Decomposition responses to phosphorus enrichment in an Everglades (USA) slough

The effects of phosphorus (P) enrichment on decomposition rates were measured in a P loading experiment conducted in an oligotrophic marsh in the northern Everglades, USA. In this study, eighteen 2.5 m(2) enclosures (mesocosms) were placed in a pristine open-water (slough) wetland and subjected weekly to 6 inorganic P loads; 0, 0.2, 0.4, 0.8, 1.6 and 3.2 g.m(-2)g.yr(-1). Phosphorus accumulated rapidly in the benthic periphyton and unconsolidated detrital (benthic floc) layer and significantly higher P concentrations were recorded after 1 yr of P addition. In contrast, a significant increase in surface soil (0-3 cm) TP concentrations was measured in the surface soil layer only after 3 yr of loading at the highest dose. Plant litter and benthic floc/soil decomposition rates were measured using litter bags, containing sawgrass (Cladium jamaicense Crantz) leaves, and cotton (cellulose) strips, respectively. Litter bag weight losses were similar among treatments and averaged 30% at the end of the 3 yr study period. Litter N concentrations increased over time by an average of 80% at P loads < 1.6 g.m(-2).yr(-1), and by > 120% at P loads greater than or equal to 1.6 g.m(-2).yr(-1). In contrast, litter P concentrations declined up to 50% in the first 6 months in all P loads and only subsequently increased in the two highest P-loaded mesocosms. Cotton strip decay demonstrated that benthic floc and soil microbial activity increased within 5 mo of P addition with more significant treatment effects in the benthic than the soil layer. The influence of soil microbial transformations was shown in porewater chemistry changes. While porewater P levels remained close to background concentrations throughout the study, porewater NH4+ and Ca2+ increased in response to P enrichment, suggesting that one significant effect of P enrichment in this oligotrophic peat system is enhanced nutrient regeneration.