A biogeochemical and isotopic view of Nitrogen and Carbon in rivers of the Alto Paraíba do Sul basin, São Paulo State, Brazil

Abstract:

The magnitude of potential flows of elements in tropical ecosystems is not well represented in the literature, even being very important. The Paraíba do Sul River drains the three more economically developed states in Brazil: São Paulo, Minas Gerais and Rio de Janeiro and its basin is considered extremely altered. Despite its economic and social importance (~ 5.3 mi inhabitants), we don’t know much about carbon and nitrogen transport into its rivers and how these are affected by soil use changes. This work aimed to investigate these nutrients, using an isotopic and a biogeochemical approach, at some third order (Paraibuna, Paraitinga and Paraíba do Sul), second and first order rivers, all inserted at the Alto Paraíba do Sul Basin. In general, the low dissolved organic carbon, dissolved inorganic carbon, total dissolved nitrogen and inorganic N concentrations found in the first order rivers, showed the lower variation, despite changes in the soil use. Forested rivers presented higher DOC (3.3 mg.L^-1) and TDN (14.2 mM) concentrations than the pasture rivers (2.6 mg.L^-1 and 13.8 mM), while these presented higher DIC concentrations than those ones (90.2 mM and 71.2 mM). In third order rivers, the concentrations were also very low. Both carbon and nitrogen contents at the fine and coarse fractions of the suspended particulate material (SPM) were lower at Paraitinga and Paraiba do Sul Rivers. At the Paraibuna River, the fine fraction of SPM presented 25% of C concentration. The concentrations found at the coarse fraction were also higher at this river. The N concentrations were higher at the fine fraction and, consequently, this fraction presented higher C:N ratio. These observations allow us to say that the coarse fraction might be related to plant material, while the fine fraction is probably related to the soils. The δ¹³C in the SPM was lower in the Paraibuna River, probably due to the predominance of forest, while in the other ones pasture was the main soil use. The Paraibuna River presented the lower δ¹⁵N in the SPM results, similar to the results found at the Montane Forest, predominant at this basin, but are lower than the δ¹⁵N results found in the soils of the region. The higher results found in the Paraíba do Sul and Paraitinga Rivers may be explained by the highest N losses that occur in pasture grasses and soils.