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a Dept. of Environmental and Resource Sciences, MS 370, University of Nevada, Reno, NV 89557 USA
b Duke Wetland Center, Duke University, Durham, NC 27708 USA
qualls{at}equinox.unr.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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The decomposition of plant litter can also be limited by the concentration of inorganic N and P (Alexander, 1977) in soil or water surrounding the decomposer microflora. Suberkropp and Chauvet (1995) found that NO3 concentration was the only variable correlated with differences in litter decomposition among six hardwater streams; however, Triska and Sedell (1976) found no decomposition response to NO3 additions. Qualls (1984) found that litter in a stream swamp decomposed much faster at sites with elevated inorganic N and PO4 from 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 whole streams increased leaf litter decomposition, but that additions of NH+4 did not. In contrast, Lockaby et al. (1996) found no increase in decomposition rates of lignin or cellulose in swamp leaf litter in response to either N or P additions. Davis (1991), comparing three sites along a nutrient enrichment gradient in Water Conservation Area 2A in the Everglades, observed that litter of Cladium and Typha decomposed faster at a nutrient-enriched site. He attributed this correlation to limitation by N or P. Since a number of factors can vary along a large-scale geographic gradient, we endeavored to experimentally test whether PO4 alone can lead to increased decomposition of leaf litter. No study of which we are aware has tested a quantitative relationship of decomposition rate across a range of nutrient concentrations.
The general explanation for the stimulation of decomposition rate by inorganic N and P is that the dead plant tissue is rich in C but contains concentrations of N and P nutrients that are less than optimal for building microbial biomass (Alexander, 1977). However, bacteria and fungi can take up exogenous inorganic N and P to supplement the nutrients in litter being decomposed. As a result of this process of immobilization, plant litter in the early stages of decomposition often takes up and stores certain exogenous nutrients. This process of immobilization has been shown to play a role in reducing concentrations of inorganic N in water of swamp streams (Qualls, 1984).
Like many of the world's wetlands that lie downstream of agricultural areas, portions of the Everglades of Florida have become enriched with P. In a section of the Northern Everglades known as Water Conservation Area 2A, a well-established nutrient enrichment gradient has been created. Several ecological changes have been attributed to nutrient enrichment: invasion of cattail and displacement of the native sawgrass, increases of more than a factor of two in net primary productivity (Davis, 1991), and an elevation in the concentration of soil P levels by up to a factor of 2.5 to three in peat accumulated since the 1960s (Craft and Richardson, 1993a, 1993b; Reddy et al., 1993; Debusk et al., 1994) in both labile and refractory forms of soil P (Qualls and Richardson, 1995). However, C/N ratios in the peat are similar along this gradient. Craft and Richardson (1993a), in comparing the accretion rates of peat to inputs from net primary productivity in enriched and unenriched areas of the Everglades, postulated that the decomposition rates must have been higher in the enriched areas than in unenriched areas because the increased net primary production should have resulted in even higher rates of accretion of peat than were observed. The question of the effects of P on decomposition and the mechanisms of P removal have also assumed additional importance because the world's largest constructed wetland for nutrient removal is currently being constructed on the periphery of the northern Everglades as part of an Everglades restoration program (Guardo et al., 1995). The process of microbial immobilization in litter could play an important role in storing P in peat accreting in these enriched marshes.
The objectives of this study were (i) to experimentally test whether PO4 enrichment alone affects plant litter decomposition rate; (ii) to test the quantitative relationship between PO4–P concentration and decomposition rate of sawgrass and cattail litter; (iii) to determine whether increased PO4 concentration results in increased immobilization of N, Cu, Ca, and K; and (iv) to determine whether the P additions resulted in large increases in the content of soil microbial biomass P; and (v) to determine the depths to which any effects would occur.
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Methods |
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200 m apart (referred to as Sites 1 and 2). Channels were constructed by sinking plastic walls into the natural substrate. One end was closed, but the downstream end was left open. Channels were 2 m wide and 8 m long. Controlled concentrations of Na2HPO4, mixed into natural water from the site, were pumped continuously at rate of 1.9 L min-1 into the closed ends of the channels. At each of the two sites, one control channel received no additional PO4 and four other channels received four different levels of PO4 additions.
Senescent leaves still attached to T. domengensis and C. jamaicense plants were gathered in September. In the subtropical climate of the Everglades, leaves of these plants are produced and also senesce at varying rates all year (Davis, 1991). During this period, a particularly strong cycle of senescence was underway. We used only leaves that were completely devoid of green color and standing above the water surface, but which had not undergone extensive decomposition. Leaves were cut into 20-cm lengths and were air dried until they reached a constant weight. Batches of leaves of 5 g were weighed to the nearest 0.01 g and were then placed into nylon mesh bags (4-mm mesh size) (Qualls, 1984). Leaves in three of these bags were dried at 70°C to estimate a conversion factor for all samples at 70°C. These were saved for chemical analysis to serve as the initial samples.
Bags of each species of plant litter were placed in each of the five experimental channels at each of the two sites late in the month of September. Bags were placed at a distance of 1 m from the source of nutrient input. Because of uptake and dilution, concentrations of PO4–P were lower than the nominal input concentrations but were measured every 2 wk at the location of the bags. Concentrations of total P, dissolved organic P, NO3 + NO2, NH4, Ca, and K were monitored monthly (Cu was not measured in water). Average concentrations in the water were calculated as the time-weighted average during the period of incubation. The ascorbic acid molybdate blue method was used to measure PO4 (Wetzel and Likens, 1991). Total P in water was measured by persulfate digestion followed by measurement of PO4 (Wetzel and Likens, 1991). Ammonia was measured by an automated phenate method (Technicon Industrial Systems, 1987a). Nitrate + NO2 was measured by automated Cd reduction and diazotization (Technicon Industrial Systems, 1987b). Calcium and K were measured by atomic absorption spectroscopy with a Perkin Elmer Model 5100 PC atomic absorption spectrophotometer (Perkin Elmer, Norwalk, CT). Temperature was monitored every 2 h in the water of each channel with probes attached to two dataloggers. Data reported for temperature excludes the final 40 d of the year of incubation because not all probes were operating. Dissolved O2 was measured approximately monthly for purposes of comparing channels at midday near the surface of the litter bags by agitating a YSI 5739 dissolved O2 probe. The pH was measured at the same time with a glass combination electrode.
Bags of Cladium litter were retrieved from the unenriched control and the channel receiving the highest level of PO4 input channels after 32, 128, 245, and 365 d of incubation in order to observe the trends in decomposition and immobilization of nutrients with time. In addition, three to four bags (in most cases) of both species from all channels were retrieved after 365 d to observe a more detailed trend in annual decomposition as a function of concentration. In several cases, not all three bags could be located, which resulted in fewer replicates. Leaves were gently and carefully brushed in a pan of water to remove any periphyton. Examination under a dissecting microscope indicated that no significant amount of leaf material was lost by this procedure. Litter was dried at 70°C to a constant weight, stored in a desiccator for 24 h and weighed to ± 0.01 g. Litter was ground and analyzed for C and N (Perkin Elmer 2400 CHN Elemental Analyzer). Litter was 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 bicarbonate-extracted P of the sample which was, or was not, initially chloroform fumigated was reported as the "chloroform-released 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 g-1 element at time t/mg g-1 element at time 0)(litter mass at time t/litter mass at time 0).
The average PO4–P concentration was the average of all PO4–P concentration samples taken during the course of the year at the location of the individual litterbag. Each individual litterbag was treated and plotted as an individual observation. An analysis of covariance was also done by using site as a categorical independent variable, average PO4–P concentration as an independent variable, and percentage original elemental content as the dependent variable in order to determine whether the slopes of the relationships differed between the two sites. To determine whether averages of any other independent variable (NH4–N, NO3–N, Ca, dissolved O2, pH, and water temperature) might explain trends, these were each regressed against the data for proportion of C remaining after 1 yr.
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Results |
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15 µg L-1, although the variability prevented a precise analysis of the characteristics of the curvilinear relationship. The regression was highly significant (P < 0.001). The polynomial gave better fits to the P remaining data (Fig. 6) than linear fits (
in the case of cattail litter and
in the case of sawgrass litter). These fits were not used to imply a particular theoretical relationship but only to test whether there was any significant increase as a function of PO4–P concentration. The maximum fraction of P remaining in sawgrass litter was much higher than that in cattail litter, but the initial P content in sawgrass litter was much lower (112 µg g-1 in cattail litter vs. 41 µg g-1 in sawgrass litter). In contrast to P, the N, Cu, Ca, and K remaining in litter after 1 yr of decomposition was not significantly related, in most cases, to the average PO4–P concentration water (Table 1)
. The one exception to this lack of trends was that the N remaining in the cattail litter after 1 yr of decomposition declined slightly but significantly
with increasing average PO4–P concentration in water (Table 1). Compared with the initial contents, there was a net increase in the N and Cu (in some cases several times more) content in the litter of both species across the range of all PO4–P treatments. In the case of Ca, sawgrass litter had gained Ca, while cattail litter had lost net amounts of Ca across the range of PO4 treatments (Table 1). However, the initial concentration of Ca was much lower in sawgrass litter (0.2% Ca) than in cattail litter (1.09%). In the litter of both species, only 10 to 20% of the initial K remained in litter after 1 yr in all channels.
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Discussion |
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TOPABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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The increase in decomposition rate due to addition of exogenous nutrient is often regarded as occurring during the initial stages of decomposition when concentrations of the leaf tissue are low (i.e., C/P or C/N ratios are high) (Alexander, 1977). As litter decomposes and N and P are concentrated in the litter, a critical C/N or C/P ratio is eventually reached, after which net mineralization and release of N and P occur. In this study, much of the acceleration of decomposition may have occurred in the early, more nutrient-limited stages of decomposition. However, there is evidence that P enrichment stimulates the decomposition rate of peat from the Everglades, since Craft and Richardson (1993a) found that the balance of net primary productivity and peat accretion would require higher rates of decomposition in the P-enriched areas than in unenriched areas. We hypothesize that the early stages of relatively rapid decomposition above the peat surface before burial is a critical stage in long-term accretion of peat and accretion of P. This is because the acceleration of decomposition while litter is still in the aerobic zone may dramatically reduce the proportion of litter buried in the anaerobic zone. In fact, the final concentration of P in the cattail litter after 1 yr in the channel receiving the highest P input was 1274 µg g-1 soil (± 282 standard deviation) (Table 3) , which is only slightly less than the P concentration in the 0- to 5-cm depth soil of the four most enriched plots (1400–1550 µg g-1 soil) along the nutrient enrichment gradient in Water Conservation Area 2A (Qualls and Richardson, 1995). This correspondence between the P concentration of the cattail litter in the most enriched channel and that found in the enriched zone of the northern Everglades not only suggests a reasonable correspondence to decay conditions there, but also that the litter approached the critical C/P ratio within a year.
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These differences among various studies in response to P additions could be related to the N/P ratio in the water and the initial N/P ratio in the litter itself. The initial N/P ratio (by mass) in our cattail litter was 37:1 and in the sawgrass litter was 73:1 (Table 3). Both of these ratios are relatively high and in a range which might suggest P rather than N limitation of litter decomposition (Vogt et al., 1968). In fact, the studies cited above in which leaf concentrations were measured, fall into a pattern of stimulation, or lack thereof, according to the initial N/P ratio. In the study of Elwood et al. (1981), the N/P ratio of the oak (Quercus spp.) leaves that exhibited a response to PO4 additions was 34:1, with a P concentration of 0.021%. In the study by Lockaby et al. (1996) which showed no response to PO4 addition, the N/P ratio was 8.1. The initial P contents of our sawgrass and cattail litter were quite low compared with those reported in many other studies (Vogt et al., 1968), but the initial P concentration of sawgrass litter was consistent with values reported by Davis (1991), who reported values of 50 mg g-1 from an unenriched area of the Everglades. These low P concentrations in litter reflect the extreme P deficiencies in the unenriched portions of the Everglades.
Additions of P have also been shown to increase microbial respiration in peat with low P concentrations from the southern Everglades (Amador and Jones, 1993). In peat with high P concentrations, however, P additions did not stimulate respiration. Additions of NH+4 did not stimulate respiration in the peat with low P content but did in peat with a high P content.
The initial P concentration in the litter also exerts an influence on the decomposition rate. Debusk and Reddy (1998) incubated samples of litter and peat gathered from a nutrient enrichment gradient in Water Conservation Area 2A in the laboratory and found that CO2 production correlated with the initial P content of the substrate, which in turn, was higher in the nutrient-enriched area of the gradient. Thus, P enrichment can speed decomposition rate of litter in the Everglades both by supplying exogenous P for microbial immobilization and by influencing the initial quality of the substrate. They also found that cattail litter decayed faster than sawgrass litter.
Immobilization of Macronutrients by Litter
The immobilization of P by litter in response to increased PO4–P supply may be an important mechanism of removing and storing excess P from the overlying water. It may also help explain the dramatically higher P content and P accretion rate of peat in the enriched areas of Water Conservation Area 2A (Davis, 1991; Craft and Richardson, 1993a; Qualls and Richardson, 1995). Craft and Richardson (1993a) found up to 2.5 to three times the P content (mg g-1 soil) in peat deposited since the 1960s in the P-enriched area of Water Conservation Area 2A compared with the unenriched areas further from the source of nutrient inputs in canal water. In litter decomposed for 1 yr, we found up to eight to 10 times the absolute mass of P in litter in the most enriched channel compared with the control channel. Since this litter will eventually be incorporated in peat, this uptake of P is likely to be very important in the P budget of the Everglades.
We had hypothesized that an increase in microbial activity stimulated by PO4 availability, and accompanied by an increase in P uptake by litter, might also result in a greater uptake of N by the microbial biomass. This might occur even under conditions where N was not limiting to decomposition simply because a higher microbial biomass would result in a higher quantity of N in the biomass. Exogenous N was apparently immobilized and accumulated in litter, as shown by the two- to fourfold increases in N remaining in the litter, but it did not accumulate differently in the P-enriched litter despite the dramatic differences in P in the litter. A greater cycling of C and P through the microbial biomass because of turnover rather than standing stock may explain why the accumulation of P was so much different from that of N. Likewise, the N content of peat was similar in the unenriched and enriched areas (Craft and Richardson, 1993a), and indeed we found that N uptake by litter was similar regardless of P treatment.
The general patterns of net uptake, and in some cases, subsequent decreases of P, N, Ca, and K, were typical of the patterns observed in litter decomposition studies, but they differed in magnitude and timing. Brinson (1977), for example, found a net increase in N, P, and Ca in the early phases of decomposition followed by a later net decrease as the weight loss of the litter progressed to the advanced stages of decomposition, presumably indicating a phase of net mineralization. Brinson (1977) also found, as we did, that K leached readily and did not accumulate in litter.
Immobilization of Copper by Litter
Copper was also taken up and accumulated by the decomposing litter to amounts that were generally between 1.5 and six times that in the initial litter (Fig. 5, Table 1). Copper is potentially a limiting nutrient for plant growth in the Everglades peat soils. Copper limits the growth of crop plants grown on drained Everglades peat, as it does in many Histosols (Coale, 1994). This process of uptake of Cu may be one that competes for available Cu with living plants and algae and may be the initial step in long-term sequestration of this potentially limiting nutrient. Humic acids bind Cu strongly, and humification of decomposing litter could be an explanation for the uptake by litter, in addition to microbial uptake and immobilization. This is the first report of which we are aware that demonstrates Cu uptake by decomposing litter. The P-enriched areas of the northern Everglades have also been enriched in Cu (Vaithiyanathan and Richardson, 1997), with a gradient in Cu content corresponding with a well-established gradient in soil P concentrations. Since crops in the Everglades Agricultural Area are fertilized with Cu (Coale, 1994), runoff may be a possible source. Thus, immobilization of Cu by litter may aid in reducing concentrations in surface water.
Microbial Biomass Phosphorus in Soil
Microbial biomass P concentration, as indicated by the chloroform-released P, was greatly elevated in the surface layers of soil. The origin of this P in the microbial biomass might have been (i) immobilization of P in microbes decomposing detritus present in the soil before the P additions began, (ii) assimilation of P from P-rich periphyton detritus deposited after P additions began, or (iii) assimilation of P from P-enriched macrophyte leaf or root litter deposited after the P additions began. While benthic algal periphyton was observed on the surface, we had shaved off the 2-mm surface layer, which appeared to eliminate any noticable green color. The restriction of the P enrichment of the microbial biomass to the near-surface layers might suggest deposition of P-rich detritus from above, but immobilization of P by microbes on older detritus might also be consistent with this observation since the more recent upper layers of soil might be expected to harbor a more active microbial community. It was notable that many macrophytes had a greater density of roots in the peat below the P-enriched layer, so it appeared that if there was any translocation of P to the roots from the aboveground parts, it had not cycled to the deeper microbial biomass at that time.
In a study of 15 different soils, Brooks et al. (1984) found no relationship between the percentage of P in microbial biomass and the bicarbonate-exchangeable PO4 in the soil. In a study of the effect of fertilizer on a cropped soil, Ghoshal and Singh (1995) found a 37% greater microbial biomass P in the fertilized soil than in the control. The dramatic increase in microbial biomass P in our study was greater in magnitude than any comparable study we could locate. Debusk and Reddy (1998) found that microbial biomass C in surficial peat was greater in the P-enriched area of Water Conservation Area 2A, suggesting that P enrichment resulted in higher microbial biomass either through effect of P on P-limited microbial growth or by influencing the replacement of sawgrass with the more rapidly decomposing cattail.
Two of the most rapid responses to P enrichment that result in P removal might be adsorption to the soil and microbial immobilization. Comparison of the small elevation of exchangeable P to the massive elevation of microbial biomass P shows the importance of the microbial processes in this peat soil with a limited adsorption capacity (Richardson and Vaithiyanathan, 1995) compared with mineral soils. This response might be contrasted to the initial removal of P in Fe- or Al-rich mineral soils by adsorption (Richardson and Marshall, 1986; Richardson, 1985).
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Conclusions |
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TOPABSTRACTINTRODUCTIONMethodsResultsDiscussionConclusionsREFERENCES |
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In the Everglades, P enrichment has been shown to increase net primary productivity, increase peat accretion, and increase litter decomposition rate — a general acceleration of C and nutrient cycling. While litter decomposition rates are increased, large increases in net primary productivity (Davis, 1991) apparently more than compensate for the increased decomposition rate in controlling the rate of peat accretion (Craft and Richardson, 1993a). Concern has also been expressed that the increased O2 demand created by these increases in decomposition may also adversely affect dissolved O2 concentrations (Amador and Jones, 1993). The remarkable capacity for extremely P-deficient litter and peat to immobilize large quantities of P also works to remove PO4 from water. These factors will all be important in the functioning of constructed wetlands (Guardo et al., 1995) meant to remove P from waters entering the northern Everglades.
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ACKNOWLEDGMENTS |
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Received for publication February 11, 1999.
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