Studies on the decomposition of plant material in soil. III. The distribution of labelled and unlabelled carbon in soil incubated with 14C labelled ryegrass

The organic matter in soils containing decomposing 14C‐labelled ryegrass was fractionated chemically. Earlier work on these soils had shown that they contained a small fraction, heavily labelled relative to the rest of the soil organic matter, that was mineralized when the partially sterilized soils were incubated. Reagents effective in extracting heavily labelled‐C included cold o.in HC1, boiling saturated CaSO4 solution, and o.in Ba(OH)2, but neither these nor any other reagent tested could extract material as heavily labelled as that mineralized when partially sterilized soil was incubated. Reagents that extract heavily labelled‐C are poor extractants for humified material and are not strongly hydrolytic: the more vigorous the hydrolysis the smaller the proportion of labelled‐C in the hydrolysate. The amounts of labelled‐C dissolved by Ba(OH)2 from soils sampled after different periods in the field were directly proportional to the amounts of labelled‐C mineralized by those soils when partially sterilized (by exposure to CHC13 vapour), inoculated and incubated.

Balance sheets are presented for the distribution of labelled and unlabelled‐C in fractions separated by hydrolysis with 6N HC1, by NaOH extraction, by neutral pyrophosphate extraction, and by oxidation with H2O2. The fraction remaining after hydrolysis with 6N HC1 was the most lightly labelled and had the widest C/N ratio. The percentage of labelled‐C in the material dissolved by alkali or by pyrophosphate was little more than in the material not dissolved, despite the presence in the soil of fractions differing at least twenty‐fold in intensity of labelling.