Qualls & Richardson: Phosphorus Effects on Decomposition in Wetland Mesocosms

affect decomposition, and values less than five can inhibit 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 chanonly by controlling H concentration, not by any other nels into which controlled inputs of five different phosphate concentrainhibitory 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 PO4 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 micaused an absolute increase in P content of the litter up to approxicroflora. Suberkropp and Chauvet (1995) found that mately ninefold across the range of water P concentrations, while NO3 concentration was the only variable correlated with immobilization of N, Ca, and K did not vary with water P concentradifferences 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 NO3 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 PO4 from concentration was restricted to the upper 12 cm of soil. agricultural hog (Sus scrofa) farm runoff than at unenriched sites. Elwood et al. (1981) and Newbold et al. (1983) found that experimental additions of PO4 to D is one of the fundamental processes whole streams increased leaf litter decomposition, but that occur in ecosystems. Decomposition of orthat additions of NH 4 did not. In contrast, Lockaby et ganic matter produced by plants is often slowed in wetal. (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 resieither N or P additions. Davis (1991), comparing three dues in wetlands: aerobic vs. anaerobic conditions, pH sites along a nutrient enrichment gradient in Water Conof 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. annutrient-enriched site. He attributed this correlation to aerobic conditions on decomposition is the decomposilimitation 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 PO4 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 concen11 Feb. 1999. *Corresponding author ([email protected]). trations. The general explanation for the stimulation of decomPublished in Soil Sci. Soc. Am. J. 64:799–808 (2000). 800 SOIL SCI. SOC. AM. J., VOL. 64, MARCH–APRIL 2000 were constructed by sinking plastic walls into the natural subposition rate by inorganic N and P is that the dead plant strate. One end was closed, but the downstream end was left tissue is rich in C but contains concentrations of N and open. Channels were 2 m wide and 8 m long. Controlled P nutrients that are less than optimal for building microconcentrations of Na2HPO4, mixed into natural water from bial biomass (Alexander, 1977). However, bacteria and the site, were pumped continuously at rate of 1.9 L min2 into fungi can take up exogenous inorganic N and P to supthe closed ends of the channels. At each of the two sites, one plement the nutrients in litter being decomposed. As a control channel received no additional PO4 and four other result of this process of immobilization, plant litter in channels received four different levels of PO4 additions. the early stages of decomposition often takes up and Senescent leaves still attached to T. domengensis and C. stores certain exogenous nutrients. This process of imjamaicense plants were gathered in September. In the subtropical climate of the Everglades, leaves of these plants are promobilization has been shown to play a role in reducing duced and also senesce at varying rates all year (Davis, 1991). concentrations of inorganic N in water of swamp streams During this period, a particularly strong cycle of senescence (Qualls, 1984). was underway. We used only leaves that were completely Like many of the world’s wetlands that lie downdevoid of green color and standing above the water surface, stream of agricultural areas, portions of the Everglades but which had not undergone extensive decomposition. Leaves of Florida have become enriched with P. In a section of were cut into ≈20-cm lengths and were air dried until they the Northern Everglades known as Water Conservation reached a constant weight. Batches of leaves of ≈5 g were Area 2A, a well-established nutrient enrichment gradiweighed to the nearest 0.01 g and were then placed into nylon ent has been created. Several ecological changes have mesh bags (4-mm mesh size) (Qualls, 1984). Leaves in three been attributed to nutrient enrichment: invasion of catof these bags were dried at 708C to estimate a conversion factor for all samples at 708C. These were saved for chemical tail and displacement of the native sawgrass, increases analysis to serve as the initial samples. of more than a factor of two in net primary productivity Bags of each species of plant litter were placed in each of (Davis, 1991), and an elevation in the concentration of the five experimental channels at each of the two sites late in soil P levels by up to a factor of 2.5 to three in peat the month of September. Bags were placed at a distance of accumulated since the 1960s (Craft and Richardson, 1 m from the source of nutrient input. Because of uptake and 1993a, 1993b; Reddy et al., 1993; Debusk et al., 1994) dilution, concentrations of PO4–P were lower than the nominal in both labile and refractory forms of soil P (Qualls and input concentrations but were measured every 2 wk at the Richardson, 1995). However, C/N ratios in the peat are location of the bags. Concentrations of total P, dissolved orsimilar along this gradient. Craft and Richardson ganic P, NO3 1 NO2, NH4, Ca, and K were monitored monthly (1993a), in comparing the accretion rates of peat to (Cu was not measured in water). Average concentrations in the water were calculated as the time-weighted average during inputs from net primary productivity in enriched and the period of incubation. The ascorbic acid molybdate blue unenriched areas of the Everglades, postulated that the method was used to measure PO4 (Wetzel and Likens, 1991). decomposition rates must have been higher in the enTotal P in water was measured by persulfate digestion followed riched areas than in unenriched areas because the inby measurement of PO4 (Wetzel and Likens, 1991). Ammonia creased net primary production should have resulted in was measured by an automated phenate method (Technicon even higher rates of accretion of peat than were obIndustrial Systems, 1987a). Nitrate 1 NO2 was measured by served. The question of the effects of P on decomposiautomated Cd reduction and diazotization (Technicon Industion and the mechanisms of P removal have also astrial Systems, 1987b). Calcium and K were measured by atomic sumed additional importance because the world’s absorption spectroscopy with a Perkin Elmer Model 5100 PC largest constructed wetland for nutrient removal is curatomic absorption spectrophotometer (Perkin Elmer, Norwalk, CT). Temperature was monitored every 2 h in the water rently being constructed on the periphery of the northof each channel with probes attached to two dataloggers. Data ern Everglades as part of an Everglades restoration proreported for temperature excludes the final 40 d of the year gram (Guardo et al., 1995). The process of microbial of incubation because not all probes were operating. Dissolved immobilization in litter could play an important role in O2 was measured approximately monthly for purposes of comstoring P in peat accreting in these enriched marshes. paring channels at midday near the surface of the litter bags The objectives of this study were (i) to experimentally by agitating a YSI 5739 dissolved O2 probe. The pH was test whether PO4 enrichment alone affects plant litter measured at the same time with a glass combination electrode. decomposition rate; (ii) to test the quantitative relationBags of Cladium litter were retrieved from the unenriched ship between PO4–P concentration and decomposition control and the channel receiving the highest level of PO4 rate of sawgrass and cattail litter; (iii) to determine input channels after 32, 128, 245, and 365 d of incubation in whether increased PO4 concentration results in inorder to observe the trends in decomposition and immobilization of nutrients with time. In addition, three to four bags (in creased immobilization of N, Cu, Ca, and K; and (iv) most cases) of both species from all channels were retrieved to determine whether the P additions resulted in large after 365 d to observe a more detailed trend in annual decomincreases in the content of soil microbial biomass P; position as a function of concentration. In several cases, not all and (v) to determine the depths to which any effects three bags could be located, which resulted in fewer replicates. would occur. Leaves were gently and carefully brushed in a pan of water to remove any periphyton. Examination under a dissecting METHODS microscope indicated that no significant amount of leaf material was lost by this procedure. Litter was dried at 708C to a Experimental mesocosms were located in the unenriched constant weight, stored in a desiccator for 24 h and weighed southern portion of Water Conservation Area 2A of the Everto 6 0.01 g. Litter was ground and analyzed for C and N glades. Two sets of five channels were constructed at two sites located ≈200 m apart (referred to as Sites 1 and 2). Channels (Perkin Elmer 2400 CHN Elemental Analyzer). Litter was QUALLS & RICHARDSON: PHOSPHORUS EFFECTS ON DECOMPOSITION IN WETLAND MESOCOSMS 801 analyzed for total P by digestion in a mixture of perchloric and nitric acid (Sommers and Nelson, 1972) and with analysis of the liberated PO4 by the molybdate method (Technicon Industrial Systems, 1988); and for Ca, K, and Cu by perchloric acid digestion followed by atomic absorption spectrophotometry (using a graphite furnace for Cu). Microbial biomass P in the soil was measured in cores taken at the 1-m distance in each of the channels except the ones receiving the lowest dose of added P. These cores were taken after 8 mo of P additions. Each core was taken by pressing a transparent butyl plastic tube with a sharpened sawtoothed end into the peat. A 2-mm slice was shaved off the soil surface to exclude benthic algae from the sample. The core was pushed up from the bottom and sliced into 3-cm increments to the 24-cm depth. Samples were stored on ice and extracted within 72 h. A portion of the soil pore water was extracted by placing the peat on a 0.45-mm pore size membrane filter, and then the PO4 concentration in the extracted water was measured. The fractionation procedure of Hedley et al. (1982) was used as detailed in Qualls and Richardson (1995) for samples taken from a nearby nutrient enrichment gradient. A moist soil sample, equivalent to 0.5 g dry mass, was treated with chloroform, which was later evaporated by vacuum. This sample was then extracted with 0.5 M sodium bicarbonate. A second parallel sample was extracted with sodium bicarbonate without chloroform fumigation. The difference between the bicarbonateextracted P of the sample which was, or was not, initially chloroform fumigated was reported as the “chloroformreleased P”. This P is proportional to the P contained in microbial biomass (Hedley et al., 1982). Walbridge (1991) found that between 37 and 46% of the microbial biomass P was extracted after chloroform fumigation of 10 acid peat soils. Since we were simply comparing several channels with initially similar soil, we will report the chloroform-released P and assume that it is proportional to microbial biomass (Hedley et al., 1982; Walbridge, 1991). The bicarbonate extracted PO4–P in the soil samples that were not chloroform fumigated is reported as “exchangeable PO4–P” (Qualls and Richardson, 1995). Statistical analysis for elemental content of bags harvested at 1 yr consisted of linear or curvilinear regressions of the percentage of the initial element remaining as the dependent variable vs. average PO4–P concentration as the independent variable. The percentage of initial element remaining was calculated as: 100%(mg g2 element at time t/mg g2 element at Fig. 1. Percentage of initial C remaining after 1 yr of decomposition time 0)(litter mass at time t/litter mass at time 0). of (a) sawgrass and (b) cattail litter as a function of average PO4–P The average PO4–P concentration was the average of all in the water column in 5 experimental channels at each of two PO4–P concentration samples taken during the course of the sites. Each data point represents one litter bag sample. year at the location of the individual litterbag. Each individual litterbag was treated and plotted as an individual observation. fraction of initial C remaining to average PO4–P concenAn analysis of covariance was also done by using site as a tration were similar for both species, but the intercept categorical independent variable, average PO4–P concentrawas less for cattail litter. Thus, the cattail decayed faster tion as an independent variable, and percentage original elethan the sawgrass, but the response to PO4 concentration mental content as the dependent variable in order to deterwas similar. The first-order decay constants for the litter, mine whether the slopes of the relationships differed between based on the predicted regression values of fraction of the two sites. To determine whether averages of any other initial C remaining, ranged from 0.46 to 1.11 yr21 for independent variable (NH4–N, NO3–N, Ca, dissolved O2, pH, sawgrass litter and 0.59 to 1.30 yr21 for cattail litter and water temperature) might explain trends, these were each regressed against the data for proportion of C remaining after across the range of PO4–P concentrations. 1 yr. Only sawgrass litter was sampled during the course of the year, and in that time the sawgrass litter in the channel enriched with the highest level of P lost more RESULTS C than litter in the unenriched control (Fig. 2). In the At the end of 1 yr of incubation, the loss of C by the channel receiving the highest input of P, the litter acculitter was strongly correlated with the average PO4–P mulated absolute quantities of P that were eight to 10 times those present in the initial litter by 250 d (Fig. concentration (Fig. 1). The slopes of the lines relating 802 SOIL SCI. SOC. AM. J., VOL. 64, MARCH–APRIL 2000 Fig. 2. Percentage of initial C remaining during 1 yr of decomposition of sawgrass litter in channels receiving either the highest level of P enrichment or no P enrichment. Data points represent individual litter bags, while lines indicate averages of data points taken from a particular channel. 3). Between 265 and 365 d, P remained constant or Figure 6 shows the remaining elemental content of both sawgrass (Fig. 6a) and cattail (Fig. 6b) after 1 yr dropped slightly in concentration. In the unenriched channel, P was lost and never accumulated above the of decomposition in all channels. The fraction of the initial mass of element remaining is plotted vs. the averquantity present in the initial litter (Fig. 3). Sawgrass litter accumulated more N than was present in the initial age PO4–P concentration in water across the year of decomposition. The P remaining was far greater at inlitter in both the enriched and unenriched channels after a 30-d period (Fig. 4). Patterns of N accumulation with creased concentrations of PO4–P in water (Fig. 6a and 6b). The P remaining in litter appeared to approach a time were variable but not different between the enriched and unenriched control channels (Fig. 4). Copper maximum or a plateau at an average PO4–P concentration water of ≈15 mg L21, although the variability prealso accumulated twoto threefold in absolute quantity compared with that present in the initial sample (Fig. 5). vented a precise analysis of the characteristics of the Fig. 3. Fraction of initial P remaining during 1 yr of decomposition of sawgrass litter in channels receiving either the highest level of P enrichment or no P enrichment. Data points represent individual litter bags, while lines indicate averages of data points taken from a particular channel. QUALLS & RICHARDSON: PHOSPHORUS EFFECTS ON DECOMPOSITION IN WETLAND MESOCOSMS 803 Fig. 4. Percentage of initial N remaining during 1 yr of decomposition of sawgrass litter in channels receiving either the highest level of P enrichment or no P enrichment. Data points represent individual litter bags, while lines indicate averages of data points taken from a particular channel. curvilinear relationship. The regression was highly sigand K remaining in litter after 1 yr of decomposition was not significantly related, in most cases, to the avernificant (P , 0.001). The polynomial gave better fits to the P remaining data (Fig. 6) than linear fits (R2 5 0.48 age PO4–P concentration water (Table 1). The one exception to this lack of trends was that the N remaining in the case of cattail litter and R2 5 0.63 in the case of sawgrass litter). These fits were not used to imply a in the cattail litter after 1 yr of decomposition declined slightly but significantly (P 5 0.05) with increasing averparticular theoretical relationship but only to test whether there was any significant increase as a function age PO4–P concentration in water (Table 1). Compared with the initial contents, there was a net increase in the of PO4–P concentration. The maximum fraction of P remaining in sawgrass litter was much higher than that N and Cu (in some cases several times more) content in the litter of both species across the range of all PO4–P in cattail litter, but the initial P content in sawgrass litter was much lower (112 mg g21 in cattail litter vs. 41 mg treatments. In the case of Ca, sawgrass litter had gained Ca, while cattail litter had lost net amounts of Ca across g21 in sawgrass litter). In contrast to P, the N, Cu, Ca, Fig. 5. Percentage of initial Cu remaining during 1 yr of decomposition of sawgrass litter in channels receiving either the highest level of P enrichment or no P enrichment. Data points represent individual litter bags, while lines indicate averages of data points taken from a particular channel. 804 SOIL SCI. SOC. AM. J., VOL. 64, MARCH–APRIL 2000 released P was approximately nine times higher than the control in the most enriched channel. Chloroformreleased P in the surface increment of the channel receiving intermediate P inputs was elevated about three times higher than that in the control. These dramatic differences declined with depth, and by the 12to 15-cm depth increment, all channels were similar. In contrast, the exchangeable P in the soil was far lower than the microbial biomass P content and was only elevated in the upper increments of the channels receiving the highest inputs (Fig. 7b).