Variation in denitrification activity with soil depth under pasture

Factors affecting the variation in soil denitrification activity with depth, at different times of the year, were studied in a well-drained sandy loam and a poorly-drained silt loam under pasture, using the denitrification enzyme activity assay. Additions of NO3−–N or glucose–C to saturated soils under anaerobic conditions were used to identify which of these factors most limited denitrification. Field denitrification rates were measured at different depths under two contrasting soil moisture contents just before and after heavy rainfall (28.5 mm). Denitrification activities were maximal in the surface soil (0–5 cm) and generally decreased exponentially with depth, regardless of sampling sites and times. For example, denitrification activities measured under anaerobic saturated conditions after amendment with both glucose–C and NO3−–N were between 10 and 20 μg N2O–N g−1 soil d−1 in the surface soils and were between 0.1 and 2 μg N2O–N g−1 soil d−1 at a depth of 20 and 40 cm. Denitrification activities in unamended soils were much greater (15×) in autumn than in spring in the 0–10 cm layer in both soils. At greater depths (20–40 cm), however, there was little difference in denitrification activities in unamended soils between seasons. At all sampling times, C limited the denitrification activity, most particularly in the subsurface soils. NO3−–N also limited the denitrification activity at some depths, especially in the surface soils when native soil NO3−–N concentrations were low in spring. In the presence of unlimited C and NO3−–N, denitrification enzyme activity also decreased with depth. However, the enzyme activity was still considerable even at a depth of 20–40 cm, which indicates that microorganisms capable 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 changes in denitrification rate in the soil profile. It is suggested that the rainfall may have washed some soluble-C and NO3−–N from the surface 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 in the presence of unlimited C and NO3−–N did not change with season.