[A-CG39] Biogeochemical cycles in Land Ecosystem

[ACG39-10] Stable carbon and nitrogen isotopes in water-extractable organic matter from air-dried soils to elucidate microbial utilization of substrate organic matter in a Japanese temperate forest

*Hirohiko Nagano¹, Mariko Atarashi-Andoh², Sota Tanaka³, Jun Koarashi² (1.Niigata University, 2.Japan Atomic Energy Agency, 3.Akita Prefectural University)

Keywords:Soil organic matter analysis, Microbial decomposition, Stable isotope, Water-extraction

Microbial decomposition of soil organic matter is crucial in terrestrial carbon (C) cycles, whereas the soil analytical approach elucidating the decomposition is still halfway to the achievements. Here, we offer the application of stable carbon and nitrogen isotopes (¹³C and ¹⁵N) in water-extractable organic matter derived from air-dried soils. Forty soils were collected from depths 0-6 cm in a Japanese temperate broadleaved forest. Air-dried soil samples were applied to water extraction and organic matter analysis. The C and N contents of the water-extractable fraction were less than 3.6% of those of the bulk soil. Those C and N contents exhibited positive correlations between the two fractions. Natural abundances of ¹³C and ¹⁵N (δ¹³C and δ¹⁵N) also exhibited positive correlations between the two fractions. However, the C/N ratio in water-extractable fractions is little correlated to that in bulk soils, exhibiting a narrow range of values around 10. Thus, water-extractable organic matter is likely derived from microbial cells. Moreover, δ¹³C enrichment in water-extractable fractions from bulk soils was negatively correlated, whereas the δ¹⁵N enrichment positively correlated to contents of organo-mineral complexes and short-range order minerals, which likely control the stabilization of soil organic matter in Japanese forest soils. Those enrichments contrasting between δ¹³C and δ¹⁵N in water-extractable fractions likely imply the microbial utilization of soil C and N under different substrate availability. Thus, the water-extractable organic matter in air-dried soils is the practical application for elucidating soil organic matter decomposition varying spatially from local to global scales.