Millimeter-scale leaf litter placement modulates carbon dioxide and methane emissions in wetlands

Abstract

Flooded soils play a crucial role in global carbon cycling, acting as significant reservoirs of soil organic carbon and sources of carbon emissions. Leaf litter, especially from local vegetation, is a key contributor to soil organic carbon formation in these ecosystems, with its decomposition driving the production of carbon dioxide (CO₂) and methane (CH₄). Although numerous studies have examined the factors influencing leaf litter decomposition and associated greenhouse gas emissions, the impact of millimeter-scale variations in leaf litter placement within the soil-water interfaces (SWI) has received little attention. This study hypothesizes that even small changes in burial depth could significantly affect the emission patterns of CO₂ and CH₄. To test this hypothesis, a microcosm experiment was conducted to monitor gas fluxes and physicochemical profiles in treatments with leaf litter placed uncovered at the SWI (LU) or covered by a thin (10 mm) soil layer (LC). The results demonstrated substantial shifts in emissions: cumulative CH₄ emissions were more than doubled in the LC group (115.3 μgC) compared to the LU group (42.5 μgC), while cumulative CO₂ emissions were reduced by 25 % in the LC group (77 mgC) versus the LU group (103 mgC). Consequently, the overall global warming potential over a 20-year horizon (GWP20) was 13.2 % lower for the LC treatment (280.7 mgCO₂e) compared to the LU treatment (323.3 mgCO₂e). These findings advance our understanding of wetland carbon dynamics by revealing that a millimeter-scale change in leaf litter placement significantly alters the spatiotemporal patterns and partitioning of CO₂ and CH₄ emissions, highlighting the critical role of micro-scale heterogeneity in regulating greenhouse gas fluxes. Moreover, our results demonstrate a decoupling of CO₂ and CH₄ responses to this small-scale environmental variation, ultimately leading to a reduction in overall global warming potential despite increased methane release.