Nitrogen oxides emission from two beech forests subjected to different nitrogen loads

Abstract. We analysed nitrogen oxides (N₂O, NO) and carbon dioxide (CO₂) emissions from two beech forest soils close to Vienna, Austria, which were exposed to different nitrogen input from the atmosphere. The site Schottenwald (SW) received 20.2 kg N ha⁻¹ y⁻¹ and Klausenleopoldsdorf (KL) 12.6 kg N ha⁻¹ y⁻¹ through wet deposition. Nitric oxide emissions from soil were measured hourly with an automatic dynamic chamber system. Daily N₂O measurements were carried out by an automatic gas sampling system. Measurements of nitrous oxide (N₂O) and CO₂ emissions were conducted over larger areas on a biweekly (SW) or monthly (KL) basis by manually operated chambers. We used an autoregression procedure (time-series analysis) for establishing time-lagged relationships between N-oxides emissions and different climate, soil chemistry and N-deposition data. It was found that changes in soil moisture and soil temperature significantly effected CO₂ and N-oxides emissions with a time lag of up to two weeks and could explain up to 95% of the temporal variations of gas emissions. Event emissions after rain or during freezing and thawing cycles contributed significantly (for NO 50%) to overall N-oxides emissions. In the two-year period of analysis the annual gaseous N₂O emissions at SW ranged from 0.64 to 0.79 kg N ha⁻¹ y⁻¹ and NO emissions were 0.24 to 0.49 kg N ha⁻¹ per vegetation period. In KL significantly lower annual N₂O emissions (0.52 to 0.65 kg N₂O-N kg ha⁻¹ y⁻¹) as well as considerably lower NO-emissions were observed. During a three-month measurement campaign NO emissions at KL were 0.02 kg N ha⁻¹), whereas in the same time period significantly more NO was emitted in SW (0.32 kg NO-N ha⁻¹). Higher N-oxides emissions, especially NO emissions from the high N-input site (SW) may indicate that atmospheric deposition has an impact on emissions of gaseous N from our forest soils. At KL there was a strong correlation between N-deposition and N-emission over time, which shows that low N-input sites are especially responsive to increasing N-inputs.