Organic matter, nitrogen and phosphorus fluxes associated with leaf litter in two small streams with different riparian vegetation: a budget approach

We studied the leaf budget in two forest stream sites: site D was located in a stream that flows through deciduous forest, and site E was located in a stream that flows through a eucalyptus plantation. Leaf inputs, transport, benthic storage and breakdown rates at both sites at different periods were measured over 5 years. Data have been combined to calculate leaf budgets with the assumption that both streams were in steady state. The total leaf input was 20 % lower at site E, but the mean benthic stock of leaves and the amount of leaves that was processed were about 1.5 times higher than at site D. Inputs of nitrogen and phosphorus associated with leaves were 50 and 20 % lower at site E, but small differences in the benthic storage of nutrients were observed between the two sites. The streams processed 31-57 % of the nitrogen, and 20-57 % of the phosphorus contained in the leaf input. Afforestation with eucalyptus has a low impact on leaf litter processing as the longer residence time of eucalyptus leaves in the stream balances their lower processing rate. However, it modifies nutrient fluxes associated with leaf litter due to the low nutrient content of eucalyptus leaves and to their role as nutrient sources during leaf decay. At both streams, alder litter was processed more efficiently than other species and was the main source of nitrogen and phosphorus for stream decomposers. We hypothesize that dependence on this fast decaying species is an indication of the disturbance of CPOM dynamics due to historical changes in our streams and that it may be difficult to isolate the effects of former forest disturbances from the impact of eucalyptus plantations.