Denitrification potential and actual rates of denitrification in soils under long-term grassland and arable cropping

Abstract

Denitrification potential and actual rates of denitrification were measured in two adjoining plots that, for 31 yr, had been either under continuous grassland or cultivated annually for arable cropping. The soil (0–10 cm) of the grassland plot had a denitrification potential, measured in the laboratory, five times greater than that of the arable plot. However in the field, actual rates of denitrification from the grassland were often much lower than those from the arable plot, despite the contents of soil nitrate being similar. Denitrification in the grassland plot was greater than in the arable plot only when the soil moisture tension was less than 5.5 kPa. However, this moisture tension was exceeded much more quickly after irrigation or rainfall on the grassland soil, due mainly to its greater porosity. At temperatures <8°C, the rate of denitrification in soil from both plots was low (< 0.2 kg N ha −1 day −1) even at moisture tensions in the range of 3–5kPa. At 20°C, however, denitrification in the grassland soil occurred at a rate > 0.2 kg N ha −1 day −1 even with a soil moisture tension as high as 8kPa, and reached 1.5kg N ha −1 day −1 at about 4.8 kPa. Denitrification from the arable soil at 20°C and 4.8 kPa amounted to about 0.7 kg N ha −1 day −1.