We investigated the long-term photochemical and microbial decomposition of biologically recalcitrant humiclike 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 d21 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 12C fraction of organic carbon and enriched the 13C of organic carbon by 6% in the residual leachate. Solar radiation also decomposed nearly completely (up to 99.7%) chromoand 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. In freshwaters and coastal waters, imported (allochthonous) organic carbon contributes much to the composition of dissolved organic carbon (DOC; Wetzel 2001). Major sources of allochthonous DOC are vascular plants in the littoral zone, wetlands, and terrestrial environments, which are hydrologically connected to open-water regions (Wetzel 1992). Because decomposers consume quickly the bioavailable parts of plant-derived DOC, biologically recalcitrant humic substances dominate the allochthonous DOC in open-water environments (Thurman 1985). Aquatic microbes consume typically only a small portion (,5%) of humic-like allochthonous DOC in #2 weeks, a time typically used to test the biological lability of DOC (Moran and Hodson 1990). Such short-term bioassays dominate the literature (Søndergaard and Middelboe 1995; del Giorgio and Davis 2003) but provide little insight regarding the fate of the .95% biologically recalcitrant allochthonous humic-like DOC in the receiving lakes and ocean. Biologically recalcitrant DOC contributes much to the DOC transported to lakes, rivers, and finally to the ocean with annual loads of 0.25 3 1015 g C (Hedges et al. 1997). Molecular tracers of terrestrial organic carbon such as lignin and 13C-depleted terrestrial-like organic carbon, however, are scarce in oceanic waters and marine sediments (Hedges 1992). This suggests that decomposition rather than sedimentation must be a major sink for terrestrial DOC. In addition to biological mineralization, photochemical reactions can mineralize at least a portion of terrestrial humic-like DOC and can potentially explain the decomposition of allochthonous DOC (Mopper and Kieber 2002). Previous photochemical experiments have been typically short (a few hours to weeks) and do not provide direct evidence for the long-term photodecomposition of allochthonous DOC in the receiving lakes or ocean with the water residence times lasting years or longer. In this study we hypothesize that long-term (years) microbial and photochemical decomposition is responsible for the disappearance of biologically recalcitrant allochthonous DOC in lakes and marine waters. Our experimental work simulates the long-term fate of wetland-derived humic-like DOM in the surface waters of a receiving open-water environment. We generated humic-like DOM from common rush (Juncus effusus), an emergent vascular wetland plant with a widespread distribution throughout the northern hemisphere, and exposed it to solar radiation (for 459 d) and to heterotrophic microbes for up to 898 d. These exposures are longer than the times typically used for testing the biological lability or the photochemical decomposition of DOC but similar to or shorter than the residence time of water in lakes and ocean.
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