Nitric oxide and nitrous oxide emission from Hungarian forest soils; link with atmospheric N-deposition

Abstract. Studies of forest nitrogen (N) budgets generally measure inputs to the atmosphere in wet and dry precipitation and outputs via hydrologic export. Although denitrification has been shown to be important in many wetland ecosystems, emission of nitrogen oxides from forest soils is an important, and often overlooked, component of an ecosystem nitrogen budget. During one year (2002-2003), emissions of nitric oxide (NO) and nitrous oxide (N₂O) were measured from Sessile oak and Norway spruce forest soils in northeast Hungary. Accumulation in small static chambers followed by gas chromatography-mass spectrometry detection was used for the estimation of N₂O emission flux. Because there are rapid chemical reactions of NO and ozone, small dynamic chambers were used for in situ NO flux measurements. Average soil emissions of NO were 1.2 and 2.1 µgNm^-2h^-1, and for N₂O were 15 and 20 µgNm^-2h^-1, for spruce and oak soils, respectively. The previously determined nitrogen balance between the atmosphere and the forest ecosystem was re-calculated using these soil emission figures. The total (dry + wet) atmospheric N-deposition to the soil was 1.42 and 1.59gNm^-2yr^-1 for spruce and oak, respectively, while the soil emissions are 0.14 and 0.20 gNm^-2yr^-1. Thus, about 10-13% of N compounds deposited to the soil, mostly as NH₃/NH₄⁺ and HNO₃/NO₃^-, are transformed in the soil and emitted back to the atmosphere, mostly as a greenhouse gas (N₂O).