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

Abstract

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 article presents experimental results that show that at least some of these compositional patterns are the outcome of selective partitioning of nitrogen-rich DOM components onto mineral surfaces. Nine laboratory experiments were conducted in which natural DOM from two rivers, one wetland, and two leachates from the Peruvian Amazon were mixed with natural suspended riverine minerals or organic-free kaolinite. Concentrations of organic carbon, organic nitrogen, and hydrolyzable amino acids were measured in both dissolved and particulate phases before and after mixing. In each of these trials, nitrogen was preferentially taken into the particulate fraction relative to the ''parent'' DOM, as were total hydrolyzable amino acids with respect to total organic carbon and nitrogen. Amino acid compositional patterns also indicated preferential sorption of basic amino acids, with positively charged nitrogen side chains, to the negatively charged aluminosilicate clay minerals. In short, sorption of natural DOM to minerals reproduced all contrasting organic nitrogen compositional patterns observed in the Amazon Basin. Although previously conjectured from FPOM-DOM compositional trends from river samples, this is the first direct evidence for preferential uptake of naturally occurring nitrogenous DOM by suspended riverine minerals. Last, nonprotein amino acids, which are commonly used as diagenetic indicators in sediments, preferentially remained dissolved, which suggests that sorptive fractionation may significantly complicate comparisons of FPOM and DOM diagenesis on the basis of interpretation of organic composition.

The role of terrestrially derived organic carbon in the coastal ocean: A changing paradigm and the priming effect

“…In fact, lignin compositional differences lignin in soils versus coastal sediments may be entirely caused by selective fractionation of OC during leaching phases and not degradation. Such fractionation processes have already been observed for free amino acids (63) and peptides (64). This may suggest that riverine DOM is less degraded than previously thought, and thus is more labile, allowing for more rapid mineralization in coastal waters.…”

Dissolved and particulate organic matter source-age characterization in the upper and lower Chesapeake Bay: A combined isotope and biochemical approach

“…However, with the exception of the high-flow UDOM sample from SQR, the Alloch : Autoch ratios indicate that autochthonous sources of DOM and POM predominate under high-flow conditions (Alloch : Autoch ,1; Table 4). In addition, the consistently greater percentage of allochthonous higher plant sterols (e.g., campesterol, stigmasterol, and C 29 D 5 ; (Aufdenkampe et al 2001).…”

Temporal dynamics of seston: A recurring nighttime peak and seasonal shifts in composition in a stream ecosystem

“…These perspectives were first reconciled by Meybeck (1982), who showed that seston in the world's rivers spanned the range from high to low carbon content with increasing seston concentration and suggested that these trends represented systematic variation in the contributions of different particle types to seston. We thus consider seston as a mixture of three fundamental particle types: inorganic particles with adsorbed terrestrially derived organic matter (Aufdenkampe et al 2001), algal cells, and fragmented leaves or woody debris. We used a comprehensive array of measurements to characterize seston, distinguish these particle types, and infer the sources of POC.…”