Abstract
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.
Highlights
Field-moist samples from the O-20 and 80-100 cm layers of the profile were fractionated according to particle size and density
Total organic C, C-to-N ratio and biomass C diminished with depth (Table 1, Fig. 1)
Basal respiration profiles (Fig. 2) show that the biomass was more active at all depths in spring than in winter or summer, Availability of organic C in particle-size and density fractions was measured as the increase in inorganic C in the solution and gas phases of suspension incubations with soil inoculum
Summary
The soil profile studied is a Gleyic Luvisol (FAO classification) in the valley of the SIone in France. The site was cropped to soya, with annual mouldboard ploughing. 20.8 19.7 19.4 17.8 16.5 water content). 8.52 * 0.43 8.17&0.61 7.?9* 1.06 6.15+0.83 5.41 +0.76 to approximately 20 cm until 199 1, when a ryegrass sward was established. Samples were taken in winter (19 December 1991), spring (23 April 1992) and summer (16 July 1992) using a coring system which prevented cross-contamination of the layers sampled. 6 cores were taken from the same 15 x 15 m plot, combined into 5 samples (O-20, 20-40, 4&60, 60-80 and 80-100 cm depth) and stored at field water content in air-tight plastic bags at 4°C. All results are given on an oven-dry soil weight basis
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