Rainfall frequency and soil water availability regulate soil methane and nitrous oxide fluxes from a native forest exposed to elevated carbon dioxide

Free-air carbon dioxide enrichment (FACE) experiments in terrestrial ecosystems have demonstrated ecological responses of key ecosystem processes to rising atmospheric carbon dioxide (CO₂). However, CO₂ fertilization responses in field conditions have seldom included methane (CH₄) and nitrous oxide (N₂O), particularly in natural and mature forests, which are expected to have an important role in climate change mitigation. Herein, we aimed to capture the effect of elevated CO₂ (eCO₂; ambient vs. +150 ppm) on long-term temporal dynamics of CH₄ and N₂O fluxes, followed by identification of climo-edaphic factors explaining feedback responses. To achieve this, continuous monitoring of greenhouse gas (GHG) fluxes using a manual chamber technique, over a 3-year period was implemented in a mature dryland Eucalypt forest FACE (EucFACE) facility in Australia. The relationship between CH₄ and N₂O fluxes with rainfall indices and soil properties was also explored since they directly impact the microbial communities in the soil responsible for CH₄ and N₂O net emissions. Our results showed that in 3 years of eCO₂ treatment, the amount and frequency of rainfall predicted GHG emissions in this native forest. We also found a significant reduction in CH₄ sink (15%–25%) for some of the years as well as an overall treatment effect index reduction in N₂O emissions under eCO₂. Higher frequency of rain events with lower intensity led to highest CH₄ sink followed by lowest N₂O emissions due to fewer wet–dry cycles. Of all the environmental variables included, soil moisture, rainfall and pH were the main predictors of net CH₄ and N₂O emissions. Methane flux was also strongly influenced by soil texture. Our findings highlight the need to account for reduced forest CH₄ sink under eCO₂ in dryland ecosystems, which has implications for GHG budget predictions under future climate conditions. A free Plain Language Summary can be found within the Supporting Information of this article.