Measurement of denitrification rate in undisturbed soil cores under different temperature and moisture conditions using 15N tracer technique, 2: Factors affecting denitrification

Using undisturbed soil cores and 15N-labelling technique, the denitrification capacity of a number of forest soils (Spodosols, Cambisols, Histosols) was determined during long-term laboratory incubation. The routine method involved adjusting the moisture content of the soil cores to approximately 100% WHC (pF less than 2), followed by incubation of the cores as 21 deg C and at constant atmospheric O2 pressure. The results can be summarized as follows: A general relationship existed between soil texture and amount of denitrification. The highest denitrification rates were found in heavy-textured clay soils, followed by well-humidified organic soils (woody fen peat). In contrast, sandy podsols, developed within glacifluvial outwash, were characterized by extremely low capacity to denitrify. For the nine experimental soils, subjected to standard water-saturation/draining procedure, the denitrification rate ranged from 11 to 162 mg N per m-2 and day-1. Related to organic N content, the denitrification rate was highest in soil material from the spodic B horizon. Soil-moisture content and incubation temperature were found to be important factors influencing the rate of denitrification. Emission of N2O and N2 decreased measurably when the moisture content dropped only slightly below 100% WHC. In the two soils studied, a decrease in moisture content to 78-89% WHC led to an average reduction in denitrification of 71%. For contrast, waterlogging of the soil stimulated denitrification. Field irrigation, corresponding to 40 mm precipitation, did not lead to any measurable denitrification loss on sandy podsol. The effect of temperature is illustrated by data showing that in lowering the incubation temperature from 21 deg C to 6 deg C the denitrification rate in the two soils incubated at a moisture content corresponding to 100% WHC decreased by 86 and 98% respectively. The above reduction in denitrification rate was significantly higher than corresponding reduction observed in soil respiration