The popular misconception that the nitrate problem is caused by farmers applying too much nitrate fertilizer is too simplistic. Nevertheless, there is now little doubt that the high concentrations of nitrate in freshwaters noted in recent years have mainly resulted from runoff from agricultural land and that the progressive intensification of agricultural practices, with increasing reliance on the use of nitrogenous fertilizer, has contributed significantly to this problem. Since 1945, agriculture in the industrialized world has become much more intensive. Fields are ploughed more frequently; more land is devoted to arable crops, most of which demand large amounts of fertilizer; grassland too receives large applications of fertilizer to ensure a high-quality silage for winter feed; stocking densities in general are higher leading to increased inputs of manure on grassland and problems of disposal of stored slurry; cattle often have direct access to water courses resulting in soil and bank erosion and direct contamination from animal waste; many low-lying fields are now underdrained, encouraging more productive use of the
September September September September September September September 1970 1976 1982 1988 1994 2000 2006
Figure 6 Long-term nitrate record 1970-2005: Slapton Wood catchment (UK).
Figure 6 Long-term nitrate record 1970-2005: Slapton Wood catchment (UK).
land and speeding the transport of leached nitrate to surface water courses. Lowland rivers close to urban areas may receive larger quantities of nitrogen from sewage effluent, but budgetting studies confirm that agriculture is the main source of nitrate in river water, except in the most urbanized river basins.
In mainland Britain mapped nitrate concentrations demonstrate a marked northwest to southeast gradient, reflecting relief, climatic conditions, and agricultural activity. Upland areas in the north and west are usually characterized by nitrate concentrations below 1 mg NO3-Nl"1. This reflects the high rainfall and low temperatures of such areas: upland soils tend to conserve organic matter and mineralization rates are low. In contrast, a decreasing ratio of runoff to rainfall and an increasing intensity of agricultural land use toward the south and east of Britain results in higher mean concentrations of nitrate in river water. Many of the lowland rivers are characterized by concentrations above 5 mg NO3-N l"1; in East Anglia and parts of the Thames basin, mean nitrate concentrations in rivers are close to the European Union limit of 11.3 mg NO3-Nl_1, a level exceeded in some spring waters especially in the Jurassic limestone of the Cotswold's and Lincolnshire Wolds.
The changing pattern of British lowland agriculture since 1945 is reflected in long-term records of nitrate for surface and groundwaters (Figure 6). Such graphs confirm the accelerated nitrogen cycling in recent decades and increasing fluxes from the terrestrial to aquatic compartments of the N cycle.
For both large and small rivers, there has been a relatively steady upward trend in nitrate concentrations, often of the order of 0.1-0.2 mg NO3-Nl_1 a"1. Analyses for relatively short time series of just a few years have shown that the upward trend may be interrupted, either because of climatic variability (drier years are associated with lower nitrate concentrations) or because of land-use change. Nevertheless, statistical analysis of long time series shows that the main effect is a steady increase in nitrate levels over time which is independent of climate. If trends continue, the mean nitrate concentration of many rivers in Europe will soon be above the EU limit; in many cases this level is already exceeded during the winter when nitrate concentrations reach their maximum. In catchments where groundwater is the dominant discharge source, this long-term trend may be prolonged since it may take years for nitrate to percolate down to the saturated zone. In such basins, nitrate pollution may remain a problem for decades to come. In recent years, a number of options have been considered as a means of halting the upward trend.
Trends in water management in Europe include moves toward catchment-level management, improved intersec-toral coordination and cooperation, and frameworks facilitating stakeholder participation. This approach is developed by the European Union in its Water Framework Directive (2000/60/EC), which sets targets for good ecological status for all types of surface water bodies and good quantitative status for groundwater. More localized schemes, like the UK Nitrate Vulnerable Zones, involve greater restrictions on farming practice, such as restricting the amount and timing of organic and inorganic fertilizer application. The EU Common Agricultural Policy is to change the way payments are made to farmers. Single-farm payments will encourage farming in a more environmentally friendly way. Financial payments may be available to farmers for loss of income or for changing farming practice such as improving slurry storage and fencing off watercourses to restrict livestock access. Much interest currently focuses on the use of riparian land as nitrate buffer zones.
The terrestrial-aquatic ecotone occupies the boundary zone between the hillslope and the river channel, usually coinciding with the floodplain. Given their position, near-stream ecotones can potentially function as natural sinks for sediment and nutrients emanating from farmland. Observed denitrification rates in floodplain sediments
The type of vegetation found on the floodplain controlling the efficiency of nitrate absorption is the subject of much debate. Several studies have argued the presence of trees is crucial, yet others state the role of surface vegetation is secondary to presence of saturated conditions together with a carbon-rich sediment. Denitrifying bacteria operate best at the junction anaerobic/aerobic zones where both carbon and nitrate are abundant. It is clear that nitrate losses may be reduced by creating a nutrient-retention zone between the farmland and the river. Given that many floodplains around the world are part of an intensive agricultural system, creating permanently vegetated buffer strips between field and water courses is an idea that should be actively promoted. However, buffer strips will only be successful nutrient sinks if they are managed in an appropriate way. Underlying artificial drainage should be broken or blocked up to prevent a direct route to the watercourse for solutes and grassland strips need maintenance to prevent them becoming choked with sediment and losing their sediment retention potential.
Solving the problem of nutrient enrichment of surface waters cannot be seen in the short-term. Long-term land-use change is needed. Taking farm land immediately adjacent to water courses out of production is one option that could go some way to allow modern agriculture and water supply to coexist in the same basin. Such proposals inevitably raise questions about who pays for them - farmers, water supply companies, or the taxpayers.
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