Phosphate cycles in energy crop systems with emphasis on the availability of different phosphate fractions in the soil

The phosphate cycle of a C-4-grass species (Miscanthus sinensis ''Giganteus'') and two different willow species (Salix viminalis ''683'' and Salix x dasyclados) were investigated on two different soil types (Gley and Grey Brown Podzolic) in Ireland. Above- and below-ground biomass and soil samples were collected once at each site during the most active growing season in May, June and July respectively. Samples were analysed for total P colorimetrically, for pH, total N, and a P fractionation was undertaken to detect the different availability of the total P pools for plant uptake. Models were drawn to show the P distribution within the crop system, the yearly P fluxes and P export through harvest of the energy crops. Digestion of the only ''inorganic'' resin fraction (Dowex 1 x 8-50, NaHCO3) gave a considerable organic P fraction, which had not been mentioned before in the literature. Outputs of P from harvest were 4% to 20.5% of the cycled P fraction respectively 58-265 kg ha(-1).