Functional organic matter components in mangrove soils revealed by density fractionation

The mechanisms underlying stabilization of soil organic matter (SOM) in vegetated coastal ecosystems, including mangrove forests, are poorly understood, limiting our ability to predict the consequences of disturbances. Here, we introduce density fractionation to mangrove soils to identify the distribution and properties of the functional components of SOM with regard to degradation state, stability, and origin, namely, the free low-density fraction (f-LF), mineral-associated LF (m-LF), and high-density fraction (HF).

Three 1-m soil cores were collected in the Fukido mangrove forest on Ishigaki Island, Japan, segmented into 10 cm intervals, and analyzed for C and N concentrations, stable carbon isotopes (¹³C), and radiocarbon isotopes (¹⁴C) followed by density fractionation. Although HF exhibited the highest abundance, the massive production of mangrove fine roots resulted in a high abundance of LFs throughout the cores, which markedly differed from terrestrial soils. The relative abundance of LFs collectively accounted for 38%–66% of total soil C. Notably, m-LF was as abundant as f-LF and 1.6 times higher in relative abundance than the global average of terrestrial soils. The C/N ratios and δ¹³C values exhibited a clear increase with depth in all fractions, attributed to the increased contribution from roots. A consistent pattern in Δ¹⁴C values across density fractions in the deepest section was observed, with HF exhibiting the oldest values (between -149‰ and -97‰), followed by m-LF (between -130‰ and -87‰), and f-LF (between -89‰ and 78‰). This implies that mineral association may play a pivotal role in long-term carbon storage in the mangrove mineral soil studied.

A further analysis of reactive iron (Fe) and aluminum (Al) present in HF through pyrophosphate (PP) and dithionite-citrate (DC) extractions suggested that PP-extractable, organically complexed metals regulate organic carbon concentrations in HF, rather than crystalline or nano-crystalline Al and Fe phases. Our analysis successfully identified meaningful functional components of mangrove SOM, yet several questions remained unanswered, including the large variability in Δ¹⁴C values among different cores.