A review of methodology used to measure leaf litter decomposition in lotic environments: Time to turn over an old leaf?

Despite the recognition of the relationship between microbial conditioning and invertebrate feeding, there has been little communication between microbial ecologists and zoologists studying the processing of leaf litter in streams. An appraisal of suitable methods is timely, to encourage workers to critically examine their experimental approach: it is often forgotten that results are determined largely by the methods used. We review some recent studies, emphasizing the application of microbiological and biochemical methods, and discuss some important decisions that must be made in experimental design and implementation. It is recommended that newly fallen, naturally abscissed leaves, bound in leaf packs and tethered to natural substrata, be used in studies attempting to simulate natural leaf decomposition. Drying and leaching leaves under extreme conditions should be avoided. Mass loss should not simply be equated with decomposition; instead, losses of the main types of chemical constituents of plant litter should be quantified. If the aim is to study decomposition rather than merely leaf breakdown, the metabolism of dissolved matter and fine particulate material lost from the decaying leaf must be addressed as well. Appropriate techniques should be used to study microbial assemblages: lipid biomarkers or nucleic-acid methods for assemblage composition, microscopy or biochemical analysis for microbial biomass, [3H]-thymidine or [3H]-leucine methods to determine growth rates. Exponential decay curves and processing coefficients should be used cautiously, especially if sample sizes are small and equal throughout the study.