Hydroperiod influence on breakdown of leaf litter in cypress-gum wetlands

Many cypress-gum wetlands in the southeast United States are isolated from rivers and streams and are seasonally inundated by rainfall. Organic matter processing in these wetlands is caused primarily by biotic components (i.e ., microbes and invertebrates), which are influenced by timing and duration of seasonal inundation, and low dissolved oxygen levels. Using litter bags , we examined breakdown of cypress (Taxodium spp.) and gum (Nyssa sylvatica var. biflora) leaves in three wetlands with different hydroperiods: (1) fl ooded exposed (FE; 5 mo flooded/6 mo litter exposed), (2) multiple flooded exposed (MFE; 6 mo flooded/exposed/flooded/exposed) and (3) permanently flo oded (PF; 11 mo flooded). Breakdown was fastest in the MFE wetland suggesti ng cycles of wetting and drying accelerated decomposition by promoting micr obial activity through aeration. Even though ergosterol content, an indicat or of fungal biomass on the litter, was similar among wetlands, we hypothes ized that within the MFE wetland microbial activity was promoted by exposed conditions, but during subsequent flooding microbial biomass was kept at a low level by invertebrate consumers. Macroinvertebrate density and biomass were comparable between litter types, but were highest in the PF wetland, followed by MFE, then FE wetlands. Chironomids, oligochaetes, Caecidotea an d Crangonyx were the dominant taxa indicating litter inputs are vital in ma intaining the aquatic foodweb in this system. C,Cypress litter (k = -1.61 y (-1)) had faster breakdown rates than gum litter (k = -1.02 y(-1)), most li kely because of plant morphology and greater surface area available to micr obial decomposers. Ergosterol (mg g(-1) AFDM leaf material) levels were hig her on cypress (34.5) than gum (22.5) litter. In both litter types initial C:N and N:P ratios were > 20, and C:P ratios were > 500, indicating a possi ble P or N/P co-limitation in express-gum wetlands. Elemental gains or loss es in litter were influenced predominantly by litter type and to a lesser e xtent by hydrologic regime. Gum leaves accumulated P, N, Ca and Ii and lost MG, whet cas cypress leaves had initial declines of these elements, follow ed by some accumulations in F and E;. Temporal patterns of P showed that th e drier sites (FE and MFE) immobilized more P than the wetter site (PF), su ggesting that exposed conditions promoted microbial activity In addition, N and F accumulations on gum leaves were highest in the summer at the time w hen wetlands would normally dry, indicating a seasonal period when moisture and temperature conditions are optimal for microbial growth. Net flux rate s to the 1 + I old component of litter indicated that the FE wetland is acc umulating more organic matter (172 g m(-2) y(-1)) than the other wetlands ( 65 and 72 g m(-2) y(-) (1)), which we attributed to higher cypress litter p roduction. We concluded that hydrologic regime influences breakdown rates a nd element accumulations, but that net productivity is mure important in de termining litter accumulation rates.