Nutrient release characteristics from droppings of grass-foraging waterfowl (Anser brachyrhynchus) roosting in aquatic habitats

Abstract

In recent decades, notably in large parts of the Western World, many waterfowl populations have increased because of increased food availability. Not uncommonly, eutrophication in these areas may occur perhaps not only because of increased dropping loads but also because nutrients are more readily released from droppings than from fresh and decomposing plant material. We conducted an experiment to study this effect comparing the decay rate and nutrient release of fresh grass leaves with that of droppings in water at 10, 20 and 30 °C. Using a two-component exponential decay model, allowing distinguishing between an easily decomposable (labile) and recalcitrant fraction of the material, we found that the labile carbon (C) fraction of droppings decomposed faster than that of grass leaves, whereas the recalcitrant C decomposition rate was similar to that of grass leaves. Higher temperature increased C decomposition rates of the labile fractions. No temperature dependence was observed for the recalcitrant fractions. Surprisingly, grass leaves and droppings did not differ in the amounts of nitrogen (N) and phosphorus (P) released in the first 90 days, i.e. more than 60% of N and 40% of P were released into the water column. Continuous mineralization of N and P predominated during decomposition of grass leaves, whereas a massive initial leaching was followed by immobilization and later mineralization during decomposition of droppings. All differences during decomposition of the litter types were attributed to the stoichiometric requirement of decomposers and their temperature sensitivity.