Dissolved organic carbon transformations and microbial community response to variations in recharge waters in a shallow carbonate aquifer

© 2016, The Author(s). In carbonate aquifers, dissolved organic carbon from the surface drives heterotrophic metabolism, generating CO2 in the subsurface. Although this has been a proposed mechanism for enhanced dissolution at the water table, respiration rates and their controlling factors have not been widely evaluated. This study investigates the composition and concentration of dissolved organic carbon (DOC) reaching the water table from different recharge pathways on a subtropical carbonate island using a combination of DOC concentration measurements, fluorescence and absorption characterisation. In addition, direct measurements of the microbial response to the differing water types were made. Interactions of rainfall with the vegetation, via throughfall and stemflow, increase the concentration of DOC. The highest DOC concentrations are associated with stemflow, overland recharge and dissolution hole waters which interact with bark lignin and exhibit strong terrestrial-derived characteristics. The groundwater samples exhibit the lowest concentrations of DOC and are comprised of refractory humic-like organic matter. The heterotrophic response seems to be controlled by the concentration of DOC in the sample. The terrestrially sourced humic-like matter in the stemflow and dissolution hole samples was highly labile, thus increasing the amount of biologically produced CO2 to drive dissolution. Based on the calculated respiration rates, microbial activity could enhance carbonate dissolution, increasing porosity generation by a maximum of 1%kyr−1 at the top of the freshwater lens

Can land use changes alter carbon, nitrogen and major ion transport in subtropical brazilian streams? Modificações no uso da terra podem alterar o transporte fluvial de carbono, nitrogênio e íons maiores?

Several studies in tropical watersheds have evaluated the impact of urbanization and agricultural practices on water quality. In Brazil, savannas (known regionally as Cerrados) represent 23% of the country's surface, representing an important share to the national primary growth product, especially due to intense agriculture. The purpose of this study is to present a comprehensive evaluation, on a yearly basis, of carbon, nitrogen and major ion fluxes in streams crossing areas under different land use (natural vegetation, sugar cane and eucalyptus) in a savanna region of SE Brazil. Eucalyptus and sugar cane alter the transport of the investigated elements in small watersheds. The highest concentration of all parameters (abiotic parameters, ions, dissolved organic carbon DOC - and dissolved inorganic carbon - DIC) were found in Sugar Cane Watersheds (SCW). The observed concentrations of major cations in Eucalyptus Watersheds (EW) (Mg, Ca, K, Na), as well as DIN and DOC, were found frequently to be intermediate values between those of Savanna Watersheds (SW) and SCW, suggesting a moderate impact of eucalyptus plantations on the streamwater. Same trends were found in relation to ion and nutrient fluxes, where the higher values corresponded to SCW. It is suggested that sugar cane plantations might be playing an important role in altering the chemistry of water bodies.<br>Diversos estudos têm sido desenvolvidos em bacias de drenagem tropicais no intuito de avaliar o impacto da urbanização e das práticas agrícolas na qualidade dos corpos d'água. No Brasil, as savanas (conhecidas regionalmente como Cerrado) representam 23% do território brasileiro, sendo uma região importante no crescimento nacional, especialmente devido às intensas atividades agrícolas. A finalidade deste trabalho é apresentar uma avaliação dos fluxos de carbono, nitrogênio e principais íons em córregos com diferentes usos do solo (vegetação, cana de açúcar e eucalipto) em uma região de Cerrado, SE Brasil. A cana de açúcar e o eucalipto alteram o transporte dos elementos investigados nessas pequenas bacias de drenagem. As concentrações mais elevadas de todos os parâmetros analisados (parâmetros abióticos, íons, carbono orgânico dissolvido e carbono inorgânico dissolvido) foram encontradas na bacia de cana de açúcar (SCW). As concentrações observadas para os cátions na bacia do eucalipto (EW) (Mg, Ca, K, Na), assim como Carbono Inorgânico Dissolvido (CID) e Carbono Orgânico Dissolvido (COD), apresentaram valores intermediários entre as bacias do Cerrado e da cana de açúcar, sugerindo um impacto moderado dessa plantação aos corpos d'água. Mesma tendência foi observada para os fluxos de íons e nutrientes, sendo, novamente, os maiores valores encontrados na bacia de cana-de-açúcar. Os autores sugerem que o cultivo da cana de açúcar representa um importante fator na modificação da química de pequenas bacias de drenagem