Effect of barley plants on the decomposition of ¹⁴ C‐labelled soil organic matter

Summary The effect of barley plants on the rate of decomposition of soil organic matter over a 6‐week period was studied using soil that had been previously labelled by incubation with 14 C‐labelled ryegrass for 1 year. The plants reduced the loss of 14 CO 2 , from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little 14 C label being associated with the shoot. Chemical fractionation of the root showed that the 14 C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of 14 CO 2 , being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.