The role of sorptive processes in the organic carbon and nitrogen cycles of the Amazon River basin

1024 681 Stroud Water Research Center

Aufdenkampe, A.K. 2002. Ph.D. Dissertation. University of Washington, School of Oceanography, Seattle, Washington.

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Dissolved, mineral-associated, and mineral-free particulate organic matter (OM) fractions are compositionally distinct from one another in a wide range of aquatic and terrestrial environments. In the Amazon River Basin in particular, differences in elemental (C, N), isotopic (13C, 14C) and biochemical (amino acid, carbohydrate, lignin) compositions are greater between these three OM fractions than the variability within a fraction along the 1,800-km reach of the lower Amazon and its major tributaries. The overall goal of this dissertation has been to determine the extent to which sorption and related processes might contribute to these consistent compositional differences.

To test these questions I conducted a suite of laboratory experiments, in which natural suspended river sediments and organic-free kaolinite were mixed with various natural dissolved OM samples from throughout the Amazon. The first round of experiments demonstrated that sorption and related processes were responsible for essentially all of the characteristic organic nitrogen compositional patterns observed in the Amazon River System. Total nitrogen, total hydrolyzable amino acids, basic amino acids and hydrophobic amino acids were all preferentially taken into the mineral-associated OM fraction relative to the parent DOM, whereas non-protein amino acids preferentially remained in solution. The second round of experiments compared sorption incubations that were inoculated with a native microbial community to sterile controls. Results demonstrated that the microbial community plays an important role in determining the extent of OM association with minerals as well as its elemental and isotopic composition, despite the fact that biomass is an insignificant fraction of this OM. Furthermore, this study presents conclusive evidence that sorption and related processes are responsible for 1–3 % enrichments of 13C of mineral-associated OM relative to the initial DOM.

Observations of OM composition in 18 rivers from the Andean headwaters to the Amazon lowlands of Peru were placed into the context of results from laboratory experiments. Downstream trends in elemental, isotopic, amino acid, and lignin compositions suggest that sorption and related processes are important to the evolution of organic matter in large rivers and that exchanges between phases are rapid relative to transport times within the river system.