Incorporation of a Wide Variety of Organic Substrate Carbons into Soil Biomass as Estimated by the Fumigation Procedure

Biodegradation of the carbon or specific carbons of a large variety of 14 C‐labeled organic substrates were followed in a closed, constantly aerated system in Steinbeck loam (pH 5.0) and Greenfield sandy loam (pH 7.0) over various periods of incubation at 22°C. Incorporation into biomass and stabilization in humus were estimated after 12 weeks of incubation. The labeled carbons in biomass were estimated using a slight modification of Jenkinson's fumigation method. Readily available substrates such as glucose, acetate, pyruvate, uracil, uridine, amino acids, and some polysaccharides were rapidly metabolized by the soil population, and after 8 to 12 weeks, about 80% or more of the substrate carbons had evolved as CO 2 . After 12 weeks, about 20 to 40% of the residual carbons of these substrates were present as hiomass, and 60 to 80% was present as new humus. Glucosamine, cysteine, cellulose, protein, and some microbial and plant polysaccharides were degraded to a somewhat smaller extend, 60 to 70%, indicating some stabilization of the intact molecules or partial degradation products by incorporation into humus or by complexing with soil colloids or metals. About 10 to 30% of the residual carbons were associated with the biomass after 12 weeks. Most microbial cells and plant residues such as straw were utilized to a still lesser extent, with about 50 to 60% loss of applied C in 12 weeks and an incorporation of about 5 to 13% of the residual 14 C into biomass. A fungal humic acid‐type melanin was still more resistant, and less of the residual C, 0.2 to 1.3%, was present in biomass at 12 weeks.