Demonstration of discrepancy in N remaining rate of organic matter evaluated by N content and ¹⁵N content in the glass fiber-filter paper bag method

¹⁵N-labeled rice straw, wheat straw, and cattle manure sawdust compost were used to evaluate the N remaining rate of organic matter by the application of the glass fiber-filter paper bag method, and to demonstrate the discrepancy between the N remaining rate of organic matter evaluated by the N content and the N remaining rate of organic matter directly evaluated by the ¹⁵N content. In all three kinds of organic matters examined, a discrepancy in the N remaining rate of organic matter was observed between the evaluation by the N content and the evaluation by the ¹⁵N content. Loss of N from organic matter was observed using the ¹⁵N content, though the N remaining rates of wheat straw and rice straw evaluated by the N content were above 100% throughout the experimental period. The N inflow into and N outflow from the glass fiber-filter paper bag occurred simultaneously. The discrepancy between the N remaining rate of organic matter evaluated by the N content and the N remaining rate of organic matter evaluated by the ¹⁵N content became larger as the C / N ratio of organic matter increased.