Phosphorus cycle of model ecosystems: significance for decomposer food chains and effect of bacterial grazers

The phosphorus cycle of microcosms consisting of decomposing plant material and bacteria (with or without bacterial grazers) was studied using a 32p tracer method and compared with computer simulations of a conceptual model of the system. It is shown that (1) The turnover of phosphorus in such systems far exceeds the actual mineralization of phosphorus from the detrital material; (2) Phosphorus cycling is more rapid in systems with protozoa, although excretion by protozoa is minor, with the increase being mainly due to a higher turnover of phosphorus in grazed than in ungrazed bacteria; (3) The mineralization of detritus is stimulated by protozoan grazing; (4) Phosphorus is partly excreted by bacteria in a dissolved, organic form and; (5) A hot-water extractable pool of phosphorus, about 50% of which consists of polyphosphates is found in bacteria, and PO4--taken up by bacteria is first found in this pool. Limited goal modelling of laboratory microcosms is discussed and found useful for the analysis of the functioning of ecosystems.