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 (15)N 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.

Continuous and unattended measurements of the site preference of nitrous oxide emitted from an agricultural soil using quantum cascade laser spectrometry with intercomparison with isotope ratio mass spectrometry

“…[4] An alternative method for the measurement of the SP and 15 N natural abundance of N 2 O, quantum cascade laser spectrometry (QCLS) has become available. QCLS provides a direct way to investigate 14 N-14 N- 16 O, 14 N-15 N- 16 O and 15 N-14 N- 16 O of N 2 O because QCLS techniques are based on the direct absorption of laser light at specific wavelengths by different isotopomers of N 2 O. Previous QCLS studies have achieved precise measurements of the SP at high N 2 O mixing ratios.…”

“…The preconcentration device was coupled to a QCLS instrument to achieve automated measurements of the SP of atmospheric N 2 O. With this system, Wunderlin et al [15] and Mohn et al [16] achieved a precision of <0.1‰ for δ 15 N α and δ 15 N β values. The method using a preconcentration unit is useful for highprecision measurements of the SP of N 2 O at its atmospheric mixing ratio.…”

“…TDLWintel software (Aerodyne Research Inc.) was used for laser control, data acquisition, and simultaneous quantification of the three main N 2 O isotopic species ( 14 N-14 N- 16 O, 14 N-15 N- 16 O, and 15 N-14 N-16 O), taking into account the path length, gas temperature (approximately 295.1 K), pressure (approximately 27.0 Torr), and laser line width (0.0002 cm -1 ). The laser spectrometer was operated in a continuous flow-through mode, with a proportional control valve (0248A-10000SV; MKS Instruments Inc., Andover, MA, USA) mounted upstream of the cell to maintain a constant cell pressure and a dry scroll vacuum pump (IDP-2; Agilent Technologies, Santa Clara, CA, USA) with a manual flow adjustment valve downstream.…”

“…With this system, Wunderlin et al . and Mohn et al . achieved a precision of <0.1‰ for δ 15 N α and δ 15 N β values.…”

Eddy covariance for quantifying trace gas fluxes from soils

“…Studies focusing on N 2 O and CH 4 are likely to become available in the near future. With the development of new laser absorption spectrometers capable of measuring the isotopic signatures of carbon, nitrogen, and oxygen in methane and nitrous oxide (Mohn et al, 2008(Mohn et al, , 2013McManus et al, 2010), new insights into soil processes producing these trace gases are now becoming possible (Wolf et al, 2014(Wolf et al, , 2015. It has to be noted that measurements of isotopes provide an additional challenge to EC flux measurements, and detailed in-depth knowledge about isotopic fractionation effects is essential (Friedman and O'Neil, 1977).…”