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

Abstract

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 > soil leachate > 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.