Productivities of microbial decomposers during early stages of decomposition of leaves of a freshwater sedge

Abstract

SUMMARY 1. We examined standing‐senescing, standing‐dead and recently fallen leaf blades of Carex walteriana in fens of the Okefenokee Swamp to determine the nature of the microbial decomposers in the early stages of decomposition, measuring both standing crops and productivities ([ 3 H]leucineprotein method for bacteria, [ 14 C]acetateergosterol for fungi). 2. Fungal standing crops (ergosterol) became detectable at the mid‐senescence stage (leaves about half yellow‐brown) and rose to 14–31 mg living‐fungal C g −1 organic mass of the decaying system; bacterial standing crops (direct microscopy) were ± 0.2 mgC g −1 until the fallen‐leaf stage, when they rose to as high as 0.9 mgC g −1 . 3. Potential microbial specific growth rates were similar between fungi and bacteria, at about 0.03–0.06 day −1 , but potential production of fungal mass was 115–512 μgC g −1 organic mass day −1 , compared with 0–22 μgC g −1 day −1 for bacteria. Rates of fungal production were about 6‐fold lower on average than previously found for a saltmarsh grass, perhaps because much lower phosphorus concentratiofis in the freshwater fen limit fungal activity. 4. There was little change in lignocellulose (LC) percentage of decaying leaves, although net loss of organic mass at the fallen, broken stage was estimated to be 59%, suggesting that LC was lost at rates proportional to those for total organics during decay. Monomers of fungal‐wall polymers (glucosamine and mannose) accumulated 2‐ to 4‐fold during leaf decay. This may indicate that an increase found for proximate (acid‐detergent) lignin could be at least partially due to accumulation of refractory fungal‐wall material, including melanin. 5. A common sequence in decaying aquatic grasses is suggested: principally fungal alteration of LC during standing decay, followed by a trend toward bacterial decomposition of the LC after leaves fall and break into particles.