## 4 Discussion  
No chemical or organic fertilizers are applied in the natural pasture area of this study, only manure and urine from grazing animals. No sowing or forage cutting is performed with machines, and the cattle in the grazing area did not receive supplementation during the study period. Therefore, the carbon input into the ecosystem is only through carbon uptake by plants or autotrophic microorganisms. This pasture represents the traditional cattle production system in the Brazilian Pompa biome, which covers roughly 6.5 million hectares of land, with an estimated herd of 5.5 million cattle. Another characteristic of this biome is that it is located in a subtropical region and therefore subject to significant meteorological variations on seasonal and yearly scales. During the data collection period, the climatic phenomena El Nino/La Nina occurred, which are responsible for higher/lower precipitation in this region, respectively. According to the ENSO classification, 2015 and 2016 were marked by the moderate action of the El Nino phenomenon, providing above-average precipitation. In 2020 and 2021, the La Nina phenomenon occurred, which explains the low precipitation in the study area [48]. Therefore, the data presented in this work reflect these climatic variations and the consequent adjustments of animal stocking rates in the area, resulting from the influence of climatic variations in pasture growth. The CO\({}_{2}\) dynamics of this pasture show marked seasonality, with higher GPP and Reco values in the spring-summer and lower in the autumn-winter (Figure 4). This behavior results from the seasonal variation in solar radiation and air temperature variation in this subtropical region. The dominant herbaceous plants in the natural pasture of the Brazilian Pompa biome are of summer growth [2]. In the autumn-winter seasons, they decrease their photosynthetic rate and accumulate senescent material. During autumn, the GPP and Reco values gradually decrease in all the analyzed years, reaching minimum values in June and July, with the lowest incidence of solar radiation and lower air temperature; this period is also marked by frost (Table 2). At the end of winter, solar radiation intensifies, gradually increasing the temperature, which is associated with regular precipitation, and increases the plant growth rate [50]. At the end of winter, GPP and Reco values increase, and the ecosystem begins to absorb CO\({}_{2}\) (i.e., NEE negative in Figure 4c). In the six years evaluated, the pasture in the rotational system behaved as a CO\({}_{2}\) sink, with an average NEE of \(-207.6\) g C m\({}^{-2}\) y\({}^{-1}\)\(\pm\) 118.0 g C m\({}^{-2}\) year\({}^{-1}\). These data have a high degree of reliability since the total uncertainty on the EC NEE data was below 10%, with a maximum of 31 g C m\({}^{-2}\) y\({}^{-1}\), thereby corroborating [37; 44; 51]. Other studies with the EC method have shown that grassland ecosystems have mostly behaved as CO\({}_{2}\) sinks. Plant growth is also stimulated as the animals remove part of the leaves on the soil surface, leading to a more significant GPP increase than Reco [15]. [PERSON] et al. [52] found an average NEE of \(-165.3\pm 50.5\) g C m\({}^{-2}\) y\({}^{-1}\) (2008-2011) in an intensively managed pasture with fertilizer addition and year-round rotational grazing in a temperate climate region of New Zealand. [PERSON] et al. [51] reported an average NEE of