RESPONSE OF EVERGLADES PLANT-COMMUNITIES TO NITROGEN AND PHOSPHORUS ADDITIONS

Nitrogen (N) and phosphorus (P) were applied to sawgrass (Cladium jamaicense), mixed sawgrass-cattail (Typha domingensis), and slough (shallow water communities dominated by Utricularia spp., Eleocharis spp., and Panicum spp.) communities in the Everglades for two years to test for N or P limitations and to investigate the plant community response. Nitrogen (as NH4+) and P (as PO43-) were applied singly and in combination at rates of 0.6, 1.2, and 4.8 g P . m(-2) . yr(-1) and 5.6 and 22.4 g N . m(-2) . yr(-1). Plant response was quantified by measuring aboveground standing crop biomass and tissue N and P concentrations each year. Everglades plant communities are P limited. Phosphorus additions at the highest rate (4.8 g . m(-2) . yr(-1)) resulted in increased P uptake and biomass production by emergent vegetation. Tissue P concentrations of sawgrass and cattail were significantly higher in response to the high P (329-684 mu . g g(-1)) and high N+P (371-594 mu g . g(-1)) treatments (control=94-256 mu g . g(-1)) in both years after the initiation of nutrient additions. Aboveground biomass also increased in response to the highest rate of P at the sawgrass (2618-3284 g/m(2); control = 1158 g/m(2)) and mixed (1387-1407 g/m(2); control = 502 g/m(2)) communities, but only after two years. At the slough site, the high P and high NSP treatments resulted in a significant decline of the Utricularia-periphyton mat after only one year of nutrient additions (16-74 g . m(-2); control=364 g . m(-2)). During the second year, the macroalga, Chara, expanded in these plots and replaced the floating mat as the major nonemergent component of the plant community. In all three communities, P additions at the highest rate resulted in a significant increase in bicarbonate-extractable and total soil P (0-5 cm depth). There was no effect of N additions on biomass production, nutrient uptake, or N enrichment of the peat during the two-year study. We observed no significant change in macrophyte species diversity or expansion of cattail in plots receiving nutrient additions during the two year study. However, the decline of the Utricularia-periphyton mat (and the subsequent increase in Chara) in slough plots receiving 4.8 g P . m(-2) . yr(-1) may serve as an early indicator of P enrichment in the Everglades.