Studies on the Decomposition of Plant Material in Soil

Summary Soil samples taken during an experiment on the decomposition of 14 C‐labelled ryegrass in soil under field conditions (see Part I) were air‐dried, irradiated, exposed to CHCl 3 or CH 3 Br vapours, oven‐dried or autoclaved. After these treatments the soils were inoculated, incubated, and the output of CO 2 measured. All these methods of partially (or, in some cases, completely) sterilizing soil rendered a small heavily labelled fraction of the soil organic matter decomposable. This fraction is postulated to be the soil biomass. Treatments involving heat or irradiation rendered small additional amounts of the soil organic matter decomposable (by processes other than the killing of organisms). Incubating unsterilized soil with partially sterilized soil did not decrease evolution of CO 2 . This suggests that partial sterilization does not increase mineralization by destroying toxic substances that inhibit microbial growth, or by disturbing a host: predator balance in the unsterilized soil. The longer the labelled ryegrass was allowed to decompose in the field, the less labelled‐CO 2 was evolved after partial sterilization. In contrast, the same amount of unlabelled‐CO 2 was evolved from a soil that had been incubated 1 or 4 years with ryegrass. The labelled part of the biomass is considered to be largely zymogenic (with a half life of approximately 1.5 years), the unlabelled part largely autochthonous, remaining almost constant over the 3‐year period. It is suggested that the size of the soil biomass can be roughly estimated from the size of the flush of CO 2 after CHCl 3 vapour treatment. Calculated on this basis, 2.3–3.5 Per cent the unlabelled‐C in these soils (i.e. the C present in the soil before the labelled ryegrass was added) was in the biomass. Of the original ryegrass C added, 10–12 per cent was in the biomass after 1 year, decreasing to 4 per cent after 4 years.