Differential responses of soil CO₂ and N₂O fluxes to experimental warming

Land-use conversions and elevated temperature can impact on carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions, both of which are important greenhouse gasses (GHGs). Afforestation activity has increased significantly over the last century with a significant focus in recent years directed at offsetting GHG emissions, as forests have a large capacity to store carbon (C) and nitrogen (N) as well as affecting CO₂ and N₂O emissions. However, the impact of warming on GHG offsetting is unclear. This study was conducted in a forest and a grassland to investigate the effect of afforestation and warming, using infrared heaters, on soil fluxes of CO₂ and N₂O. Warming significantly increased the daily mean soil temperatures at a depth of 5 cm by 1.7 °C and reduced the soil moisture by ∼5% in the forest from March 2014 to February 2016. In the grassland, there were no significant increases in temperature and moisture with warming and no impact on the soil fluxes of CO₂ and N₂O. In the forest, elevated soil temperature enhanced the average soil CO₂ efflux by 23% but had no effect on soil N₂O fluxes. Warming decreased the temperature sensitivity by 13% and 23% at the forest and grassland, respectively. The soil fluxes of CO₂ increased exponentially with temperature and decreased linearly with the reduction in soil moisture, and were much larger in the grassland compared to the forest. However the grassland proved to be a larger sink for N₂O than the forest. Irrespective of warming treatments, all measured pools were significantly larger in the grassland compared to the forest. Our results imply that afforestation may have a bigger effect than warming on soil CO₂ and N₂O fluxes within the range of temperatures used and that afforestation dramatically lowers the inorganic, organic and microbial C and N pools, that could, in turn, impact on the responses of forest soils to future global warming.