|Ph.D Student||Zohar Iris|
|Subject||Phosphorus Transformations in Reclaimed Wastewater Irrigated|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Emeritus Abraham Shaviv|
|Full Thesis text|
Use of organic waste for fertilizing cultivated soils may impose an environmental threat, as excess labile phosphorous (P) accumulating on top soils may leach into adjacent water bodies, and cause ecological imbalance and eutrophication.
This study attempted to determine the mechanisms that govern P lability in soils, irrigated with reclaimed wastewater (RW) and freshwater (FW), and the relevant major transformations of P in the tested soils. The study included pairs of soils treated with RW or FW (denoted hereafter: RWs and FWs, respectively) and focused on model soils - a pair of calcareous alluvial clay soils (Grumusol, Acre), sampled on different occasions and included soils from a Lyzimeters experiment (Grumusol and sandy-loam (Hamra)).
In all the tested pairs of soils, P was more labile in the RWs than in the FWs of the same source. The biggest difference was for P dissolved in De-ionized (DI) water. When the same P concentration was applied in DI water to soils, P sorption was lower and weaker in the RWs than in the FWs samples. However, applying P in RW resulted in its higher sorption, probably due to high concentration of cations (e.g., Ca) introduced to the soil by the applied RW. P compounds detected by 31P NMR acquisitions of soil solutions, extracted sequentially, indicated higher lability of P-diesters and stronger sorption of P-monoesters, independent of P compounds detected in the applied RW and very similar in the model RWs and FWs.
A variety of evidences, including geochemical calculations, indicate that P mineral phases, of different solubility, govern P lability in the model RWs and FWs and not adsorption saturation. The nature of the OM compounds introduced by the RW (e.g., humic and fulvic acids) has probably affected the more soluble nature of precipitated mineral phases in the RWs.
A modified procedure for analysis of the isotopic composition of oxygen in phosphate (d18Op) in different P fractions of soils was developed and d18Op was distinguished and tracked in an irrigation-incubation experiment, further elucidating P biogeochemical transformations and different enzymatic activities in the soil solution and on soil surfaces (oxides and P minerals). Similar transformations were observed in the model soils although biological activity was more pronounced in the RWs compared to the FWs. Organic P appears to serve as an intermediate pool and a pool for other nutrients (e.g., C, N) and energy.