Lignin Methoxyl δ¹³C Reveals Particle-Size–Dependent Sources and Degradation in Forest Soils

Lignin is a major component of plant-derived organic matter in soils, and the stable carbon isotopic composition of lignin-derived methoxyl (δ ¹³C LMeO) groups provides a distinct molecular fingerprint for identifying sources and their relative contributions to soil organic matter. This study investigates δ ¹³C LMeO values in soil profiles from surface horizons to bedrock in  a deciduous and coniferous forest in Switzerland, with the aim of estimating the relative contributions of lignin from photosynthetic and non-photosynthetic plant tissues. Analyses were conducted on two particle-size fractions (<63 µm and 63–200 µm), and the influence of ¹³C isotopic fractionation during lignin degradation was evaluated for both size fractions.

Preliminary source apportionment results, not accounting for isotopic fractionation during degradation, indicate that the coarse fraction at the coniferous site is dominated by lignin derived from non-photosynthetic plant tissues, approaching a 100% contribution. In contrast, the fine fraction at the coniferous site and both particle-size fractions at the deciduous site comprise approximately 60% lignin from non-photosynthetic tissues.

In contrast to bulk δ¹³C and other compound-specific stable isotope tracers, δ ¹³C LMeO  values exhibited a systematic isotopic depletion in the fine (<63 µm) fraction. This depletion suggests preferential stabilization of the more easily degradable lignin from photosynthetic tissues. In the coarse (63–200 µm) fraction, δ ¹³C LMeO values showed a clear relationship with the extent of degradation, consistent with isotopic fractionation during lignin loss. In contrast, no systematic degradation-related trend was observed in the fine fraction. Together, these results highlight contrasting controls of degradation and stabilization on lignin across soil particle-size fractions and underscore the importance of accounting for isotopic fractionation when applying δ ¹³C LMeO for soil organic matter source attribution.