Sorptive fractionation of dissolved organic nitrogen and amino acids onto fine sediments within the Amazon Basin

A consistent observation of river waters in the Amazon Basin and elsewhere is that suspended fine particulate organic matter (FPOM) is compositionally distinct from coexisting dissolved organic matter (DOM). The present artic le presents experimental results that show that at least some of these comp ositional patterns are the outcome of selective partitioning of nitrogen-ri ch DOM components onto mineral surfaces. Nine laboratory experiments were c onducted in which natural DOM from two rivers, one wetland, and two leachat es from the Peruvian Amazon were mixed with natural suspended riverine mine rals or organic-free kaolinite. Concentrations of organic carbon, organic n itrogen, and hydrolyzable amino acids were measured in both dissolved and p articulate phases before and after mixing. In each of these trials, nitroge n was preferentially taken into the particulate fraction relative to the "p arent" DOM, as were total hydrolyzable amino acids with respect to total or ganic carbon and nitrogen. Amino acid compositional patterns also indicated preferential sorption of basic amino acids, with positively charged nitrog en side chains, to the negatively charged aluminosilicate clay minerals. In short, sorption of natural DOM to minerals reproduced all contrasting orga nic nitrogen compositional patterns observed in the Amazon Basin. Although previously conjectured from FPOM-DOM compositional trends from river sample s, this is the first direct evidence for preferential uptake of naturally o ccurring nitrogenous DOM by suspended riverine minerals. Last, nonprotein a mino acids, which are commonly used as diagenetic indicators in sediments, preferentially remained dissolved, which suggests that sorptive fractionati on may significantly complicate comparisons of FPOM and DOM diagenesis on t he basis of interpretation of organic composition.