Availability of organic carbon in soluble and particle-size fractions from a soil profile

In an agricultural soil profile, the availability of soil organic C for microbial activity decreased from 0 to 100 cm depth in winter, spring and summer. Availability was defined as the ratio of respiration rate to total organic C, or the ratio of microbial biomass C to total organic C. The amount and availability of organic C in various soil fractions was measured. Fractions consisted of aqueous extracts from throughout the profile, and particle-size and density fractions from the 0–20 and 80–100 cm layers. The fractions were incubated with soil inoculum for 24 or 28 days. Availability of organic C in soluble fractions was measured by the decrease in dissolved organic C. Availability of organic C in insoluble fractions was measured by the increase in inorganic C. Availability of organic C in all fractions was similar at each depth, except for the weakly-adsorbed and clay fractions, in which organic C was less available at depth. Availability was least in the silt fractions. Approximately half of the organic C mineralized during incubations 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 μm, d< 2 g cm−3) fractions, due to a larger proportion of total organic C in those fractions. At depth, a higher proportion was in the extracted fractions. The decrease in the availability of total organic C with depth appeared mainly to be due to a decrease in the accessibility of organic C to microorganisms, and a decrease in the availability of clay fraction organic C.