NITROGEN AND CARBON MINERALIZATION RATES OF COMPOSTED MANURES INCUBATED IN SOIL

Rates of decomposition and inorganic N release from composted manures should be known and predictable on the basis of their composition for their proper use as a source of available N. Four composted cattle manures were analyzed for their inorganic N, soluble organic C and N, and total N and organic matter contents. The composts were incubated in soil for 32 wk at 30 degrees C and 60% water-holding capacity. Inorganic N and CO2 evolution were determined periodically. Inorganic N released from the composts after 32 wk ranged from 11 to 29% of their total N content, 2 to 12% of total N were initially inorganic, and 1 to 5% were soluble organic N. The rates of decomposition of the composts were computed by the simulation model NCSOIL by minimizing the deviation of the model output from the periodically measured CO2 and inorganic N release and by using the soluble and insoluble organic C and N contents of the composts as input. The soluble components decomposed before the end of the first week. The decomposition rate constant obtained for the insoluble components of three composts was 4 to 5 x 10(-4) d(-1), whereas for one compost it was 1 x 10(-8) d(-1), indicating that this compost was much more stable. To obtain universal rate constants that will fit any compost, components of the insoluble material must be better defined.