Seasonal fluctuations in live and dead biomass of Phragmites australis as described by a growth and decomposition model:: implications of duration of aerobic conditions for litter mineralization and sedimentation

We developed a model of Phragmites australis growth and decomposition to evaluate the material budget and nutrient cycles of a reed stand in Neusiedlersee, Austria. The model describes the growth of each organ of R australis, the collapse of standing dead shoots, the decomposition of leaves and stalks, and nutrient uptake and release during these processes. The model was calibrated using growth and decomposition data from the literature, and subsequently applied to predict the effects of R australis stands on a marsh ecosystem. From the start of its decomposition in water, the litter was assumed to stay in the aerobic water layer for 6, 12 or 24 months before entering the anaerobic sediment layer. Because decomposition increases with increasing oxygen and temperature, the aerobic decomposition rate (before the litter was transferred to the anaerobic substrate) increased markedly, especially from spring to autumn. The model predicted that between 33 (6 months aerated) and 48% (24 months aerated) of the annual aboveground production would decompose within I year, while the rest would remain in the anaerobic substrate. Rates of nitrogen and phosphorus release were 1.4 times higher between late spring and the end of summer than during autumn and winter. A higher proportion of phosphorus than nitrogen was expected to remain trapped in the anaerobic layer. The uptake of nitrogen and phosphorus during the growing season exceeded release during decomposition 4-6 and 5-7-fold, respectively. The model is useful for quantifying the nutrient cycles of reed-dominant marshes. (C) 2002 Elsevier Science B.V. All rights reserved.