Nitric oxide emission from a Norway spruce forest floor

Although the available data on soil NO emission are increasing, only few me asurements have been made in temperate forests. Emissions from these forest s are generally low, but recent studies have shown that NO emission from N- affected forests can be much higher. In this study the emission of NO from the door of a moderately N-affected. Norway spruce forest in Denmark was me asured using dynamic chambers during two seasons (April 1995 and September 1995). At the same time, fluxes of CO2 and NO2, soil water content inside a nd outside the chambers, soil temperature at 10 cm depth, soil surface temp erature and air temperature were, recorded. The emission of NO in April was low (< 0.3 ng N m(-2) s(-1)), whereas it was significantly higher in Septe mber (21 ng N (-2) s(-1) with a range 0-82 ng N m(-2) s(-1)). In April the soil surface temperature was low (< 6"C) and the soil moisture was high (26 % vol/vol). In September the soil surface temperature was high (16 degrees C) and soil moisture was significantly lower (14% vol/vol) than in April. T here was a high correlation between CO2 emission and NO emission. The fluxe s of NO2 were very small and not related to NO emission. Generally, the var iation within a single plot was small, whereas the variation between plots was large. About half of the total variation of NO emission in September co uld be explained by a combination of soil moisture, distance to tree trunks , and moss cover. No significant relationship could be found between NO emi ssion and soil surface temperature or soil temperature. Other parameters su ch as the C/N dynamics in the soil must be taken into account to provide a better mechanistic understanding of the NO emission. An annual emission of 3.15 kg NO-N ha(-1) yr(-1) was estimated by the amount of NH4+ in throughfa ll. Compared to most of the previous published measurements of NO emission from temperate forest soils, the values presented here are high and of the same magnitude as found in N-affected forests in Central Europe.