, Volume 55, Issue 2, pp 123-131

Effect of composting time on phosphate exchangeability

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Because of their high concentrations in organic matter and nutrients, composts have been used as soil amendments for years. However, information on their P availability is scarce. The effect of the composting time on phosphate exchangeability of composts was assessed on three substrates (House Refuse Compost, HRC; Sewage Sludge Compost, SSC; and Food Waste Compost, FWC) using the isotopic exchange kinetic method proposed by Fardeau (1996). Results were then interpreted by a pluricompartmental analysis and compared to those yielded by a sequential extraction. Preliminary results confirmed that the isotopic exchange kinetic method was appropriate to assess phosphate exchangeability of composts. Composts were shown to have a low buffering capacity (r(1)/R) for inorganic P (P i ) and high concentration in water extractable P i (Cp) and in P i isotopically exchangeable within 1 min (E 1min ) compared to soils. Their concentra tion in P i isotopically exchangeable between 1min and 3 months (E i1min−3months ) and in P i which cannot be exchanged within three months (E >3months ) was a function of their origin. Composting of HRC, SSC, and FWC, systematically led to decreases in Cp and E 1min with time and in some cases to increases in E i1min−3months and/or in E >3months . These changes were related to the leaching of water soluble P i from the HRC and FWC composts and, for the SSC and FWC composts, to the formation of phosphate precipitates with Ca, Mg and/or Fe during composting. Most of the changes in P i exchangeability occurred during the first month of composting, i.e., during the most intense period of organic matter mineralisation. The slight increase in total organic P content observed after 180 d of composting in FWC and SSC indicates that the immobilisation of P in orga nic forms was not a major pathway for P transformation.