Extractability of inorganic phosphorus during the composting of agro-industrial waste and sewage sludge

Phosphate rock is a not-renewable resource, in this frame increasing attention is recently paid to the recycling of phosphorus (P) from organic sources. A better knowledge of the potential P availability for plant nutrition from composts from different type of waste play a key role in their better utilization for agronomical purposes. In this work two compost piles were formed by mixing starch waste (SW) or sewage sludge (SS) both (at 20% V/V) with garden trimming (80% V/V). These were composted over 60 days and weekly turned until the oxygen uptake rate (OUR) showed a suitable stabilization level. The samples collected at the beginning of the process (day 0) and after 30 and 60 days were sequentially extracted following the Hedley protocol with increasing strength solutions (H2O, NaHCO3 0.5M, NaOH 0.1M, HCl 1M); on the extracts was determined the inorganic P (Pi) via Murphy and Riley method. At day 0 both SW and SS compost showed similar total P content (4.80 and 4.50 mg g-1); during the composting process these values increased up to 30% following the organic matter mineralization regardless of the mixture; in the same time frame the OUR decreased from 80 to 20 mmol O2 kg-1 VS h-1 in SW and from 40 to 10 mmol O2 kg-1 VS h-1 in SS. Inorganic P is recognized to be the most easily utilizable form by plants, in this light its speciation in composts can give more information for their rational agricultural utilization. Water soluble inorganic P (H2O-Pi) beside to the sodium bicarbonate extractable P (NaHCO3-Pi) is known to be the most easily utilizable by plants. Following we can find the alkali extractable P (NaOH-Pi), which can be utilized by plants in a longer period. At last the HCl extractable P (HCl-Pi), generally ascribed to sparingly soluble not available calcium-P compounds. Sequential extraction showed SW had the highest H2O-Pi (10% of total P) at day 0, being three-fold SS at the same sampling time (H2O-Pi 3%). At the beginning of the process SW showed also the highest NaHCO3-Pi (30% of total P), being higher than SS (20%). Conversely SW showed lower NaOH-Pi in comparison to SS (20 vs. 30%). Both mixtures showed similar HCl-Pi at day 0 (9%) and day 60 (20%), having similar increase. Composting process showed increased whole Pi extractability over time (from 70 up to 80%) regardless of mixtures; this was related to the increase in the sparingly fraction of P (HCl-Pi ) very likely due to the precipitation with calcium. It appears therefore a P shifting from the different fraction during the stabilization process. The utilization of agro-industrial waste or sewage sludge influenced the potential availability of Pi during composting. SW showed decreased H2O-Pi, which can be related to the highest microbial activity (immobilization) following the addition of very unstable material; at the same time the other easily accessible P fraction (NaHCO3-Pi) increased up to 16% in the end. SS on the contrary, more stable from the beginning showed a shifting from the medium-available P forms (NaOH-Pi; -23%) to the more available fraction (NaHCO3-Pi; +24%) suggesting a different mechanism rather than microbial immobilization. As a results of the whole process SW showed increased the easily and intermediate available P (H2O + NaHCO3 + NaOH; +3%), oppositely SS showed 3% decrease. The final output in stable compost for inorganic P fractionation was in SW: NaHCO3 (33%)> NaOH (24%)> HCl (20%)> H2O (3%); SS: NaOH (30%)> NaHCO3 (28%)> HCl (23%)> H2O(4%). Further investigation about the organic P forms in the different fraction is needed beside to the validation with biological test with plant.