Spatial variability of NO and NO2 flux rates from soil of spruce and beech forest ecosystems

In order to evaluate differences in the magnitude of NO and NO2 flux rates between soil areas in direct vicinity to tree stems and areas of increasing distance to tree stems, we followed in 1997 at the Hoglwald Forest site with a fully automated measuring system a complete annual cycle of NO and NO2 fluxes from soils of an untreated spruce stand, a limed spruce strand, and a beech stand using at each stand measuring chambers which were installed onto the soils in such a way that they formed a stem to stem gradient. Flux data obtained since the end of 1993 from measuring chambers placed at the interstem areas of the stands, which had been used for the calculation of the long year annual mean of NO and NO2 flux rates from soils of the stands, are compared to both (a) those obtained from the interstem chambers in 1997 and (b) those from the stem to stem gradient chambers. Daily mean NO fluxes obtained in 1997 were in a range of 0.3-280.1 mug NO-N m(-2) h(-1) at the untreated spruce stand, 0.5-273.2 mug NO-N m(-2) h(-1) at the limed spruce stand and 0.5-368.8 mug NO-N m(-2) h(-1) at the beech stand, respectively. Highest NO emission rates were observed during summer, lowest during winter. Daily mean NO2 fluxes were in a range of -83.1-7.6 mug NO2-N m(-2) h(-1) at the untreated spruce stand, -85.1-2.1 mug NO2-N m(-2) h(-1) at the limed spruce stand and -77.9 to -2.0 mug NO2-N m(-2) h(-1) at the beech site, respectively. As had been observed for the years 1994-1996, also in 1997 NO emission rates were highest at the untreated spruce stand and lowest at the beech stand and liming of a spruce stand resulted in a significant decrease in NO emission rates. For NO2 no marked differences in the magnitude of flux rates were found between the three different stands. Results obtained from the stem to stem gradient experiments revealed that at all stands studied NO emission rates were significantly higher (between 1.6- and 2.6-fold) from soil areas close to the tree stems and decreased-except at the beech stand-with increasing distance from the stems, while for NO2 deposition no marked differences were found. Including the contribution of soil areas in direct vicinity to the beech stems in the estimation of the annual mean NO source strength revealed that the source strength has been underestimated by 40% in the past.