Efeitos da reinundação de sedimentos expostos por secas prolongadas sobre as emissões de carbono em reservatórios da região semiárida

Abstract

The increased occurrence and intensity of drought events in the semiarid regions has contributed considerably to the regular and prolonged exposure of reservoir sediment to the atmosphere. This extreme hydrological scenario may affect the processing of organic matter and alter greenhouse gas (GHG) emission rates in reservoirs, as these aquatic ecosystems play an essential role in carbon cycling (C). In addition, during extreme drought events in the Brazilian semi-arid region, exposed sediment from the dry reservoir drainage area are transformed into agricultural cultivation sites due to the large amount of stored phosphorus. This practice defines new biochemically active areas that may influence the emission rates of two GHGs, carbon dioxide (CO2) and methane (CH4). To determine how re-flooding events act on C emission rates, we experimentally estimated CO2 and CH4 flux in exposed sediment from the drainage area of four reservoirs in the Brazilian semiarid region (Marechal Dutra - Gargalheiras, Passagem das Traíras, Ministro João Alves - Boqueirão de Parelhas and Cruzeta) with different physicochemical properties after re-flooding simulation and compared their response with emissions in dry sediment. The intensity of C emissions varied strongly in and among all reservoirs. The observed CO2 and CH4 fluxes ranged from 10-40 mg C m-² day-¹ and 0.7-1.3 mg C m-² day-¹ in dry sediment and from 3-78 mg C m-² day-¹ and 0.4-13.5 mg C m-² day-¹ in rehydrated sediments, respectively. Nutrient concentrations and organic matter contents boosted CO2 flux and no variable showed a strong relationship with CH4 flux. In Dourado reservoir, we compared the responses of CO2 and CH4 flux after re-flooding simulation in exposed sediment with and without agricultural cultivation. CO2 and CH4 flux in cultivated areas were about three and two times higher than the flux in the non-cultivated areas, respectively. Organic matter content and nutrient concentration (nitrogen and phosphorus) in water and sediment in cultivated areas were found to be drivers of CO2 and CH4 flux. Understanding GHG flux in reservoirs that experience the drying and reflooding cycle is fundamental for the development of management strategies for these environments. Thus, C emissions from exposed and re-flooded sediment should be explicitly considered in the total emissions from river networks, particularly under predicted scenarios of global climate change, which should increase the temporality and spatiality of rainfall.