Tree stem methane emissions are regulated by site-level biogeochemistry over species identity in Amazon floodplain...

Tree stems in Amazonian floodplains emit substantial methane (CH₄), yet controls on emission variability remain unclear. Emissions span orders of magnitude between várzea (nutrient-rich) and igapó (nutrient-poor) forests and among trees, suggesting controls beyond flooding. We tested whether site-level biogeochemistry better explains stem CH₄ variability than species identity by measuring emissions from two co-occurring species with contrasting wood densities - Eschweilera coriacea and Hevea spruceana - across várzea and igapó forests. Emissions were paired with porewater chemistry (electrical conductivity, dissolved oxygen, dissolved CH₄, and dissolved organic carbon), methane production potential (MPP), and root biomass. Stem CH₄ emissions were significantly higher in várzea than in igapó, independent of species or stem height. Várzea porewaters displayed higher conductivity, dissolved CH₄ and MPP, near-neutral pH, and lower oxygen, with fine roots concentrated in the 0- to 50-cm soil layer, indicating a shallow CH₄ supply zone. Basal stem emissions in várzea correlated with shallow porewater chemistry and fine-root biomass, whereas relationships in igapó were weak. These findings show that Amazonian floodplain stem CH₄ emissions are governed by shallow site-level biogeochemistry, rather than species identity alone and should be incorporated into basin-scale CH₄ budgets and process models to capture spatial variability.