Estimation of organic matter degradation rate and accumulation in soil using ¹⁵N-labeled cattle manure compost

Composted organic matter application is used to supply nutrients, such as nitrogen, and to accumulate carbon in the soil. However, the organic constituent dynamics derived from compost in soil have not been sufficiently clarified. The compost degradation rate depends on the adsorption forms to soil materials, such as clay and amorphous materials. A degradation test of compost using glass-fiber tube in three typical Japanese soils (Yellow soil, Gray lowland soil, and Andosol) added ¹⁵N-labeled cattle manure compost for 3 years was conducted to investigate the relationship between the degradation rate of organic constituents from compost and its adsorption form to soil materials. The changes in the constituents' adsorption forms were analyzed by sequential extraction using three solutions (KCl, Phosphate buffer, NaOH). As a result, the KCl-extractable and phosphate buffer-extractable organic matter had a half-life of approximately 2.2 years, while the non-water-extractable (hydrophobic) organic matter had a half-life of approximately 5.3 years. Conversely, the NaOH-extractable organic matter amounts were unchanged for 3 years and accumulated about 0.25 g-Nkg^-1 in each of the three soils. They may have gained resistance to degradation through adsorption on clay minerals and amorphous soil materials. Furthermore, the clear increase in NaOH-extractable organic matter by compost application was only 1g-Ckg^-1 and 0.1g-Nkg^-1 in soils for 3 years. Therefore, the organic matter accumulation by compost application alone is not expected. In order to accumulate more organic matter in upland soil, it is necessary to increase the adsorption sites for organic matter, which requires the addition of materials, such as clay and amorphous aluminum. (author)