Long-term measurements of NOx and O3 soil fluxes in a temperate deciduous forest

A long-term campaign devoted to the measurement of fluxes of nitrogen oxides and ozone at the surface of a temperate deciduous forest was carried out from 1st September 2012 to 31st December 2013, using automated dynamic chambers. Soil temperature and water content profiles were recorded near the chambers in order to clarify the short-term response of the fluxes to these two variables. Bulk soil resistances against NO, NO2 and O3 emission or deposition were also estimated in order to understand their daily and seasonal variations. Nitric oxide was almost exclusively emitted and the annual average soil emission of 27.4mgNm−2 h−1 was higher than those for other deciduous European forests. The maximum daily peaks of NO emission exceeded 100mgNm−2 h−1 during summertime and were close to zero in wintertime. On annual basis the N balance of the ecosystem as NOx showed a net emission towards the atmosphere of 1.1 kgNha−1 yr−1. This result can be explained by the strong N input through wet deposition observed at this site during the last decades. NO emission fluxes depended exponentially on soil temperature and decreased at high levels of soil water content during summertime, suggesting that the main contribution was due to aerobic nitrification processes. Soil deposition fluxes of NO2 and O3 were mainly governed by atmospheric processes related to the atmosphere/soil concentration differences. Soil resistances against NO2 and O3 deposition showed both diurnal and seasonal variations which disagree with the constant values often used in model studies.