Reviews and syntheses: When does moss act as a methane biofilter? Transport pathways and environmental controls in peatlands

Abstract. Peatlands are critical global carbon stores and the largest natural source of atmospheric methane (CH₄). Bryophytes, particularly Sphagnum and submerged brown mosses, can act as effective biological CH₄ filters by hosting methanotrophs within their hydrated tissues and providing photosynthetically derived oxygen. In diffusion-dominated systems, moss-associated oxidation can consume 61–99% of upward CH₄ flux. However, the presence of moss cover alone does not guarantee ecosystem-scale emission reduction. Synthesizing evidence from 122 studies across field observations, mesocosms, and manipulation experiments, we evaluate the moss biofilter as a realized, state-dependent transport-and-oxidation process. We show that biofilter effectiveness emerges from competition among three distinct pathways: diffusion through the living moss layer, vascular-plant bypass via aerenchyma, and episodic ebullition. Only the diffusive fraction is consistently exposed to the moss oxidation zone. This pathway partitioning is shaped by microtopography and successional trajectories and is highly vulnerable to global-change stressors. Hydrology acts as the master regulator: modest water-table drawdown expands the oxic oxidation layer, whereas severe desiccation damages moss structure and microbial habitats. Furthermore, warming and chronic nitrogen enrichment can destabilize moss–methanotroph metabolic coupling and promote graminoid expansion, exacerbating vascular bypass. For peatland management and paludiculture, hydrological rewetting is necessary but insufficient; without rapid re-establishment of a functional moss layer and mitigation of nutrient legacies, rewetted sites can become bypass-dominated CH₄ hotspots. Techniques such as topsoil removal and the moss layer transfer technique (MLTT) can accelerate recovery, provided full CO₂–CH₄ trade-offs are evaluated. Finally, we highlight priorities for Earth system modeling, emphasizing that models must explicitly separate diffusive, vascular, and ebullitive pathways rather than treating moss cover as a static oxidation parameter.