Understanding soil N2O emissions and production pathways in a changing climate by coupling automated chambers with isotope measurements

Soils are the dominant global source of the important greenhouse gas nitrous oxide (N₂O). The anthropogenic input of nitrogen (N) into soil ecosystems increases the rate of soil N cycling, and thus enhances soil N₂O emissions. N₂O is produced during microbial N transformation processes, mainly via oxic nitrification and anoxic denitrification processes. These predominant pathways depend heavily on soil environmental conditions, such as soil moisture, aeration and substrate availability, which are modulated by weather and climate conditions, atmospheric composition and land use. Consequently, N₂O emission rates and pathways are likely to be affected by future global changes in climate and atmospheric composition. However, the combined effects of elevated carbon dioxide (eCO₂) and elevated air temperature on both N₂O emission rates and pathways are unclear, as the effects can be synergistic, antagonistic or additive, and they can be further influenced by additional interacting disturbances (e.g. summer drought).

Here we test how soil N₂O fluxes and emission pathways respond to environmental changes in a multifactorial climate manipulation experiment, combining warming and eCO₂, as well as precipitation manipulation to simulate an extreme drought during the growing season in a managed montane grassland. For the first time, we combine in-situ surface N₂O flux measurements with online high-time resolution isotopic measurements, soil N₂O isotope depth profiles, molecular microbial ecology, and complementary soil and microclimate measurements. Under future global change conditions, we expect increasing N₂O emission rates, as well as an increasing importance of denitrification, due to the effect of large emission pulses following rewetting. In addition, we hypothesize that drought effects overrule other environmental change factors. Our results will provide an unprecedented insight into the effects of global changes on soil N dynamics and soil N₂O emissions in managed montane grasslands. Furthermore, these findings will help to improve the modelling of N dynamics at the atmosphere-biosphere interface, which will be used to derive soil N₂O production and consumption pathways, based on soil N₂O isotope measurements, and to upscale the results to examine their potential global relevance.