The presence of Fe-oxidizing bacteria in the rhizosphere of four different species of wetland plants was investigated in a diverse wetland environment that had Fe(II) concentrations ranging from tens to hundreds of micromoles per liter and a pH range of 3.5 to 6.8. Enrichments for neutrophilic, putatively lithotrophic Fe-oxidizing bacteria were successful on roots from all four species; acidophilic Fe-oxidizing bacteria were enriched only on roots from plants whose root systems were exposed to soil solutions with a pH of <4. InSagittaria australis there was a positive correlation (P < 0.01) between cell numbers and the total amount of Fe present; the same correlation was not found for Leersia oryzoides. These results present the first evidence for culturable Fe-oxidizing bacteria associated with Fe-plaque in the rhizosphere.

It has been hypothesized that periodically flooded forests have higher rates of aboveground net primary production than upland forests and near-continuously flooded forests, but a competing hypothesis holds that the benefits of periodic inputs of nutrients and water may be diminished by stresses associated with anaerobic soils or drought. To test these hypotheses, we measured groundwater table depths and aboveground productivity in floodplain forests of South Carolina and Louisiana. We established paired plots on locally dry, intermediate, and wet topographic positions across three hydrologic transects in each state. These plots encompassed upland hardwood, bottomland hardwood, and cypress swamp forests. Measurements of leaf litterfall, wood production, and groundwater table depth were made in 1987 and 1988. We then used mean growing-season water depth (MWD) to group the plots into three classes: wet (Ͼ0 cm), intermediate (0 to Ϫ60 cm), and dry (ϽϪ60 cm).Aboveground net primary production (NPP) on wet plots (2-yr mean Ϯ 1 SD ϭ 675 Ϯ 271 g·m Ϫ2 ·yr Ϫ1 ) was significantly lower than on intermediate and dry plots (P Յ 0.02). There was no significant difference between intermediate and dry plots (107 Ϯ 189 and 1038 Ϯ 91) g·m Ϫ2 ·yr Ϫ1 , respectively). In addition, aboveground NPP on intermediate plots was not significantly different from 22 temperate upland forests in the literature.Combining our data with data from the literature, we found that aboveground NPP on wet plots was negatively related to MWD with a slope of Ϫ5 g·m Ϫ2 ·yr Ϫ1 ·cm Ϫ1 . On sites with evidence of hydrologic disturbance (Ͼ25% dead stems) the slope of this line was 5 times greater (Ϫ24 g·m Ϫ2 ·yr Ϫ1 ·cm Ϫ1 ).We conclude that the subsidy-stress hypothesis does not adequately describe patterns of NPP across Southeastern U.S. floodplain forests. Conditions of periodic flooding and flowing water do not often lead to high rates of productivity compared with upland forests. However, extensive flooding is nearly always a significant stress on forest productivity, particularly when the flooding regime has been recently perturbed through levee construction or impoundment. Our data support a more complex interaction between subsidy and stress factors.Key words: flooding regime and net primary production; flooding stress and floodplain forest production; floodplain forests, southeastern United States; hydrologic disturbance and net primary production; hydrologic disturbance in floodplain forests; Louisiana, USA; net primary production in bottomland hardwood forest; South Carolina, USA.

[1] We investigated the relationship between CH 4 production and oxidation in two tidal freshwater wetland forests in order to determine whether CH 4 oxidation efficiency was limited by O 2 or CH 4 . Methane oxidation was measured in situ over a 16-month period with bi-monthly applications of the inhibitor CH 3 F. Oxidation consumed 52 ± 10 and 81 ± 9% of diffusive CH 4 emissions on the two sites. Methane oxidation rates were linearly related to gross CH 4 emissions on both sites (r 2 = 0.96), demonstrating the process was CH 4 -limited. This interpretation is consistent with the fact that the apparent activation energies for the potential CH 4 production and oxidation differed by <4 kJ mol À1 . Apparent activation energies calculated from field emissions data were also similar for the two processes. The high CH 4 oxidation efficiency on these sites may be attributed to relatively low rates of methane production, a deep oxidizing zone (5-10 cm), and low cover of understory vegetation capable of CH 4 transport. If our results are typical of forested wetlands, CH 4 oxidation efficiency in forested wetlands will not change in response to soil warming.

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