Interannual variability of CO2 fluxes in a natural grassland in the southern Brazilian Pampa biome

The Pampa is a specific biome located in southern America, with vegetation mainly composed of grasses interspersed with small gallery forests and riparian forests. The use of the pampa Biome native fields for livestock has been used for centuries and is seen today as the main tool to combat environmental degradation, maintaining the local flora and fauna protected from the advance of commercial agriculture. Although several studies suggest that natural pastures can act as important sinks of atmospheric carbon dioxide (CO2), the livestock productivity of the fields must be improved from an economic point of view. In this sense, studies have been carried out to maximize livestock productivity, reducing the environmental impacts on the native Pampa. One system that has shown productivity increase is the rotational management, which consists of dividing the total area into small subdivisions through which the herd will be directed to maintain an amount of biomass that conserves the flora diversity of the fields with an adequate stocking rate. In this work, we analyzed six years of the net ecosystem exchange (NEE) of CO2, through Eddy Covariance (EC) measurements, in a livestock area with rotational management in the Pampa biome in the State of Rio Grande do Sul, southern Brazil. The ecosystem was a sink of atmospheric CO2 with annual accumulated values for 2015, 2016, 2017, 2018, 2019, and 2020 of -519, -83, -372, -144, -138, and -321 g C m-2 y-1, respectively. The NEE annual variability is related to the region's climate variability, whose climate, according to Köppen's classification, belongs to the Cfa zone, humid temperate with hot summers and cold and mild winters, susceptible to drought and frost events. The highest annual CO2 absorptions occurred in years with higher volumes of precipitation and less severe winters, while the lowest absorptions occurred in years with lower annual volumes of precipitation, and more severe winters, i.e., with a higher number of frosts. These results are important for understanding the CO2 exchange dynamics and showing that the Pampa biome can be an important sink of atmospheric CO2, producing quality animal protein and preserving the local fauna and flora.