Microbial processes regulating carbon cycling in subtropical wetlands

Wetlands host complex microbial communities including bacteria, fungi, protozoa and viruses. The size and diversity of microbial communities are related directly to the quality and quantity of the resources (i.e., nutrients, energy sources) available in the system. Microbial biomass and activity is highest in habitats where these resources are concentrated, including periphyton mats, plant detritus, and surface soils. Microbial processes regulate major nutrient cycles in wetlands and, therefore play an important role in determining water quality and ecosystem productivity. Many freshwater wetlands are open systems receiving inputs of carbon (C) and nutrients from adjacent agricultural watersheds and urban areas. Prolonged nutrient (such as N and P) loading to wetlands can result in distinct gradients in floodwater and soil. The degree of nutrient enrichment depends on mass loading and hydraulic retention time. This enrichment effect can be seen in many sub-tropical freshwater wetlands, most notably in the Everglades. The Florida Everglades wetlands are historically low-nutrient systems, but are currently impacted by nutrient loading from the adjacent Everglades Agricultural Area (EAA). Microbial communities respond to this enrichment with increased biomass and community and accelerated rates of various processes regulated by microbes. Because of short life cycles of microbes, they respond rapidly to any changes in nutrient or energy source status of wetlands and, thus, provide an early warning signal of eutrophication. In oligotrophic wetlands, nutrients are the primary factor affecting the microbial communities and the processes that control decomposition and nutrient cycling rates. Compared with reference areas of the marsh, nutrientenriched areas are characterized by the rapid turnover of organic matter, and by open elemental cycling, where nutrient inputs often exceed demand for primary and secondary production. These changes have important environmental and ecological consequences including: (1) a conversion from a P-limited to an N-limited system because of high P availability and increased biological demand for N, and (2) an accumulation of low N:P ratio detritus and accelerated rates of decomposition and nutrient cycling. Many biogeochemical processes that affect plant productivity and water chemistry are accelerated by P enrichment, resulting in the release of other plant nutrients such as N. The accumulation of P and other nutrients in the soils and biota coupled with accelerated cycling rates may maintain eutrophic conditions in already enriched areas for some time following load reductions. REDDY ET AL. 17 WCSS, 14-21 August 2002, Thailand 982-2