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Richardson ABSTRACT affect decomposition, and values less than five can in- hibit decomposition of leaf litter (Qualls and Haines, Like many wetland ecosystems, areas of the northern Everglades 1990). While many bogs exhibit very acid conditions of Florida, USA, have been influenced by P eutrophication. Our objective was to determine if P enrichment of water influences the that are potentially inhibitory to decomposition, fens litter decomposition rate and nutrient immobilization by litter and, such as the Everglades often have soil pH values near further, to determine the quantitative relationship of these responses neutrality and near the optimum for decomposition. across a range of P concentrations in surface water. In addition, we High concentrations of dissolved organic acids are a determined whether P additions rapidly elevated microbial biomass common characteristic of many wetlands and can inhibit P in the soil. In order to isolate the effects of P enrichment, we placed decomposition by creating highly acid water. Qualls and bags containing cattail (Typha domengensis Crantz) and sawgrass Haines (1990) showed that this affected decomposition (Cladium jamaicense Pers.) litter into two sets of experimental chan- only by controlling H + concentration, not by any other nels into which controlled inputs of five different phosphate concentra- inhibitory properties of the organic acid molecules. tions were added continuously. After 1 yr of incubation, litter was The decomposition of plant litter can also be limited analyzed for C, P, N, Cu, Ca, and K content. Loss of C at the end of 1 yr increased linearly with increasing average PO 4 content in the by the concentration of inorganic N and P (Alexander, channels with a similar slope for both species of litter. Immobilization 1977) in soil or water surrounding the decomposer mi- caused an absolute increase in P content of the litter up to approxi- croflora. Suberkropp and Chauvet (1995) found that mately ninefold across the range of water P concentrations, while NO 3 concentration was the only variable correlated with immobilization of N, Ca, and K did not vary with water P concentra- differences in litter decomposition among six hardwater tions. During decomposition, litter exhibited a net uptake of Cu (a streams; however, Triska and Sedell (1976) found no nutrient potentially limiting plant growth on peat soils). The microbial decomposition response to NO 3 additions. Qualls (1984) biomass P was up to nine times higher in the surface soil of the most found that litter in a stream swamp decomposed much enriched channel compared with the control, but this elevation in faster at sites with elevated inorganic N and PO 4 from concentration was restricted to the upper 12 cm of soil. agricultural hog (Sus scrofa ) farm runoff than at unen- riched sites. Elwood et al. (1981) and Newbold et al. (1983) found that experimental additions of PO 4 to D ecomposition is one of the fundamental processes whole streams increased leaf litter decomposition, but that occur in ecosystems. Decomposition of or- that additions of NH + 4 did not. In contrast, Lockaby et ganic matter produced by plants is often slowed in wet- al. (1996) found no increase in decomposition rates of lands, particularly in those that form histosols. Several lignin or cellulose in swamp leaf litter in response to factors are likely to control decomposition of plant resi- either N or P additions. Davis (1991), comparing three dues in wetlands: aerobic vs. anaerobic conditions, pH sites along a nutrient enrichment gradient in Water Con- of the water, temperature, and perhaps availability of servation Area 2A in the Everglades, observed that inorganic nutrients to decomposers. One of the world’s litter of Cladium and Typha decomposed faster at a most dramatic examples of the effect of aerobic vs. an- nutrient-enriched site. He attributed this correlation to aerobic conditions on decomposition is the decomposi- limitation by N or P. Since a number of factors can vary tion and subsidence of peat in the drained histosols of along a large-scale geographic gradient, we endeavored the Everglades (Tate, 1980). The pH of water can also to experimentally test whether PO 4 alone can lead to increased decomposition of leaf litter. No study of which R.G. Qualls, Dept. of Environmental and Resource Sciences, MS 370, University of Nevada, Reno, NV 89557; C.J. Richardson, Duke we are aware has tested a quantitative relationship of Wetland Center, Duke University, Durham, NC 27708. Received decomposition rate across a range of nutrient concen- 11 Feb. 1999. *Corresponding author (qualls@equinox.unr.edu). trations. The general explanation for the stimulation of decom- Published in Soil Sci. Soc. Am. J. 64:799–808 (2000).