AVAILABILITY OF ORGANIC-CARBON IN SOLUBLE AND PARTICLE-SIZE FRACTIONSFROM A SOIL-PROFILE

In an agricultural soil profile, the availability of soil organic C fo r microbial activity decreased from 0 to 100 cm depth in winter, sprin g and summer. Availability was defined as the ratio of respiration rat e to total organic C, or the ratio of microbial biomass C to total org anic C. The amount and availability of organic C in various soil fract ions was measured. Fractions consisted of aqueous extracts from throug hout the profile, and particle-size and density fractions from the 0-2 0 and 80-100 cm layers. The fractions were incubated with soil inoculu m for 24 or 28 days. Availability of organic C in soluble fractions wa s measured by the decrease in dissolved organic C. Availability of org anic C in insoluble fractions was measured by the increase in inorgani c C. Availability of organic C in all fractions was similar at each de pth, except for the weakly-adsorbed and clay fractions, in which organ ic C was less available at depth. Availability was least in the silt f ractions. Approximately half of the organic C mineralized during incub ations originated from the clay fraction in both the 0-10 and 80- 100 cm layers. Of the remainder, at the surface a higher proportion was in the silt and light (> 50 mum, d<2 g cm-3) fractions, due to a larger proportion of total organic C in those fractions. At depth, a higher p roportion was in the extracted fractions. The decrease in the availabi lity of total organic C with depth appeared mainly to be due to a decr ease in the accessibility of organic C to microorganisms, and a decrea se in the availability of clay fraction organic C.