RATES OF DECOMPOSITION IN SOIL AND RELEASE OF AVAILABLE NITROGEN FROMCATTLE MANURE AND MUNICIPAL WASTE COMPOSTS

Two different composts, a cattle manure compost (CMC) and a municipal waste compost (MWC), were applied at a rate of five percent or 15 perc ent to two soils, differing in their mineralization capacity, and incu bated for 33 weeks at 30 degrees C and optimal soil-water content. Per iodically, CO2 evolution rates and inorganic N concentrations were mea sured in the soils. The rate of compost-N recovered as inorganic N was independent of the soil and compost application rate. The recovery af ter 33 weeks (w) was 22 percent of MWC-N and 23 to 27 percent of CMC-N , of which 13 percent was initially inorganic. The recovery of compost -C as CO2 depended on the compost application rate and to a lesser ext ent on the soil, reaching values of 13 to 15 percent and eight percent for the low and high application rates, respectively. The rates of de composition of the composts were computed by the model NCSOIL by minim izing the deviations between simulated and measured data of CO2 and in organic N. The soluble and insoluble organic C and N contents of the c omposts were used as input in the model, representing two components d iffering in their rates of decomposition. The decomposition rate const ants of the insoluble components were 6.4x10(-4) d(-1) for CMC and 8.9 x10(-4) d(-1) for MWC, assuming that the soluble component decomposed rapidly during the first week. The small difference between the compos ts indicated that a similar decomposition rate constant could fit the insoluble component of any compost. Better definitions of the insolubl e material could improve the prediction of decomposition of composts. The wider C/N ratio of MWC explains the smaller rate of inorganic N re lease although its decomposition rate constant was larger.