Mineralization of Composted¹⁵N-Labelled Farmyard Manure During Soil Incubations

Fresh farmyard manure (C-to-N ratio = 20.3) was composted in the presence of 15N-labelled (NH4)2SO4 using a bench-scale reactor under temperature 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°C and 100% of WHC for 76 weeks. C mineralization of compost was assumed to come from a labile and a recalcitrant fraction which decomposed according to 1 and 0 order kinetic reactions, respectively[Cm = Clabile.(1-e−Klabile.t)+Kresistant.t]. The size of the labile fraction (Ciabiie) represented 31.3 and 19.9% of compost-C for the two soils, and its mineralization rate constant (kiabiie) ranged from 0.03 to 0.09 day−1. N-mineralization was assumed to come from only one labile fraction [Nm = Niabiie.(l-e−Klabile.t)]. This fraction ranged from 34 to 25% of compost-N for the two soils; its mineralization rate constant did not vary with soils (0.006 day−1). Composted farmyard manure produced under these laboratory conditions may not have been completely mature when added to soil and totally representative of comparable material composted in a pile. Nevertheless, it was considered as fairly mineralizable and its mineralization was greatly influenced by soil type.