Effect of Nitrogen Fertiliser on the Nitrogen Cycle of Two Upland Arable Soils of Contrasting Textures

Keywords: Nitrogen cycle, Upland arable soils, Nitrogen losses, Nitrogen fertilisation.

The results of 3 years work on the N balance of two upland arable sites with contrasting textures (a clay loam and a sandy loam overlying loamy sand) are presented. The N application rate varied from 0 to 210 kg N ha/ha as NH₄NO₃. Nitrate leaching at both sites was measured using drained plots. N release from soil organic matter was studied using ¹⁵N and control (zero N) plots. Denitrification was estimated using acetylene block and balance sheet methods.

There was little influence of soil texture on N leaching and the effect of excess N fertiliser was also small, but timing of cultivation on the heavy soil was important. Large quantities of N (around 50 kg/ha) were unaccounted for at optimal fertiliser rates at both sites. Denitrification is the likely main cause of loss at the clay loam site, and this is supported by ¹⁵N balance data, but this explanation is inadequate at the sandy loam site, because a full ¹⁵N recovery in crop and soil occurs, and virtually no denitrification loss could be detected using the acetylene block method. The most likely explanation is immobilisation of fertiliser N, but subsoil denitrification may also be involved.

The interpretation of the results was aided by concurrent chloride leaching experiments, the Cl^– acting as a conservative tracer behaving in a similar way to nitrate. These experiments showed that the residence time of a substantial proportion of chloride (40-50%) was longer than a year.

A J A Vinten and R S Howard, Soil Science Department, SAC, West Mains Road, Edinburgh, UK.

48 pages, 23 figures, 8 tables, 29 references.