Decadal variability of soil CO₂, NO, N₂O, and CH₄ fluxes at the Höglwald Forest, Germany

Abstract. Besides agricultural soils, temperate forest soils have been identified as significant sources of or sinks for important atmospheric trace gases (N₂O, NO, CH₄, and CO₂). Although the number of studies for this ecosystem type increased more than tenfold during the last decade, studies covering an entire year and spanning more than 1–2 years remained scarce. This study reports the results of continuous measurements of soil-atmosphere C- and N-gas exchange with high temporal resolution carried out since 1994 at the Höglwald Forest spruce site, an experimental field station in Southern Germany. Annual soil N₂O, NO and CO₂ emissions and CH₄ uptake (1994–2010) varied in a range of 0.2–3.0 kg N₂O-N ha⁻¹yr⁻¹, 6.4–11.4 kg NO-N ha⁻¹yr⁻¹, 7.0–9.2 t CO₂-C ha⁻¹yr⁻¹, and 0.9–3.5 kg CH₄-C ha⁻¹yr⁻¹, respectively. The observed high fluxes of N-trace gases are most likely a consequence of high rates of atmospheric nitrogen deposition (>20 kg N ha⁻¹yr⁻¹) of NH₃ and NO_x to our site. For N₂O, cumulative annual emissions were ≥ 0.8 kg N₂O-N ha⁻¹yr⁻¹ in years with freeze-thaw events (5 out 14 of years). This shows that long-term, multi-year measurements are needed to obtain reliable estimates of N₂O fluxes for a given ecosystem. Cumulative values of soil respiratory CO₂ fluxes tended to be highest in years with prolonged freezing periods, i.e. years with below average annual mean soil temperatures and high N₂O emissions (e.g. the years 1996 and 2006).

Furthermore, based on our unique database on trace gas fluxes we analyzed if soil temperature, soil moisture measurements can be used to approximate trace gas fluxes at daily, weekly, monthly, or annual scale. Our analysis shows that simple-to-measure environmental drivers such as soil temperature or soil moisture are suitable to approximate fluxes of NO and CO₂ at weekly and monthly resolution reasonably well (accounting for up to 59 % of the variance). However, for CH₄ we so far failed to find meaningful correlations, and also for N₂O the predictive power is rather low. This is most likely due to the complexity of involved processes and counteracting effects of soil moisture and temperature, specifically with regard to N₂O production and consumption by denitrification and microbial community dynamics. At monthly scale, including information on gross primary production (CO₂, NO), and N deposition (N₂O), increased significantly the explanatory power of the obtained empirical regressions (CO₂: r² =0.8; NO: r² = 0.67; N₂O, all data: r² = 0.5; N₂O, with exclusion of freeze-thaw periods: r² = 0.65).