MINERALIZATION OF COMPOSTED N-15-LABELED FARMYARD MANURE DURING SOIL INCUBATIONS

Fresh farmyard manure (C-to-N ratio = 20.3) was composted in the prese nce of 15N-labelled (NH4)2SO4 using a bench-scale reactor under temper ature conditions which reproduce the evolution of compost in a pile. C and N mineralization of composted farmyard manure (C-to-N ratio = 12. 5) was monitored in two silty soils during an aerobic incubation at 28 -degrees-C and 100% of WHC for 76 weeks. C mineralization of compost w as assumed to come from a labile and a recalcitrant fraction which dec omposed according to 1 and 0 order kinetic reactions, respectively [Cm = Clabile.(1 - e(-klabile.t)) + kresistant.t]. Tle size of the labile fraction (Clabile) represented 31.3 and 19.9% of compost-C for the tw o soils, and its mineralization rate constant (klabile) ranged from 0. 03 to 0.09 day-1. N-mineralization was assumed to come from only one l abile fraction [Nm = Nlabile.(1 - e(-klabile.t))]. This fraction range d from 34 to 25% of compost-N for the two soils; its mineralization mt e constant did not vary with soils (0.006 day-1). Composted farmyard m anure produced under these laboratory conditions may not have been com pletely mature when added to soil and totally representative of compar able material composted in a pile. Nevertheless, it was considered as fairly mineralizable and its mineralization was greatly influenced by soil type.