Plant Litter Abundance Decay and Microbial Growth

In most wetlands the majority of plant production is not consumed during the lifetime of the plant parts but enters a detritus-based food web (Odum et al., 1984) and serves as a growth substrate for various microbes and food for many benthic invertebrates (Vos et al., 2002). In the tidal freshwater wetlands of the Hudson, quantities of plant litter vary among marsh types, along elevational gradients within a marsh and among plant communities (Kiviat and Beecher, 1991;Mihocko et al., 2003). Litter cover was highest in the high marsh zone with values approaching 100 percent; in the marshes overall, median litter cover was 1 percent with half the observations in the range 0-25 percent. As suggested by the coarser-scale sampling, maximal litter abundance occurs in communities of intertidal emergent plants with standing stocks of litter on the order of 500-1000+ g dry mass • m-2. These high standing stocks are probably due to a combination of high rates of litter production, relatively slow decay, and limited opportunity for export. Lower intertidal areas occupied by broadleaf (e.g., spatterdock, arrow arum) or floating-leaf plants (e.g., water-chestnut) have much less recognizable litter and what is shed from the plants is rapidly decomposed or exported with tidal waters (Findlay, Howe, and Austin, 1990).

Decay of litter by microbes is typically a major fate of detritus and rates of decomposition are known to vary significantly among plant species (Odum and Heywood, 1978; Brinson, Lugo, and Brown, 1981). Loss of mass from recognizable fragments of plant litter occurs via a numberofseparate processes including mechanical fragmentation into smaller, unrecognizable particles, leaching of dissolved organic components, and mineralization to carbon dioxide (CO2) by bacteria and fungi. Decay rates for cattail, reed, and loosestrife are such that less than half the litter decays within a year from plant senescence (exponential mass loss coefficients 0.3-0.4 • year-1). As a general rule, the "softer" broadleaf plants have the highest rates of mass loss such that litter decomposes completely within a year (or less). In contrast the predominant grasslike plants of intertidal marshes generate litter with much slower decay and, for instance, roughly 50 percent of the litter from reed is still retained in coarse-mesh litter bags three years after the death of aerial stems (Findlay, Dye, and Kuehn, 2002b). For many emergent species, plants will spend as much as a year as standing dead material where the culms may serve as habitat for invertebrates (Krause, Rietsma, and Kiviat, 1997) and provide physical structure on the marsh. In contrast to previous work in other wetland systems (e.g., Newell and Porter, 2000), there appears to be no mass loss from the standing deadphase of common reed although there is an abundant fungal community. Also, litter mass loss over the winter months appears to be minimal.

There is a clear transition from fungal-dominated communities on relatively large litter as is common through the first few years of decay to a bacteria-dominated community in the sediments and associated with finer particles of organic matter (Sinsabaugh and Findlay, 1995; Findlay et al., 2002b). In relatively intact litter over the first two to three years of decay, fungal biomass can be hundreds of times greater than bacterial biomass and fungal production is much greater than bacterial production. Bacterial production in surface sediments of the Hudson wetlands is well within the range for bacterial growth reported from other wetland and shallow aquatic habitats (Austin and Findlay, 1989).

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