Total P in Ogeechee surface soils (349.7 ± 21.3 and 375.5 ± 29.0 mg kg-1; Table 1) was at the low end of Pt values reported for wetlands (300–1400 mg kg-1) by Faulkner and Richardson (1989); soil N/P ratios were within the range reported for forested wetlands in the southern USA by Bedford et al. (1999). Resin-extractable Pi in Ogeechee floodplain soils (9.5 ± 1.4 mg kg-1; Table 1) was similar to values cited by Stanturf and Schoenholtz (1998) for extractable P in blackwater river floodplain soils (10–11 mg kg-1; extraction method not defined by authors). Microbial P (CHCl3-fumigated/NaHCO3-extractable P less NaHCO3-extractable P and adjusted with a kp of 0.4) concentrations may seem high, both in absolute amounts and in relation to other fractions (Table 1). However as a percentage of total P, microbial P (29.2%) was comparable to values reported for NC Coastal Plain bay forest soils (37.4%) (Walbridge, 1991) and Appalachian forest soils (18–24%) (Walbridge et al., 1991), and was markedly less than microbial P in wetland wastewater treatment systems (60%) (Lee et al., 1975; Sloey et al., 1978). Microbial P in Ogeechee floodplain soils (109.6 ± 10.7 mg kg-1; Table 1) was above the range reported by Brookes et al. (1982) for nonnative (added) microorganisms in a variety of soils, including those under native grasses, deciduous forest, and cultivated crops (5.3–72.3 mg kg-1), but comparable to levels of microbial biomass P reported by Joergensen and Scheu (1999) for a mineral forest soil (86–114 mg kg-1).
We hypothesized that P availability would increase with increased flooding duration and periodicity, because of the flooding-induced release of P from Fe and Al phosphates. Phosphorus availability did increase in response to artificial flooding. As indexed by daily supply to in situ AER, P availability was either significantly greater in flooded versus control soils, and/or decreased significantly following drainage, in each of the five flooding treatments at some time during the 
6-mo study (Fig. 2), while total P supply to in situ AER was significantly greater in flooded versus control mesocosms regardless of treatment (Table 2). However, during the course of the experiment, we observed no significant changes in Fe and Al (NaOH I and II) P in any treatment (Wright, 1998) that suggested the observed increases in P availability were the result of the solubilization of P from phosphate minerals. Similarly, Darke et al. (1997) found no significant changes in soil Fe or Al chemistry in response to flooding in this same experiment.
Previous research on upland soils flooded for rice cultivation has suggested that increases in P availability following flooding can be attributed to the reduction and dissolution of Fe (III) phosphates, the hydrolysis and dissolution of Fe and Al phosphates, or the release of clay-associated phosphates through anionic exchange (Ponnamperuma, 1972; Gambrell and Patrick, 1978). Because we observed no flooding-induced changes in Hedley P fractions representing Fe and Al phosphates in these Ogeechee floodplain soils, there is little evidence to suggest that observed increases in P availability with flooding (Fig. 2) were caused by the release of P from geochemical sources.
Alternative explanations for the observed increases in P availability include: (i) P inputs received in river water; (ii) a decrease in biological P demand under anaerobic conditions; or (iii) release of labile P from microbial biomass. With average Pt concentrations in Ogeechee River water of 59 µg L-1 (Meyer, 1986) and the low flow rates used in this artificial flooding experiment, river water is an unlikely explanation for the increases in P availability observed with flooding. Similarly, because the quality of river waters used to flood our mesocosms was uniform across treatments, flooding-induced increases in P availability associated with river water should have been more uniform across treatments than was observed (Fig. 2).
The decrease in biological P demand that occurs as aerobic microorganisms and plants become dormant under anaerobic conditions (Gambrell and Patrick, 1978; Schlesinger, 1997), at least partially explains the increases in P availability observed with flooding. While investigating the effects of flooding on decomposition dynamics in another aspect of this study, Lockaby et al. (1996a) found that significant P release from litter only occurred after drainage, suggesting that microbial activity was inhibited by flooding. Microbial N demand is known to decline under anaerobic conditions (Gambrell and Patrick, 1978). Since microbial N/P ratios are relatively constant over a wide range of soil conditions (Schlesinger, 1997), changes in microbial P demand should be consistent with those observed for N. In addition, many plants are known to tolerate short-term flooding through dormancy (cf., Mitsch and Gosselink, 2000).
A third explanation might be the release of labile P from microbial biomass following flooding, because of either the lysis of aerobic microorganisms under anaerobic conditions or the activities of facultative anaerobes specially adapted to fluctuating oxic and anoxic environments (Fleischer, 1985; Davelaar, 1993). In both wastewater treatment plants (Wentzel et al., 1986) and lake sediments (Boström et al., 1988; Gächter et al., 1988; del Carmen Doria-Serrano et al., 1992; Davelaar, 1993; Gächter and Meyer, 1993; van Veen et al., 1993; Waara et al., 1993), facultative anaerobes have been shown to accumulate Pi in polymer form ("poly-P") under aerobic conditions, hydrolyzing this stored poly-P under anoxic conditions when oxidative phosphorylation is no longer possible. The hydrolyzed Pi builds up within the cells and is released to the environment via diffusion (Wentzel et al., 1986). While poly-P accumulation has been of particular interest to the wastewater treatment industry because of pollutant P removal possibilities, microbes capable of these activities are also common in natural systems, including surface waters and soils (Gächter and Meyer, 1993). Boström et al. (1985) found a large decline in lake sediment residual P (defined as organic and inert P) with anoxia, but no significant fluctuations in NaOH–extractable P, and attributed the decline to a sediment bacteria P release. A recent laboratory study (Khoshmanesh et al., 1999) of wetland sediments also suggested a microbial accumulation of P in aerobic conditions and a subsequent release of P in anaerobic conditions when a proper fermentation product, such as acetate, was present.
In our Ogeechee soils, we observed significant decreases in microbial biomass P over time in two flooding treatments (Fig. 4E and G), and significantly lower microbial P concentrations in flooded versus control mesocosm soils during the first 3 mo of flooding, regardless of treatment (Table 3). Because experimental flooding treatments were initiated in late June, aerobic and facultative anaerobic soil microbial communities would probably have been well-developed.
In these natural floodplain soils, P availability increased in response to flooding. We found no evidence to suggest that flooding-induced increases in P availability were caused by the release of P from Fe and Al phosphates. We did observe significant decreases in microbial biomass P over time in two treatments, and significantly lower microbial P concentrations in flooded soils during the first three months of flooding, regardless of treatment. In natural wetland soils, where periodic flooding is a major factor driving soil pedogenesis, Fe and Al phosphates are probably liberated from parent materials by flooding fairly early during soil development. In this natural floodplain wetland, biological mechanisms provide a more probable explanation for observed flooding-induced increases in P availability.
ACKNOWLEDGMENTS
The authors thank T.M. Williams for providing baseline water table elevations, T. Carpenter for conducting soil physical analyses, M. Thomas for providing unpublished USDA Soil Survey information for Effingham County, Georgia, and C.D. Sutton for providing assistance with statistical analyses. S.L. Beach, R.C. Jones, J. Lawrey, and two anonymous reviewers provided helpful comments on an earlier draft of this manuscript. This research was supported in part by a USDA Competitive Research Grant to Mark R. Walbridge.
Received for publication August 23, 1999.
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