SOIL MICROBIAL ECOPHYSIOLOGY OF A WETLAND RECOVERING FROM PHOSPHORUS EUTROPHICATION

Soil phosphorus (P) enrichment from external inputs can result in considerable changes in wetland ecosystem structure. What is not known is whether many of the microbial physiological measures that are effective at determining ongoing impact are equally sensitive to reductions in the soil P content. The study was conducted over a two year period (1999–2000) in two areas located in Blue Cypress Marsh Conservation Area (BCMCA), an area (Enriched) with historically elevated soil P (1,544 mg kg−1 in 1995, 877 mg kg−1 in this study) and a reference area (Reference) with background soil P contents (698 mg kg−1). Nutrient loading to this wetland was terminated in 1994. Microbial ecophysiology measures were obtained quarterly and consisted of soil microbial biomass carbon (MBC) content, β-glucosidase and acid phosphatase, and end products of anaerobic microbial metabolism (CO2 and CH4). All measures exhibited significant temporal variation with higher values for MBC content, enzyme activities, and respiration rates during summer months (June and September) and lower in the winter months (December and March). We found no significant differences between site mean MBC (Enriched: 7.24, Reference: 8.22 mg−1 kg−1), CH4 production (Enriched: 4.41, Reference: 4.73 µmol CH4 gr−1 d−1), or β-glucosidase activity (Enriched: 56.14, Reference: 57.70 µg MUF gr−1 h−1). The site mean acid phosphatase (Enriched: 56.92, Reference: 78.74 µg MUF gr−1 h−1) and CO2 production rates (Enriched: 11.00, Reference: 13.69 µmol CO2 gr−1 d−1) were found to be significantly different. Microbial communities at the two sites were different in terms of their metabolic activities, but not in terms of C-pathways. We also found that enzyme profiles at the enriched site did not change appreciably over the two year period. The results obtained in this two year study suggest that most microbial community ecophysiology measures were not responsive to decreasing concentrations of P. However, at both sites, β-glucosidase and anaerobic microbial activities were higher in the second year (41.80 vs. 68.27 µg MUF gr−1 h−1 and 10.34 vs.13.85 µmol CO2 gr−1 d−1) and acid phosphatase activities lower (72.54 vs. 63.11 µg MUF gr−1 h−1). A drawdown that took place in the winter months of late 1999 and early 2000 might have released labile soil components, resulting in increases in overall metabolic activities and repression of the acid phosphatase activities. This has management consequences as P from the enriched are