Limits to denitrification in two pasture soils in a temperate maritime climate

Abstract

Factors limiting denitrification in pasture soils under the climatic conditions typical of western Britain were investigated. Water and temperature were expected to be the most important factors in the control of denitrification but pasture management, season and spatial variability were also studied. Soil cores were incubated in bottles to examine the relative importance of water and temperature. Soils were incubated at field temperature with and without acetylene to block the reduction of nitrous oxide (N 2O) to nitrogen gas (N 2). Two pasture systems were studied: grass/fertiliser receiving 150 kg N ha −1 and grass/clover. The average rate of denitrification was 10 g N ha −1 day −1 under grass/ clover. Fertiliser increased the average rate to 60 g N ha −1 day −1 with a maximum of 500 g N ha −1 day −1. There was a larger potential for denitrification than could be realised, in part, by rainfall and solar warming because an average of 1600 g N ha −1 day −1 was measured from wet soils at room temperature. When these rates were corrected for the initial lag because of enzyme synthesis, the maximum rate was 5000 g N ha −1 day −1. Dung and urine from sheep caused spatial diversity. Under grass/clover in the autumn 70% of the denitrification loss came from only 14% of the area. Seasonal variations were confounded with variations caused by pasture management. Most denitrification losses occurred after the fertiliser applications on the grass/fertiliser pasture. Under grass/clover the fastest rates were measured in the autumn. Nitrous oxide formed 40–80% of the denitrification product from grass/clover. Its response to fertiliser varied widely. In March, fertiliser apparently caused the value to exceed 100% nitrous oxide; an artifact of the acetylene blocking method attributed to the release of nitrous oxide during nitrification. The value later fell to 20% nitrous oxide. A new model of denitrification was derived from published data, specifically to check the validity of the incubation data. In the model, denitrification depended on soil water content, temperature and nitrate supply. In these soils, the nitrogen supply limited field denitrification more than any other factor, followed in importance by soil moisture and temperature.