Studies on the decomposition of plant material in soil .IV. The effect of rate of addition

Different amounts of ryegrass roots and tops, both uniformly labelled with 14C, were mixed with soil and allowed to decompose for 155 days under controlled conditions in the laboratory at 25°C. Initially the roots decomposed more slowly than the tops but by 155 days this difference had disappeared. About a third of the added plant C remained in the soil at the end of 155 days, about as much as when the same plant materials were incubated in the same soils for 6 months in the field.

To a first approximation, the amount of labelled CO2–C evolved was directly proportional to the amount of labelled plant C added. This held throughout the incubations. However, a slightly smaller percentage of the added plant C was evolved with small additions than with large, although this effect was on the limits of detection. Slightly more labelled plant C was retained in a soil rich in organic matter (2.43% C) than in an otherwise similar soil with less organic matter (0.97% C).