Soil fluxes of methane, nitrous oxide, and nitric oxide from aggrading forests in coastal Oregon

Soil exchanges of greenhouse and other gases are poorly known for Pacific Northwest forests where gradients in nutrient availability and soil moisture may contribute to large variations in fluxes. Here we report fluxes of methane (CH₄), nitrous oxide (N₂O), and nitric oxide (NO) over multiple seasons from three naturally N-rich, aggrading forests of coastal Oregon, USA. Mean methane uptake rates (3.2 mg CH₄ m⁻² d⁻¹) were high compared with forests globally, negatively related to water-filled pore space (WFPS), but unrelated to N availability or temperature. Emissions of NO (6.0 μg NO–N m⁻² h⁻¹) exceeded N₂O (1.4 μg N₂O–N m⁻² h⁻¹), except when WFPS surpassed 55%. Spatial variation in NO fluxes correlated positively with soil nitrate concentrations (which generally exceeded ammonium concentrations, indicating the overall high N status for the sites) and negatively with soil pH, and at one site increased with basal area of N₂-fixing red alder. Combined NO and N₂O emissions were greatest from the site with highest annual net N mineralization and lowest needle litterfall C/N. Our findings of high CH₄ uptake and NO/N₂O ratios generally >1 most likely reflect the high porosity of the andic soils underlying the widespread regenerating forests in this seasonally wet region.