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 20.2kg N ha-1y-1 and Klausenleopoldsdorf (KL) 12.6kg 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 (N 2O) 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 S W ranged from 0.64 to 0.79kg N ha-1y-1 and NO emissions were 0.24 to 0.49kg N ha-1 per vegetation period. In KL significantly lower annual N2O emissions (0.52 to 0.65 kg N 2O-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 (0.32kg 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.