Controls of Land Use and the River Continuum Concept on Dissolved Organic Matter Composition in an Anthropogenically Disturbed Subtropical Watershed

About 250 Tg of dissolved organic carbon are annually transported from inland waters to coastal systems making rivers a critical link between terrestrial and ocean carbon pools. During transport through fluvial systems, various biogeochemical processes selectively remove or transform labile material, effectively altering the composition of dissolved organic matter (DOM) exported to the ocean. The river continuum concept (RCC) has been historically used as a model to predict the fate and quality of organic matter along a river continuum. However, the conversion of natural landscapes for urban and agricultural practices can also alter the sources and quality of DOM exported from fluvial systems, and the RCC may be significantly limited in predicting DOM quality in anthropogenically impacted watersheds. Here, we studied DOM dynamics in the Altamaha River watershed in Georgia, USA, a fluvial system where headwater streams are highly impacted by anthropogenic activities. The primary goal of this study was to quantitatively assess the importance of both the RCC and land use as environmental drivers controlling DOM composition. Land use was a stronger predictor of spatial variation (∼50%) in DOM composition defined by both excitation–emission matrix–parallel factor analysis (EEM–PARAFAC) and ultrahigh-resolution mass spectrometry. This is compared to an 8% explained variability that can be attributed to the RCC. This study highlights the importance of incorporating land use among other controls into the RCC to better predict the fate and quality of DOM exported from terrestrial to coastal systems

Quantitative assessment of photo- and bio-reactivity of chromophoric and fluorescent dissolved organic matter from biomass and soil leachates and from surface waters in a subtropical wetland

Dissolved organic matter (DOM) reactivity plays a central role in ecosystem function and the global carbon cycle. In this study, a suite of biomass and soil leachates together with surface water samples from the Florida coastal Everglades (FCE) were investigated to quantitatively assess the photo- and bio-reactivity of DOM via dissolved organic carbon (DOC), ultraviolet–visible absorbance (UV–Vis) measurements, and excitation-emission matrix (EEM) fluorescence combined with parallel factor analysis (PARAFAC). The decomposition patterns observed were diverse, but dominated by first order decay for most of the samples studied. The reactivity rate constants obtained based on a first order multi-pool kinetic model, suggest that the DOM in the Everglades is largely refractory on time scales relevant to this ecosystem (2 months). The relative reactivity of different fluorescent DOM pools determined as labile, semi-labile and refractory were in the order of biomass leachate \u3e soil leachate \u3e surface water. Photo-reactivity was found to be a more important process in controlling the fate of FDOM in this system, while FDOM was mainly refractory to bio-degradation. Degradation patterns and rate constants for samples of different origin displayed vast differences for the same PARAFAC component, suggesting a complex and heterogeneous composition of fluorophores for each component, most likely the result of different structures, speciation and conformation, and molecular weight distribution. In addition, a terrestrial humic-like and two protein-like components were found to have the potential to serve as indicators of photo-degradable and bioavailable DOM

A New Perspective on the Apparent Solubility of Dissolved Black Carbon

Black carbon (BC), pyrogenic organic matter generated from the incomplete combustion of biomass, is ubiquitous in the environment. The molecular structures which comprise the BC pool of compounds are defined by their condensed aromatic core structures polysubstituted with O-containing functionalities (e.g., carboxyl groups). Despite the apparent hydrophobicity of BC molecules, a considerable portion of BC is translocated from terrestrial to aquatic systems in the form of dissolved BC (DBC). However, the specific biogeochemical mechanisms which control the transfer of BC from the land to the water remain elusive. In the current study, the apparent solubility of DBC was inferred from octanol-water partition coefficients (Kow) modeled for proposed DBC structures with varying degrees of polycondensation and polar functionality. Modeled Kow values indicated that DBC molecules with small aromatic ring systems and high degrees of hydrophilic functionality may be truly solubilized in the aqueous phase. However, large and highly condensed DBC structures yielded high Kow values, which suggested that a considerable portion of the DBC pool which has been quantified in aquatic environments is not truly dissolved. We hypothesized that other DOM components may act as mediators in the solubilization of condensed aromatic molecules and serve to increase the solubility of DBC via hydrophobic, intermolecular associations. This hypothesis was tested through controlled leaching experiments to determine whether the mobilization of DBC from particulate soils and chars became enhanced in the presence of DOM. However, we observed that characteristics inherent to each sample type had a greater influence than added DOM on the apparent solubility of DBC. In addition, the direct comparison of molecular marker (benzenepolycarboxylic acids) and ultrahigh resolution mass spectral data (FT-ICR/MS) on leachates obtained from the same set of soils and char did not show a clear overlap in DBC quantification or characterization between the two analytical methods. Correlations between FT-ICR/MS results and BPCA were not significant possibly due to differences in the methodological windows and/or small sample size. Our results were unable to provide evidence in support of proposed hydrophobic interactions between DOM and DBC, suggesting that other physical/chemical mechanisms play important roles in the dissolution of BC