Interannual and seasonal variability of CH4 and N2O exchange over a temperate mountain grassland

The quantification and understanding of the greenhouse gas (GHG) exchange between terrestrial ecosystems and the atmosphere is crucial when trying to assess the effect of anthropogenic and biogenic controls on a future climate. Using the eddy covariance method, fluxes of CO2 have been measured over a wide array of ecosystems, while measurements of the other two major GHG, methane (CH4) and nitrous oxide (N2O), were only conducted by few groups due to expensive scalar sensors and their time-consuming maintenance. These first measurments mainly focused on ecosystems that were believed to represent significant sources for CH4 (e.g. wetlands) or N2O (e.g. heavily fertilized crops). With CH4 and N2O measurement devices now being widely available, more measurements are made over sites that are characterized by relatively small and often close-to-zero fluxes, and despite recent advances in sensor sensitivity and stability, the quantification of these two GHG remains challenging. Here we report on the CO2, CH4 and N2O exchange measured over 2 years at a temperate mountain grassland managed as a hay meadow near the village Neustift in the Stubai Valley, Austria, by means of the eddy covariance method. The three wind components, the speed of sound and the CO2 mole densities were acquired at a time resolution of 20 Hz and used to calculate true eddy covariance CO2 fluxes. CH4 and N2O mixing ratios were recorded at 2 Hz by a quantum cascade laser absorption spectrometer (QCL-AS), resulting in a disjunct time series when compared to the 20 Hz wind data. Fluxes of both compounds were then calculated using the virtual disjunct eddy covariance method (vDEC). Mixing ratios of CH4 and N2O were then corrected for the cross-talk effect of water as described in earlier studies. The net ecosystem exchange of CO2 at the study site is monitored continously since 2001, while the measurement of CH4 and N2O fluxes started in April 2010. During the vegetation period, typical concentration values for CH4 and N2O were around 2.0 and 0.3 ppm, respectively, exhibiting distinct diurnal cycles. Emission and uptake could be observed for both compounds, and exchange rates showed a clear diurnal cycle during certain time periods. During the day, relatively large amounts of methane were emitted in September 2011, with peak emission rates of 24.9 nmol m-2 s-1 around noon, while relatively strong emissions of nitrous oxide could be observed in August 2011 (0.9 nmol m-2 s-1). However, during calm and stable nighttime conditions, mixing ratios of both compounds were often erratic with distinct spikes and high ambient concentration values, which propagated into the flux calculations and resulted in similarly erratic exchange rates, making further despiking necessary. The drivers of of CH4 and N2O exchange patterns during undisturbed conditions and the effect of management events like harvesting and the spreading of manure are investigated and set in relation to previously published studies on this matter.