The mineralisation and fate of nitrogen following the incorporation of grass and grass-clover swards

Abstract

Field and laboratory experiments were carried out between 1992 and 1994 to study the release and fate of nitrogen (N) following the incorporation of grass and grass- clover swards. The effects of sward type, quality of sward residue and previous grazing management on the N cycle were studied. Techniques for the measurement of mineralisation and nitrate (NO3") leaching on medium textured soils were also assessed.

Initial work on the measurement of mineralisation using 1 p00j dilution techniques revealed major problems with practical application in the field. Mineral N release was therefore estimated by combining soil mineral N, NO3' leaching, plant uptake and gaseous N loss data.

Evaluation of estimated leaching loads and specific tests comparing porous cups with three other estimates of soil water N0 3--N concentrations cast serious doubts on the quantitative accuracy of porous cups on medium textured soils. Results suggest that cups were sampling relatively immobile water and, depending on the source of the solute, this led to over- and under-estimates of leaching loads.

The ploughing out of previously grazed swards released a substantial amount of N in the first eighteen months following incorporation (ca. 370 kg N ha-*), particularly during the first two months. Swards subject to an unfertilised cutting regime released much less N following ploughing. This was attributed to a reduction of soil organic matter (SOM) "degradability" and, to a lesser extent, plant residue N supply. Clover residues were found to have a notable effect on mineral N release only when sward clover contents were over 20% of total dry matter (DM) and grass residues were of much lower N content.

The ploughing out of grassland produces a considerable short term increase in nitrous oxide (N2O) emissions thought to be initiated by the intimate mixing of readily available carbon sources into the soil during rotavation. The influence of clover residues on N2O emissions was greater than their influence on mineral N release patterns.

Manipulation of the post-ploughing N cycle may be possible by altering sward management prior to ploughing. The cessation of grazing for a short period before sward incorporation could reduce mineral N release and N2O emissions. However, the capacity to reduce N2O emissions is constrained by weather conditions at critical times following incorporation.