Phosphorus Recycling from Wastes

Abstract

The plant phosphorus (P) uptake by Italian ryegrass using organic wastes such as sewage sludge, biochar, composted sewage sludge and spent wetland filters as fertilizer was investigated in pot experiments after manipulating the wastes pH by mixing with other wastes giving acidification by acetic acid and compost leachate, and liming by concrete waste and lime. Pots with no fertilizer and with mineral fertilizer served as control. Available P was measured both with passive diffuse gradient thin film (DGT) samplers and by P uptake in the grass. The pH in the treated waste was about 4 in the acid treatment, and more than 11 in the high pH treatment. The pH in the pot during the grass production was adjusted to normal levels. The P uptake increased up to 56% after pH treatment of the waste. The P uptake responded both to the high and low pH treatments, probably due to the prevalence of different P species. The more extreme pH treatments gave the highest uptake. The DGT uptake gave the same broad picture in the pots fertilized with biochar but not in pots with sludge, and, also, without the separation between high and low treatments. The passive samplers correlated relatively well with the measured grass uptake. A principal component (PCA) analysis showed that the P uptake as measured by the DGT correlated with P, potassium (K) and silicon (Si) concentrations, at to a lower degrade with iron (Fe) and lead (Pb), and was uncorrelated with nickel (Ni) and cupper (Cu). In summary we can say that the pH treatment of the different organic wastes increased the plant P availability. The smallest increase was in the wetland filter that also had the lowest P total uptake. Also the lupin treatment increased the plant P uptake.