Landscape patterns of soil oxygen and atmospheric greenhouse gases in a northern hardwood forest landscape

Abstract. The production and consumption of the greenhouse gases, carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄), are controlled by redox reactions in soils. Together with oxygen (O₂), seasonal and spatial dynamics of these atmospheric gases can serve as robust indicators of soil redox status, respiration rates, and nitrogen cycling. We examined landscape patterns of soil oxygen and greenhouse gas dynamics in Watershed 3 at the Hubbard Brook Experimental Forest, NH, USA. We analyzed depth profiles of soil O₂, CO₂, N₂O, and CH₄ approximately bimonthly for one year. Soil gas depth profiles were obtained from several different soil types encompassing a range of topographic positions, drainage classes, and organic matter content. Soil O₂ was a good predictor of greenhouse gas concentrations. Unsaturated soils always had O₂ concentrations >18 %, while saturated soils had O₂ ranging from 0 to 18 %. For unsaturated soils, changes in CO₂ were nearly stoichiometric with O₂. High concentrations of CH₄ (>10 μL L⁻¹) were typically associated with saturated soils; CH₄ was typically below atmospheric concentrations (<1.8 μL L⁻¹) in unsaturated soils. High concentrations of N₂O (>5000 nL L⁻¹) were found only in well-aerated soils after summer rainfall events and in marginally-anoxic soils; N₂O was consumed (<200 nL L⁻¹) under anoxic conditions. The production and consumption of greenhouse gases were linked to functionally distinct biogeochemical zones of variable redox conditions (hotspots), which exhibit dynamic temporal patterns of redox fluctuations (hot moments). These soil redox hot phenomena were temporally driven by climate and spatially organized by soil type (reflective of topographic position) further constrained by subsurface hydrology.