Leaching and microbial degradation of dissolved organic matter from salt marsh plants and seagrasses

Abstract

Dissolved organic matter (DOM) is outwelled from highly productive salt marshes, but its sources and fates are unclear. To examine common salt marsh plants as sources of coastal DOM, two dominant salt marsh vascular plants Spartina alterniflora and Juncus roemarianus, and two major coastal seagrasses Syringodium filiforme and Halodule wrightii, were collected from a Florida salt marsh and studied using laboratory incubation experiments. We investigated the leaching dynamics of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and chromophoric dissolved organic matter (CDOM) from these plants, in conjunction with our field investigations of the sources and outwelling of DOM from Florida salt marshes. The leaching of DOM and CDOM from the plants was a rapid process, and leaching rates were 65-288 lM/ g dry weight/day for DOC and 3.8-16 lM/g dry weight/ day for TDN from different plants in the bacteria-inhibited incubations. DOC was proportional to TDN in the leachates, but the quantity of C and N leached was dependent on the species and growth stage of the plants. At the end of the 25-day experiments, 5.4-23 % and 10-45 % of solid phase C and N were released into DOC and TDN pools, respectively. Bacteria played an important role during the leaching process. The majority of DOC and TDN leached from marsh plants and seagrasses was labile and highly biodegradable with 56-90 % of the leached DOC and 44-72 % of the leached TDN being decomposed at the end of the experiments. The fluorescence measurements of CDOM indicate that organic matter leached from marsh plants and seagrasses contained mainly protein-like DOM which was degraded rapidly by bacteria. Our study suggests that leaching of DOM from salt marsh plants and seagrasses provide not only major sources of DOC, TDN, and CDOM that affect many biogeochemical processes, but also as important food sources to microbial communities in the marsh and adjacent coastal waters.

Release and Microbial Degradation of Dissolved Organic Carbon and Nitrogen from Phragmites australis and Suaeda salsa in the Wetland of the Yellow River Estuary

“…These results are consistent with the data reported by Wang et al who examined the CDOM release from salt marsh plants and seagrasses. Bacteria utilized both protein-associated and humic-like CDOM released from P. australis and S. salsa, suggesting that these CDOM components are biologically labile for bacterial degradation in coastal waters [8,36,[46][47][48]. These fluorescence results also support our DOC results as discussed above.…”

“…Based on the different concentrations of DOC at the end of the incubations (Figures 1a and 1b), it is calculated that 89% and 83% of the DOC released from P. australis and S. salsa were consumed by bacteria. However, this calculation could not be applied to DN because in the bacteriaactive incubations, the observed increase of DN in the later incubation was likely due to the conversion of organic N into inorganic N such as NH during DOM degradation [8,17,[39][40][41]. Both labile organic substrates and useable forms of inorganic and organic N could promote bacterial production, thus complicates the release dynamics of DN.…”

“…More importantly, from geochemical viewpoints, wetlands and salt marshes are significant sinks for the atmospheric CO 2 due to the high rates of annual carbon sequestration in vegetation in these ecosystems. On the other hand, salt marshes also export large amount of dissolved organic matter (DOM) and nutrients, thus influence the primary production and biogeochemical processes in the coastal waters as well [5][6][7][8].…”

“…Export of dissolved organic carbon (DOC) and dissolved nitrogen (DN) from salt marsh and wetland to coastal waters has been considered an important biogeochemical link between these coastal ecosystems and the ocean [5,[12][13][14][15][16]. Contribution of DOC from marshes and wetlands is largely through the leaching and decomposition of plant biomass, especially during the late fall "dumping" period when plants began to die [8,17,18]. Previous studies have shown that DOC output from Southeastern U.S. salt marshes alone could contribute an equal amount of DOC as river inputs to the continental shelf [19,20].…”

“…Studies have shown that salt marsh and wetland plants are important sources contributing chromophoric dissolved organic matter (CDOM) to estuarine and coastal waters [8,17,18,21]. CDOM is the optically measureable component of DOM and is important for most photochemically-mediated processes in coastal waters [22,23].…”