Keywords: environment, N losses, denitrification
The chemistry and biochemistry of the processes resulting in denitrification and emissions of nitrogen oxides by soils are reasonably well understood, although some of the finer details still have to be established beyond doubt. Attempts to quantify the amounts of mineral nitrogen lost to the atmosphere have met with less success.
Two factors are principally responsible: the great variability - spatial and temporal - of the rates of emission and, in the case of denitrification, the difficulty of measuring what is often the major product, N2, in the presence of the atmospheric background.
As methods of measuring denitrification improve (and in particular as more experience is obtained with 15N tracer methods in this area of research), we may expect to see more reliable and consistent data emerging. It will not be surprising if, then, we find in agronomic studies that a very significant proportion of the “unaccounted for” entry in nitrogen balance sheets becomes attributed unequivocally to denitrification, especially in relation to crops grown in heavy clay soils. Under such conditions gaseous losses have probably been under- rather than over-estimated, hitherto.
Environmental pressures have already promoted a shift of emphasis towards the study of nitrous oxide (N2O) emissions per se, rather than as a detail within the study of denitrification. As the environmental problems to which N2O contributes are truly global ones, there is a need to acquire more representative information about emissions, and the processes causing them, world-wide. Up to now, a disproportionate amount of information has come from the developed temperate regions of the northern hemisphere, compared with the rest of the world, and from managed agricultural systems compared with forests and other natural ecosystems. Enough is known, however, to conclude that increased use of nitrogen fertilisers has contributed to the increased emissions of N2O (and NO), directly from agricultural land, and indirectly following the transfer of nitrogen to other environments.
Much release of N2O from soils has nothing to do with fertilisers or manures, but comes from mineralisation processes, especially in tropical forests. However, the very large reductions in emissions being called for by bodies such as the Intergovernmental Panel on Climate Change mean that no significant source can reasonably be ignored, and reductions in emissions from fertilisers will be highly desirable.
The emissions of N2O from fertilisers vary greatly with fertiliser form and soil conditions, and with different combinations of these two factors. There is considerable scope for reducing them (just as nitrate leaching can be reduced) by modification to existing fertiliser use and soil management practices. The procedures which optimise crop recovery of applied N will generally also be those which minimise losses by denitrification, and more can undoubtedly be done to achieve their more widespread adoption. The effect of fertiliser form on the emission rate of N2O from nitrification appears to be a significant one, and any consequent pressures to change the balance of the products used are likely to have major implications for the international fertiliser industry. The subject promises to give rise to a lively debate during the next few years.
K A Smith and J R M Arah, The Edinburgh School of Agriculture, West Mains Road, Edinburgh EH9 3JG, UK
34 pages, 11 figures, 6 tables, 124 references