Flow of Nitrogen in Grassland

CONCLUSIONS AND RESEARCH NEEDS

Considerations of the efficiency of nitrogen (N) use in grassland clearly indicate that the ruminant has a profound effect on the flow of N. This effect has been largely overlooked in research conducted during the past 10 to 20 years, evidence of which is seen in recent authoritative reviews. This has arisen because (i) research has been concentrated on the utilisation of N upstream of the ruminant, particularly at the point of fertiliser addition and (ii) it is only recently that methods have been developed to measure some of the most important pathways of N loss from grazed swards.

The importance of perturbations to the N cycle caused by ruminant production is likely to increase if further intensification of management occurs. On theoretical grounds alone it is apparent that a large proportion of the N not recovered in products is lost from the system. This has been confirmed in recent studies in grazed swards which cast N more as a waste product of intensive pastoral systems rather than a largely re-usable resource. When this is viewed against information that suggests a trend towards annual rates of fertiliser application in the range 200 to 400 kg N/ha it can be seen that there are grounds for serious concern. It is essential, therefore, that increased effort is devoted to studies of the flow of N downstream of the ruminant. This will be achieved only by adopting integrated research involving the whole soil-plant-animal complex.

The need for such work is emphasised not only by recent changes in agricultural policy affecting the grassland sector but also by increased concern about the effects of intensive agriculture on the natural environment. Outflows of N as ammonia (NH₃) to the atmosphere and as nitrate to ground and surface waters are of particular concern. The potential effects of increased nitrate levels in water resources are well documented. In contrast, the environmental effects of NH₃ loss are largely unknown. Such loss inevitably contributes to increased wet and dry deposition of N over whole regions, may accentuate the problems associated with acid rain and increase the formation of aerosols in the atmosphere by reaction with sulphur dioxide. Possibly the least damaging loss of N is that through denitrification. However, increased denitrification has longer term and global environmental implications as a proportion of this loss occurs as nitrous oxide (N₂O) which is thought to be involved in the depletion of stratospheric ozone. The proportion of nitrous oxide formed during denitrification is increased when the nitrate content of soil is large, a feature characteristic of grazed swards, particularly the urine-affected areas.

This review has pointed to a shortfall in information on the flow of N in complete and representative grassland systems. Techniques are now available to study each aspect of the flow of N thereby permitting the determination of complete balances for N. At present, there are few data on the volatilisation of NH₃ from grassland, particularly as it is affected by grazing management in systems relevant to north west Europe. An assessment is also required of the inter-dependence of leaching and denitrification in the dispersal of nitrate from soils below grazed swards. In that soil organic matter acts as a ‘buffer’ against N loss, more detailed studies of factors affecting the accumulation of organic N in soils are needed, in particular the inter-relationships between the flow of C and N in the soil-plant-animal complex. The extent to which organic N can accumulate in soil below permanent pasture during intensification through increased input of N also requires more critical evaluation. Detailed studies are also needed of the fate of N following the application to land of slurries and manures, in particular the management factors affecting the retention and loss of N.

In conjunction with these more detailed aspects of the flow of N in grassland, case studies are required on farms that have already reached high levels of production and on those where development to increase production are being initiated. In addition, it is important that the efficiency of the flow of N in fertiliser-based systems is compared with that in systems based on grass-clover swards. Preliminary studies at Hurley suggest that the flow of N in grazed grass-clover swards is more precisely regulated than that in ryegrass swards receiving fertiliser N. Further studies of the management of animal wastes are also required with particular attention being given to the conservation of their N content. Such studies are now possible due to recent developments in the techniques available to measure the inputs, transformations and losses of N as they occur in the field. Studies of the type outlined above will provide information on the flow of N upstream and downstream of the ruminant, the ultimate objective being its use in the development of models to describe the flow of N in representative systems. Only when adequate information is available will it be possible to identify and test realistic management to optimise the flow of N in grassland and minimise its impact on the environment.

John C Ryden, The Grassland Research Institute, Hurley, Maidenhead, Berkshire, UK.

44 pages, 10 figures, 10 tables, 148 references.