Carbon dioxide production during a 367‐day period was used in assessing the rate of carbon oxidation when wood (W), bark (B), and wood‐bark (WB) mixture from Engelmann spruce (Picea Engelmannii Parry) were each mixed in different proportions with anaerobically digested sewage sludge and each mixture applied to a Nunn clay loam at rates of 22.4 to 224 metric tons/ha.Generally, as the application rate of organic material increased the CO2 production increased, but nonadditively for most time intervals. At the 22.4‐metric tons/ha rate, there were no consistent differences in CO2 production with variations in the proportion of wood materials to sludge. At the higher application rates, as the percentage of wood materials increased and sludge decreased, the CO2 accumulation increased up to 75% wood material — 25% sludge, then dropped with 100% wood material. The 22.4‐metric tons/ha application of 100% sludge also resulted in lower CO2 production rates than the mixtures of wood materials and sludge. The percentage decomposition of the added residues increased with decrease in application rate of the treatments, with values ranging from 11 to 55%.In concurrent studies, attempts were made to correlate CO2 production with N mineralized in laboratory incubation studies and with wheat growth in a greenhouse pot experiment. The appropriate regression equations were derived to describe the relationships obtained. It was concluded that microbial respiration was a good index of plant‐available N in soil, except where plant nutrients were deficient due to microbial immobilization, or where toxic factors were limiting to microbial activity.