Functional organic matter components in mangrove soils revealed by density fractionation

25 The mechanisms underlying stabilization of soil organic matter (SOM) in coastal 26 ecosystems, including mangrove forests, are poorly understood, limiting our ability to predict the 27 consequences of disturbances. Here, we introduce density fractionation to mangrove soils to 28 identify the distribution and properties of the functional components of SOM with regard to 29 degradation state, stability, and origin, namely, the high-density fraction (HF), free low-density 30 fraction (f-LF), and mineral-associated LF (m-LF). Three soil cores (1 m) were collected in a 31 mangrove forest on Ishigaki Island, Japan and cut into 10 cm intervals and analyzed. The massive 32 production of mangrove fine roots resulted in a high abundance of LFs throughout the cores, 33 which markedly differed from terrestrial soils. Relative abundance of LFs together accounted for 34 38%–66% of total soil C. The m-LF was as abundant as f-LF and 1.6 times higher in relative 35 abundance than the global average of terrestrial soils. The C/N ratios and δ 13 C values clearly 36 increased with depth in all fractions, which was attributed to the increased contribution from roots. 37 We found a consistent pattern in Δ 14 C values of density fractions. HF was the oldest with Δ 14 C 38 between -149‰ and -97‰ followed by m-LF (between -130‰ and -87‰) and then f-LF 39 (between -89‰ and 78‰), suggesting that mineral association may be pivotal in long-term carbon 40 storage in the mangrove mineral soil. Our analysis successfully identified meaningful functional 41 components of mangrove SOM, yet several questions remained unanswered, including large