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 ^{1 4} C, 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).