Effect of different forage plants on denitrification potential of Horotiu soil

Denitrification potential (DNP),water-soluble soil carbon, soil nitrate, and plant weights were measured in lysimeters growing cocksfoot, tall fescue, chicory, Yatsyn ryegrass, or Ruanui ryegrass in a New Zealand soil. The soil-related parameters were measured in the topsoil and upper- and lower-subsoil zones of the lysimeters. Significantly lower rates of DNP were measured under Ruanui ryegrass than under the other plants and there was a tendency for higher rates of DNP to be associated with bigger root masses. Water-soluble carbon was higher in the topsoil than in the upper- or lower-subsoil, but soluble carbon did not differ significantly among the cultivars. DNP responded positively to addition of glucose to the soil, demonstrating that soluble carbon rather than the resident populations of denitrifying micro-organisms were limiting DNP. Lysimeters growing cocksfoot had the lowest soil nitrate levels and the largest root systems but only intermediate rates of DNP. The quality of root residues entering the soil under cocksfoot is probably lower than for the other plants.