Comparison of the contribution to soil organic matter fractions,particularly carbohydrates,made by plant residues and microbial products

Soil was incubated with ¹⁴C-ryegrass or ¹⁴C-glucose, fractionated and the distribution of radioactivity in six fractions and in their constituent sugars was compared. The fractions were (1) the light fraction (d < 2.00), (2) heavy fraction which was subdivided into (3) HCl soluble, (4) humic acid, (5) fulvic acid, and (6) humin. Sugars were released from soluble fractions by hydrolysis with 0.5M H₂SO₄, and from insoluble fractions by hydrolysis with 2.5M H₂SO₄ and 12M H₂SO₄ followed by 0.5M H₂SO₄. The light fraction contained the greater part of the radioactivity and the sugars in both types of incubation. The proportion of the radioactivity accounted for by sugars in the light fraction of the ryegrass incubation, initially 44%, decreased to 30% after 6 months incubation and to 19% after a year. In one of the glucose incubations, done with soil that had been dried prior to remoistening to 25% w/w and incubating, the proportion was 17% after incubation for one month; in the other, done on soil that had not been given a drying treatment and had a 35% w/w moisture content, it was 32%. In the ryegrass incubation, the sugars with the greatest specific activities were present in the light fraction. In contrast, in the glucose incubation, sugars in the HCl-soluble, fulvic and humic fractions had the highest specific activities. Carbon from ryegrass contributed more to the humic acid and humin fractions of the soil than did the carbon from glucose, whereas carbon from glucose made a greater contribution to the acid-soluble organic matter.