Long-term photochemical and microbial decomposition of wetland-derived dissolved organic matter with alteration of 13C : 12C mass ratio

We investigated the long-term photochemical and microbial decomposition of biologically recalcitrant humic-like dissolved organic matter (DOM) leached from a vascular wetland plant, the common rush (Juncus effusus). Although the leachate would have been characterized as biologically recalcitrant by short-term (<14 d) bioassays, microbes decomposed 51% of its organic carbon in 898 d with a first-order biological decomposition coefficient of 0.0008 d(-1) in darkness. Solar radiation exposure accelerated the decomposition of leachate. Under 459-d exposure to surface solar radiation, up to 90% of organic carbon was mineralized. During the exposure, the photochemical reactions preferentially mineralized the C-12 fraction of organic carbon and enriched the C-13 of organic carbon by 6% in the residual leachate. Solar radiation also decomposed nearly completely (up to 99.7%) chromo- and fluorophores of DOM. A 439-d bioassay following the solar radiation exposure resulted in up to 97.3% mineralization of organic carbon. Solar radiation together with microbial metabolism can completely mineralize at least some forms of wetland-derived DOM in surface waters with sufficiently long residence times.