Linking moss cover, soil organic matter fractions, and carbon stabilization pathways in temperate forest ecosystems

Abstract. Moss cover is increasingly recognized as a biotic driver of soil organic carbon (SOC) dynamics in forest ecosystems, yet its influence on C stabilization across functionally distinct soil organic matter (SOM) fractions remains poorly quantified. How moss cover interacts with forest type and parent material to regulate C partitioning into mineral-associated organic matter (MAOM), the most persistent SOC fraction, is largely unknown in temperate forests. To address this gap, we investigated SOC distribution among free particulate organic matter (fPOM), occluded POM (oPOM), and MAOM across moss-covered and no moss soils in two contrasting temperate forest types differing in parent material. A total of 42 topsoil samples (0–2 cm) were collected from a coniferous mixed forest on sandstone and loess parent materials (Baden-Württemberg) and a pine forest on sandy parent material (Brandenburg). Physical density fractionation was applied to resolve SOM fraction distribution, and total nitrogen (TN) and C:N ratios were determined alongside carbon stabilization efficiency (CSE), calculated as mineral-associated organic matter carbon (MAOM-C) / SOC × 100, as an integrative index of stabilization capacity.

Moss-covered soils showed significantly higher SOC and TN concentrations than no moss soils, with the strongest effects observed in the coniferous mixed forest on sandstone parent material and negligible in loess. MAOM-C concentrations significantly increased under moss cover in the coniferous mixed–sandstone environment where CSE nearly doubled compared to no moss soils. Biotic factors, in particular the interaction between moss cover and forest type, were the primary predictors of stable C, while parent material exerted no independent effect. These results demonstrate that moss cover is linked to distinct C stabilization pathways, favoring mineral-associated stabilization in coniferous mixed forests and accumulation of labile C fractions in pine forests, underscoring the importance of incorporating moss cover as a biotic driver in SOC models and temperate forest C accounting frameworks.