Effect of composting time on phosphate exchangeability
1999
Traore, O. | Sinaj, S. | Frossard, E. | Kerkhove, J.M. van de
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 concentration in P(i) isotopically exchangeable between 1 min and 3 months (E(1min-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(1min-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 organic forms was not a major pathway for P transformation.
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