Nitrogen Economy of Grazed Grassland

Keywords: Nitrogen cycle, Grazed grassland, Leaching, Denitrification, Nitrogen balance.

Nitrogen is the key element which drives production of temperate grassland systems. A grassland sward producing 15 tonnes of dry matter/ha/yr will transfer approximately 375 kg of nitrogen from soil to harvestable biomass but the grazing animal will retain only a small fraction of this. This results in substantial losses of nitrogen in grazed grassland systems. Whilst these losses may be regarded as wasteful in economic terms, or potentially damaging to the environment, it is important to realise that they are also part of natural recycling mechanisms within the global nitrogen cycle. The inevitability of such natural loss processes sets limits on what can be achieved through improved channelling of nitrogen from its input to the outputs of milk and meat from grassland systems.

Knowledge of the nitrogen economy of grazed grassland is fundamental to improving nitrogen use efficiency. An integrated study of grazed grassland in Northern Ireland is described in which different levels of nitrogen input have provided balance, response curve and time course data together with information on the inter-relationship of nitrogen cycle processes. The data are compared to those from other studies and related to regional differences in the grassland nitrogen cycle. Opportunities for controlling and directing loss processes are considered.

M K Garrett, C J Watson, C Jordan, R W J Steen and R V Smith, The Department of Agriculture for Northern Ireland and the Queen’s University, Belfast, UK.

32 pages, 11 figures, 6 tables, 53 references.