Mineralization of labelled microbial compounds after partial sterilization

Samples of two soil types amended with uniformly labelled 14C glucose were incubated for six weeks, and reached a stable and low respiration rate. After drying, the samples were treated with chloroform vapours to kill partially the living microbial population. After a second amendment with unlabelled glucose, the samples were a second time incubated for 16 d. Newly labile compounds liberated from drying and chloroform fumigations caused release of labelled CO2 at the beginning of the second incubation. Twenty-five per cent of the label remaining after the first incubation was mineralized during the second incubation. These mineralized compounds are less available than the added glucose, because in unlabelled glucose-amended samples their mineralization was delayed. Nitrogen amendment at a C/N ratio of 25 had no effect on respiration. Increased stages of acid hydrolysis followed by alkaline extraction before and after the second incubation showed a large diminution of compounds extractable with weak acid reagents, whereas the basic extract remained stable. A protective effect of the Ca2+ saturated clays appears to retard the mineralization of acid hydrolysable compounds. (author)