Experimental Studies on the Behavior of Fine Organic Particles and Water-Soluble Organic Matter in Mineral Soil Horizons

Abstract

The following three series of experiments were carried out to analyze the behavior of the fine organic particles and water-soluble organic matter (FWOM) in mineral soil horizon. 1) The FWOM suspension obtained from decayed Crypfomeria japonica leaves was passed through the columns filled with three different samples of mineral soil horizons and sea sand, and C and N contents were determined for the effluent, and the soil samples and sea sand after elution. 2) The water-soluble organic matter fraction separated from the FWOM was examined for its adsorbability on three different soil samples. 3) The freeze-dried FWOM was incubated with the soil samples and sand at 50% of maximum water holding capacity for 2 months at 30°C, and analyzed for C decomposition rate and humus composition. The results obtained are summarized as follows. 1) Column experiment. The FWOM penetrated into soil columns by water percolation, while coarse organic particles accumulated on the surface of the columns. The FWOM which penetrated was mostly retained in the soil sample and nearly half of the FWOM applied to a sea sand column flowed out with water. It was concluded that the fine organic particles and the water-soluble organic matter constituting the FWOM may be retained in soil samples by the ”sieving effect“ and adsorption, respectively. The distribution of the organic matter translocated into columns decreased with depth and the C/N ratios of the organic matter which penetrated down to the lower parts of columns were lower than those in the upper parts. 2) Adsorption experiment. The water-soluble organic matter separated from the FWOM was mostly adsorbed by the soil samples with a soil/C ratio above 2,000 approximately. 3) Incubation experiment. About 2–10% of the carbon incubated in the form of FWOM with the soil samples or sea sand was decomposed and the decomposition rate was higher for sea sand and the soil samples with a sandy texture. Humus composition and the degree of humification of humic acids were altered by incubation in different ways, i.e., there was an Increase in the degree of humification of the humic acids by decomposition of non-humic substances in the case of sea sand or a sandy soil samples, and a decrease in the degree of humification by the partial transition of humic acids into humin fractions in clayey soil samples.