Some factors influencing denitrification and nitrogen immobilization in a clay soil

Nitrate-N, enriched with ¹⁵N, was added to small cores of the 0–10 cm layer of a clay soil. The base of each core was sealed, then water, equivalent to 0, 10, 20 or 30mm of rain, was added to the soil surface. The cores were incubated for 1 week at 10, 20, or 30°C in the presence or absence of wheat straw. The recovery of ¹⁵N in the soil mineral-N and organic-N fractions was then measured.No significant losses of ¹⁵N were detected in the cores which received 0–10 mm of added water, and in which the soil water content was close to 0.56 g g^?1 (?10 kPa). However, ¹⁵N losses, assumed due to denitrification, were rapid from cores receiving 20 or 30 mm of water and incubated at 20–30°C. The onset of denitrification was quite sudden as the amount of added water increased from 10 to 20 mm. In this range, a small increment of added water apparently sealed a relatively large volume of soil from atmospheric O₂ diffusion. This phenomenon was strongly temperature-dependent since no losses were detected from any cores at 10°C even though the 30mm addition of water produced a thin layer of free water across the soil surface.The addition of straw did not promote denitrification in soil at water contents close to 0.56 g g^?1. At high soil water contents, adcling straw increased immobilization of labelled NO₃^? and so reduced denitrification losses. The response of immobilization to changing soil water and temperature conditions was very different from that of denitrification.