 m\({}^{2}\) m\({}^{-2}\) with mean biomass (FM) values of 75.06, 71.17, 303.2, 217.6, 62.4 g m\({}^{-2}\) for the key, wheat, maize, rapeseed and barley agroecosystems, respectively. According to previous outcomes [29], the ecosystem LAI depends on the environmental conditions that determine the seasonal intensity of the physiological processes at different growth stages. The highest rates of the maize LAI exhibited the most intensive growth and C assimilation, thus exceeded 41% in ley, 28% in wheat, 21% in rapeseed and 57% in barley LAI. Similar LAI tendencies based on long-term research was observed by former researchers as well [5; 15].