N₂O emissions and source processes in snow-covered soils in the Swiss Alps

Abstract

Nitrous oxide (N₂O) emissions from snow-covered soils represent a significant fraction of the annual flux from alpine, subalpine or cold-temperate regions. In winter 2010–2011, we investigated the temporal variability of N₂O emissions and source processes from a subalpine valley in the Swiss Alps. The study included regular measurements of N₂O snow profiles at a fixed location and an intensive sampling campaign along a transversal cut through the valley with grassland at the bottom and coniferous forest at the slopes. During the intensive campaign, recently developed laser spectroscopy was employed for high-precision N₂O isotopomer analysis. Maximum N₂O fluxes (0.77±0.64 nmol m⁻² h⁻¹) were found for periods with elevated air temperature and, in contrast to our expectations, were higher from forest than from grassland in mid-February. At maximum snow height (63 cm) the main N₂O source processes were heterotrophic denitrification and nitrifier denitrification. The reduction of N₂O by heterotrophic denitrifiers was much more pronounced for the grassland compared with the forest soil, as indicated by the ¹⁵N site preferences of 16.4±11.5 ‰ (grassland) and−1.6±2.1 ‰ (forest). This illustrates the potential of laser spectroscopic N₂O isotopomer analysis for the identification of source processes even at low emission rates in nutrient poor ecosystems.

Keywords:

• biogeochemistry
• denitrification
• isotopomer
• laser spectroscopy
• nitrification
• nitrogen-15
• nitrous oxide
• snow-covered soil

Acknowledgements

Funding from the Swiss National Foundation for Scientific Research (SNF) and the State Secretariat for Education and Research (SER) within COST-ES0806 and ES0804 is gratefully acknowledged. Additional funding was provided by GHG-Europe (FP7, EU contract No. 244122). Béla Tuzson is highly acknowledged for his continuous instrumental support. The authors would like to thank Tobias Wyler for his help during the field campaign on 21 February. Naohiro Yoshida and Sakae Toyoda from Tokyo Institute of Technology are acknowledged for analysis of the site-specific isotopic composition of our primary N₂O calibration gases. Tobias Jonas is acknowledged for providing meteorological data.