Towards a budget of leaf litter decomposition in a first-order woodland stream

To construct a budget of carbon transformations occurring during leaf decomposition, alder leaves were placed in a woodland stream, later retrieved at weekly intervals, and rates of fungal and bacterial production, microbial respiration, and release of dissolved organic matter (DOM) and fine particulate organic matter (FPOM) were determined during short laboratory incubations. Carbon dioxide was the major decomposition product, explaining 17% of the microbially mediated leaf mass loss. DOM and FPOM were also important products (5 and 3% of total mass loss, respectively), whereas carbon flow to microbial biomass was low (2%). Fungal biomass in leaves always exceeded bacterial biomass (95-99% of total microbial biomass), but production of bacteria and fungi was similar, indicating that both types of microorganisms need to be considered when examining leaf decomposition in streams. Comparison of leaf mass loss in coarse and fine mesh bags revealed, in addition, that the shredder, Gammarus pulex, had a major impact on leaf decomposition in this study.