VARIATION IN DENITRIFICATION ACTIVITY WITH SOIL DEPTH UNDER PASTURE

Factors affecting the variation in soil denitrification activity with depth, at different times of the pear, were studied in a well-drained sandy loam and a poorly-drained silt loam under pasture, using the den itrification enzyme activity assay. Additions of NO3--N or glucose-C t o saturated soils under anaerobic conditions were used to identify whi ch of these factors most limited denitrification. Field denitrificatio n rates were measured at different depths under two contrasting soil m oisture contents just before and after heavy rainfall (28.5 mm). Denit rification activities were maximal in the surface soil (0-5 cm) and ge nerally decreased exponentially with depth, regardless of sampling sit es and times. For example, denitrification activities measured under a naerobic saturated conditions after amendment with both glucose-C and NO3--N were between 10 and 20 mu g N2O-N g(-1) soil d(-1) in the surfa ce soils and were between 0.1 and 2 mu g N2O-N g(-1) soil d(-1) at a d epth of 20 and 40 cm. Denitrification activities in unamended soils we re much greater (15x) in autumn than in spring in the 0-10 cm layer in both soils. At greater depths (20-40 cm), however, there was little d ifference in denitrification activities in unamended soils between sea sons. At all sampling times, C limited the denitrification activity, m ost particularly in the subsurface soils. NO3--N also limited the deni trification activity at some depths, especially in the surface soils w hen native soil NO3--N concentrations were low in spring. In the prese nce of unlimited C and NO3--N, denitrification enzyme activity also de creased with depth. However, the enzyme activity was still considerabl e even at a depth of 20-40 cm, which indicates that microorganisms cap able of denitrification occurred at depth in these soils under pasture . Soil moisture contents before and after the rainfall influenced the soil air-filled porosities and consequently influenced the temporal ch anges in denitrification rate in the soil profile. It is suggested tha t the rainfall may have washed some soluble-C and NO3--N from the surf ace into the subsurface soils and this was one of the reasons for the increase in field denitrification rate at lower depths after rainfall. However, the denitrification enzyme activities in the soil profiles i n the presence of unlimited C and NO3--N did nor change with season. ( C) 1998 Elsevier Science Ltd. All rights reserved.