# Seasonal Net Carbon Exchange in Rotation Crops in the Temperate Climate of Central Lithuania
## 2 Materials and Methods  
_Measurement object and location_. Lithuania is located in the cold temperate zone (5-6) with moderately warm summers and medium cold winters. The average temperature in July is approximately 17 \({}^{\circ}\)C, and in winter, it is approximately \(-5\)\({}^{\circ}\)C; the interval between the temperatures is approximately 20 \({}^{\circ}\)C [24]. Investigations into the seasonal C exchange of conventional farming (CF) leg (L), winter wheat (_Tritcium aestivum_ L.) (W), maize (_Zea mays_ L.) (M), spring rapeseed (_Brassica napus_ L.) (R) and barley (_Hordeum vulgare_) + ley undercrop) (B) were carried out during the growth period in 2013-2016 at the Training Farm of Vytautas Magnus University (former Aleksandras Stulginskis University, 54\({}^{\circ}\)52' N, 23\({}^{\circ}\)49' E), Kaunas district (Table 1). The cropland soil types were _Hapli-Epihypogleyic Luvisol_, _LVg-p-w-ha_, or _Albi-Epihypogleyic Luvisol_, _LVg-p-w-ab_) [25]. Measurementsites were set up every 50-100 m in linear transects oriented in the N-S direction in the fields, at a distance of 20 to 25 m from the edge to avoid the margin effect. The measurement plots in 6 replications were installed at each site. The ley (50% red clover (_Trifolium pratense_ L.) 'Start' and 50% timothy-grass (_Phleum pratense_) 'Jumis') was undersown in the oat 'KWS Contender' (170 kg ha\({}^{-1}\)) and pea (_Pisum sativum_) 'Kiblukai' (50 kg ha\({}^{-1}\)) mixture on 7 May 2013. A 2-cut system was applied in ley (4 June and 11 August 2014; 2 June and 6 August 2015; 8 June and 4 August 2016). To evaluate the crop photosynthetic surface, the crop density (un. M\({}^{-2}\)) and leaf surface area (cm\({}^{2}\) m\({}^{-2}\)) were determined and the leaf area index (LAI, m\({}^{2}\) m\({}^{-2}\)) was calculated in plots of 0.25 m\({}^{-2}\) (0.5 m \(\times\) 0.5 m) in six replications. Fresh plant biomass (FM, g m\({}^{-2}\)) and dry matter content (DM, g m\({}^{-2}\)) were determined by the weighing method. Dry matter content was determined by drying plant samples (80 \({}^{\circ}\)C thermostat (Tritec, Hannover, Germany). _C exchange investigation._ Agroecosystems' seasonal C exchange was investigated by measuring the rate of gross primary production (_GPP_, \(\mu\)mol m\({}^{-2}\)s\({}^{-1}\)) and respiration emissions of soil and autotrophs (R\({}_{s+a}\), \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)) in situ. CO\({}_{2}\) exchange was measured by applying the closed chamber method [16] using LCpro + System analyser (ADC Biosecientific LTD, UK) with a standard 2.5 \(\times\) 2.5 cm = 6.25 cm\({}^{2}\) chamber area for broadleaved species every 7-10 days between 11:00 and 14:00 with regard to the environmental conditions and plant growth stages (BBCH-scale) [26]. For the measurement of soil respirational emissions, the plastic collar fitted to the soil chamber was used. The plastic collar was put into the soil at a depth of 20 mm until it was sealed. Since then the programming console was connected to the soil respiration chamber. _GPP_ presents the total amount of CO\({}_{2}\) that is fixed by the plant in photosynthesis and was assessed biometrically. The light-saturated photosynthetic rate was measured at the saturating irradiance photosynthetic photon flux density PPFD (1500 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)) and an ambient temperature, humidity and CO\({}_{2}\) concentration. The carbon exchange of each agroecosystem was evaluated by net ecosystem production (_NEP_, \(\mu\)mol m\({}^{-2}\)s\({}^{-1}\)) [20] which was calculated as follows [27]:  
\[NEP_{ij}=GPP_{ij}-R_{ij}^{s+a} \tag{1}\]  
where \(NEP_{ij}\) is the net ecosystem production, \(GPP_{ij}\) is the gross primary production and \(R_{ij}^{s+a}\) for crop \(i\) during month \(j\). The values of \(GPP_{ij}\) and \(R_{ij}^{s+a}\) are calculated as the averages over the six replications of measurements as described above. _Meteorological conditions._ Meteorological conditions determine the vegetation of crops, seed germination, plant growth, development, maturity and yield.