Preconditioning of leaves by solar radiation and anoxia affects microbial colonisation and rate of leaf mass loss in an intermittent stream

SUMMARY 1. Seasonal sow intermittence in streams often coincides with early leaf abscission of riparianvegetation due to water stress. When accumulated on dry stream beds or in remaining pools, leavesare exposed to solar radiation or fermentation processes, respectively. However, very little informa-tion exists on how these preconditioning processes could affect leaf decomposition when stream sowhas recovered.2. We simulated natural preconditioning of leaves by irradiation with UV-VIS suorescent lamps andincubation under anoxic conditions. Mass loss rates of preconditioned leaf litter from deciduous trees(Alnus glutinosa , Fraxinus excelsior, Populus tremula and Quercus petreae) were quantiÞed in a tempo-rary stream during base sow conditions. Coarse and Þne mesh litter bags were used to study theeffect of benthic macroinvertebrates and microorganisms on leaf mass loss.3. Preconditioning reduced the concentration of macronutrients such as P, K and Mg and increasedthe relative cellulose content of the leaves. Preconditioning changed the fungal community structure(analysed by DGGE) depending on leaf species and sampling date. Preconditioning in anoxic condi-tions also suppressed fungal decomposer biomass (measured as ergosterol) by 42% resulting in 33%lower mass loss rates in Þne mesh bags. In contrast, mass loss rates were not affected by precondi-tioning when macroinvertebrate decomposers had access to the leaf litter.4. In streams exhibiting seasonal sow intermittence, preconditioning will insuence organic carbondynamics towards lower rates of microbially mediated turnover and towards poorer quality ofdownstream-transported material.Keywords : aquatic fungi, nutrient leaching, photodegradation, seasonal sow intermittence, UV radiation