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

Abstract

<p>Soils are the dominant global source of the important greenhouse gas nitrous oxide (N<sub>2</sub>O). The anthropogenic input of nitrogen (N) into soil ecosystems increases the rate of soil N cycling, and thus enhances soil N<sub>2</sub>O emissions. N<sub>2</sub>O is produced during microbial N transformation processes, mainly via <em>oxic</em> nitrification and <em>anoxic</em> 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, N<sub>2</sub>O 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 (<em>e</em>CO<sub>2</sub>) and elevated air temperature on both N<sub>2</sub>O 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).</p><p>Here we test how soil N<sub>2</sub>O fluxes and emission pathways respond to environmental changes in a multifactorial climate manipulation experiment, combining warming and <em>e</em>CO<sub>2</sub>, 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 N<sub>2</sub>O flux measurements with online high-time resolution isotopic measurements, soil N<sub>2</sub>O isotope depth profiles, molecular microbial ecology, and complementary soil and microclimate measurements. Under future global change conditions, we expect increasing N<sub>2</sub>O 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 N<sub>2</sub>O 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 N<sub>2</sub>O production and consumption pathways, based on soil N<sub>2</sub>O isotope measurements, and to upscale the results to examine their potential global relevance.</p>