Fluxes of NO3-, NH4+, NO, NO2, and N2O in an Old Danish Beech Forest

The fluxes of the major nitrogen compounds havebeen investigated in many ecosystem studies over the world.However, only in few studies has attention been drawn to theimportance of the fluxes of minor gaseous nitrogen compoundsto complete the nitrogen cycle. In Denmark a detailed study onthe nitrogen cycle in an old beech forest has been implementedin 1997 at Gyrstinge near Sorø, Zealand. The study includesthe fluxes of the gases NO, N 2 O and water mediatedtransport of NO 3 - and NH 4 + . Measurementsof the fluxes of the gaseous compounds are performed withmicro-meteorological methods (eddy-correlation and gradient)and with chambers. Water mediated fluxes encompass rain,throughfall, stem-flow and leaching from the root zone. Thehydrological model is verified by TDR measurements. The findings show that the total water mediated N input tothe forest floor with throughfall and stemflow was 25.6 kg Nha -1 yr -1 , and open field wet deposition withprecipitation was 19.0 kg N ha -1 yr -1 . The internalcycling of N in the ecosystem measured as turnover oflitterfall and plant uptake was 100 kg N ha -1 yr -1 and 14 kg N ha -1 yr -1 , respectively. The fluxes ofthe gaseous N compounds NO and N 2 O were of minorimportance for the total N turnover in the forest, NO x emission being <1 kg N ha -1 yr -1 and N 2 Oemission from the soil being 0.5 kg N ha -1 yr -1 withno significant difference between wet and dry soils.Concentrations of NO 3 - and NH 4 + in thesoil solution beneath the rooting zone are very small andconsequently the N leaching is almost negligible. It isconcluded that the nitrogen mass balance of this old beechforest ecosystem mainly is controlled by the input by dry andwet deposition and a large internal N cycle with a fast litterturnover. The nitrogen input tothe forest ecosystem which currently exceeds the critical loadby 5 kg N ha -1 yr -1 is mainly accumulated in the soil and no significant nitrateleaching is occurring.