Trace Gas Fluxes from a Warm-Temperate Savanna: Influence of Vegetation Cover and Simulated Rainfall Events

Woody plant encroachment in drylands is among the most significant global land cover changes during the past century, likely driven by land use activities, rising atmospheric CO₂, and climate change. In the U.S. southern Great Plains, juniper and oak cover has expanded and increased soil C and N stores significantly beneath their canopies; however, responses of soil trace gas fluxes have not been examined. We hypothesized that: (1) Larger soil C and N pool sizes under woody plants would stimulate microbial activity and lead to higher rates of soil trace fluxes; and (2) Simulated precipitation pulses would stimulate anaerobic processes leading to increased CH₄ and N₂O fluxes. We quantified daytime soil CO₂, CH₄, and N₂O fluxes seasonally using in situ vented static chambers in oak-juniper savanna in west-central Texas, U.S. Fluxes were measured diurnally under oak, juniper, and grassland canopies. Plots at ambient soil moisture had highest CO₂ and N₂O fluxes during spring when soil moisture was high (CO₂ = 590 μg m^-2 min^-1; N₂O = 0.03 μg m^-2 min^-1), whereas summer fluxes were water limited (CO₂ = 372 μg m^-2 min^-1; N₂O = 0.0 μg m^-2 min^-1), and winter fluxes were temperature limited (CO₂ = 325 μg m^-2 min^-1; N₂O = 0.01 μg m^-2 min^-1). Methane fluxes in ambient soil moisture plots were strongly temperature controlled and exhibited net consumption throughout the year, with highest consumption in winter (-0.55 μg m^-2 min^-1) and lowest in summer (-0.28 μg m^-2 min^-1). Vegetation cover did not significantly affect seasonal fluxes of any trace gases in plots at ambient soil moisture. Plots receiving 2.5 cm of simulated rainfall had significantly higher CO₂, N₂O, and CH₄ fluxes than ambient plots during each measurement period, and rates decreased in the order summer > spring > winter. Flux rates of CO₂ and N₂O (but not CH₄) in simulated rainfall plots were affected by a vegetation x season interaction, with grasslands and oak having highest rates in summer, and juniper plots having highest rates in winter. In ambient moisture plots, soil moisture is the key driver of CO₂ and N₂O fluxes, and temperature is more important for CH₄ flux. In simulated rainfall plots, pulsed precipitation events interacted significantly with vegetation type to increase CO₂ and N₂O fluxes. Results should facilitate development of trace gas budgets for dryland ecosystems.