Organic matter and microbial biomass in a soil incubated in the field for 20 years with 14C-labelled barley straw

Abstract

14C-labelled barley straw was incubated under field conditions in a sandy soil. After 8 yr, 16% of the labelled C added remained in the soil; after 20 yr 9.3%. During the 8 to 20 yr period the labelled organic matter in the soil decayed at a rate corresponding to a half life of 15 yr. The percentage of residual labelled C in amino acids remained almost constant during the period, being on average 21%. The soil contained 1.9% native C in organic matter; during the 8 to 20 yr period this decayed at a rate corresponding to a half life of 91 yr. The percentage of native C in amino acids increased significantly, from about 14% to about 16%. During the 8 to 20 yr period, microbial biomass was determined yearly by the chloroform fumigation technique. The proportion of total labelled C in biomass remained nearly constant during this period, on average 2.7%. Labelled C in biomass decreased at a rate corresponding to a half life of 8 yr. The native biomass increased during the same period from 0.7 to 1.4% of the total native soil C. As measured by CO 2 produced during periods of 3 months, laboratory incubation increased the rate of decay of the labelled organic matter by a factor of 1.2 and that of the native organic matter by a factor of 4.3.