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Nitrogen in agriculture
Maria PAU VALL, Claude VIDAL (Eurostat)
Nutrients such as nitrogen and phosphorous are removed from soils by plant growth and need to be replaced. Nitrogen from mineral fertilizer is the major source of N input in EU countries although inputs from animal manure remain important, especially in regions of high livestock density. Excessive nitrogen surpluses, the difference between inputs and removals by crops, can pose a threat to the environment, leading to pollution of water, air and soil. The integration of environmental concerns into agricultural policies and practices aims to reduce current and potential pollution. The monitoring of nitrogen surpluses from agriculture can be a useful tool to highlight those areas vulnerable to nutrient pollution.
Nitrogen is essential for crop growth
In 1995 around 43% (138 million hectares) of the total area of the European Union (EU-15) was used for agriculture. Between 1985 and 1995, total utilised agricultural area (EU-15) declined by 2.7%, whereas the volume of agricultural output increased by 5%. This improved productivity is the result of a number of factors, among them the nutrient inputs needed to reach the optimum crop yield, and a cheap supply of livestock feedstuffs, about 30% of which are imported from third countries.
Fertile soils are rich in nutrients, essential components which play a key role in plant metabolism and growth. Crops take the nutrients they need from the soil, with the result that soils can become impoverished if there is no process to replace these nutrients. Impoverished soils reduce crop yields, and ultimately the economic viability of the farm itself. Traditionally, rotation of crops and regular fallow periods, together with spreading of animal manure, allowed the land to recover some of its fertility, but today the main method used to restore nutrients to soil and to increase crop yields is the application of mineral fertilizers. Nitrogen in commercial fertilizer is particularly soluble, to facilitate uptake by crops, and ease of storage and handling means that it can be easily applied at the times when plants most need it. Mineral fertilizer is now the major source of nutrients applied to soil in EU countries, although input of animal manure remains important, especially in regions of high livestock density. However excessive application of nutrients can pose a threat to the environment and in extreme cases to the fertility of the soil itself.
Losses to the environment can be minimised if a reasoned fertilization is used, together with sustainable agricultural practices, such as crop rotation, planting cover crops, and ploughing in crop residues. Reasoned fertilization means applying fertilizers, whether mineral or organic, in the correct weather conditions, (to avoid run off) at the appropriate stage in crop growth (so that plants take up the nitrogen quickly) and at the correct doses.
As stated in AGENDA 21 - THE FIRST 5 YEARS', the 1992 reform of the Common Agriculture Policy goes some way towards reversing the intensification trend, but much more needs to be done to integrate environmental concerns into agricultural policies and practices.
Excess nitrogen a risk to the environment
Nutrients which are not taken up by plants may be metabolised by micro-organisms in the soil to improve soil fertility. This is a slow process however, and the major risk is that nutrients, particularly nitrate which is very soluble, will run off into surface water or percolate into groundwater.
Moreover, some nitrogen, particularly from animal manure, is lost to the air through volatilisation of ammonia (a contributor to acidification, see chapter 'Acidification and Agriculture') or as N2O (a powerful greenhouse gas, see chapter 'Agriculture and Climate Change'). Phosphorous, primarily in the form of phosphate, is not as soluble as nitrate and is primarily transported by sediment in runoff, often ending up in rivers and streams.
Excessive concentrations of nitrogen and phosphorus in water bodies can result in eutrophication of slow flowing rivers, lakes, reservoirs and coastal areas. This manifests itself through a proliferation of blue-green algae, reduced light penetration, depletion of oxygen in surface water, disappearance of benthic invertebrates, and production of toxins which are poisonous to fish, cattle and humans.
Soils are also at risk of eutrophication in cases where excessive nutrients deplete oxygen in the soil. The result is that the natural micro-organisms cannot functioning properly and soil fertility is affected. Eutrophied soils are also sources of N2O.
Ground water is mainly at risk from nitrates, which at excessive concentrations are considered a health risk. Highly contaminated ground water cannot be used as drinking water. This results in extra costs for the water industry to remove nitrates from ground water sources of drinking water. The Nitrates Directive (91/676/EEC) aims to protect groundwater from excessive contamination by nitrates from agriculture, and in particular, from manure (Box 1).
Where does the nitrogen come from?
For the EU as a whole, the main source of nitrogen input to agricultural land is mineral fertilizer, with livestock manure a close second. However the situation varies considerably from one country to another. For example, in 1995, mineral fertilizers accounted for 50% or more of total nitrogen input in Denmark, Germany, Greece, France, Luxembourg, Finland and Sweden. On the other hand in Belgium and the Netherlands livestock manure was responsible for more than 50% of nitrogen inputs (Figure 1).
There are several other minor sources of nitrogen input to soils, such as deposition of nitrogen and ammonia from the atmosphere, nitrogen fixed biologically by crops such as leguminous plants, and sewage waste which is disposed of by spreading on land. Although the importance of these varies from country to country, on average they account for a small percentage of nitrogen inputs compared with mineral fertilizers and livestock manure.
Sewage sludge as a potential source of nitrogen has not been taken into account here because of lack of data for most European countries. Sewage sludge is collected at municipal wastewater treatment plants and might be used for agriculture. The use of sewage sludge in agriculture is regulated by Council Directive 86/278/EEC in such a way as to prevent harmful effects on soil, vegetation, animals and man, thereby encouraging the correct disposal of such sludge.
Mineral fertilizers: the major source of nitrogen input in agriculture
Within the EU mineral (commercial) fertilizers are applied to agricultural soils mainly as straight nitrogen fertilizers in the form of ammonium nitrate. Nitrogen in commercial fertilizer is particularly soluble to facilitate uptake by crops, but this also makes it vulnerable to run-off after heavy rainfall, and to leaching to groundwater.
Since the early 90s, total consumption of nitrogen fertilizers has exceeded the total consumption of potash and phosphate fertilizers. Use of nitrogen fertilizers soared in the 1970s and early 1980s (Figure 2), a period when agricultural output in Europe expanded, with a major increase in cereal production following the introduction of high yield crops, an increase in production of oil crops (e.g. sunflower and colza) and an intensification of fodder crops, at the expense of permanent pastures.
A closer look at the data shows that although nitrogen fertilizer use decreased between 1985 and 1995 in the EU-15, both in terms of volume and in application rate, the major decrease in application rate occurred between 1989 and 1992. Since then, the rate, expressed as kg per hectare of agricultural land, has increased slightly, though never returning to the level of 1991.
This slight increase can be explained by a number of factors. Although total agricultural area decreased between 1992 to 1995, the area of certain crops with high application rates, such as wheat and oilseed rape, had increased during the same period. Increasing rates of fertilizer application are reflected in higher crop yields, although other factors, such as improvements in plant varieties and the use of plant protection products, influence crop yields and, together with product prices, condition farmers decisions on fertilizer application rates.
Phosphate use has declined steadily since the early '80s, and in 1995 was 38% down on 1980 figures. This reflects a growing trend within the EU towards soil analysis, which assesses the soil's need for additional phosphates, which are less mobile than nitrogen.
Within a country, fertilizer use varies from one region to another for many reasons, not least because cropping patterns differ between regions. However, only a few countries carry out regular surveys of fertilizer use per crop and region (Box 2), and it is currently only possible to present a regional breakdown of fertilizer inputs to agricultural land.
Future trends in use of mineral fertilizers will depend on a number of factors, in particular on future agricultural and environmental policies of the EU. Industry forecasts up to 2006/7 (EFMA 1994) show that fertilizer application rates are expected to fall over that period.
Some of the actions likely to influence nutrient use in EU countries are:
Livestock manure: the second source of nitrogen
Livestock manure is the second most important source of nutrient inputs to agricultural land. The nutrient content of manure varies from country to country and from one region to another within a country. It depends on the type of livestock, the grazing systems, and the nutrient content of the different fodder and feedstuffs used for livestock.
The nitrogen input from manure is calculated by multiplying the numbers of the different livestock types by a manure coefficient specific to that type and country. The same coefficients are used for all years. (Table 1)
In most countries a reduction in total numbers of livestock since 1990 has resulted in a slight reduction in the amount of manure to be disposed of (Figure 3).
Not all the nitrogen contained in excreted manure is spread on the land. A certain amount is lost through volatilisation of ammonia from stables and during storage. This can vary considerably, depending on the type of manure and type of storage facilities. In these calculations volatilisation of ammonia from stables and during storage, estimated at between 15-20%, has been deducted, based on information provided by the countries themselves. Ammonia is a contributor to acidification, see chapter 'Agriculture and Acidification', and for this reason, some countries have introduced measures to reduce volatilisation. These measures include injecting the slurry into the soil, instead of spreading it on top of the soil. The result is less volatilisation, but a higher level of nitrogen in the soil and a greater risk of leaching.
In 1995 the Netherlands and Belgium, both of which have high livestock density, had the largest input of nitrogen from livestock manure per ha, followed by Denmark, Ireland and Luxembourg (Figure 4).
However, if low national figures are interpreted to mean that no manure problem exists, then this is misleading, especially for large countries, such as France, Spain, Italy and the UK, where the pattern of agriculture can vary widely from one region to another (Map 1).
Comparing figure 3 with map 1, it can be seen that low national figures for input of manure nitrogen per ha can mask the fact that there are some regions with high manure loading, again because of high livestock densities in those regions. For example, levels of nitrogen inputs from manure in Galicia are four times higher than the Spanish national average, Brittany three times higher than the French average, Lombardia close to three times higher than the Italian average, and North West England twice the UK national average. All four regions are well above 100kg/ha.
Removals of nitrogen from agricultural soils
Nitrogen is spread on agricultural land because it is an important factor in plant growth and is absorbed by crops during the growing season. The quantity of nitrogen absorbed by arable crops is calculated as a function of the area grown for each crop, the yield and the nitrogen content (coefficient) of each of the harvested crops. Only nitrogen contained in the grain or harvested part of the crop is calculated, as the straw is generally left in place, ploughed in or used as a substrate for effluent and therefore spread on the land at a later date, together with manure.
A particular problem arises for grass, as it is difficult to know the amount of grass harvested and/or grazed. Statistics on harvested grass are only available for a few countries. To overcome this, some estimations are needed, based on calculations of the animals theoretical fodder needs. This assumption will tend to over-estimate the removal via grass.
Using these estimates, grass is seen to be the most important crop in all EU countries, in terms of removing nitrogen from the soil. In twelve of the fifteen countries it accounts for more than 50% of the nitrogen removed. Wheat is the second most important for most of the countries (24% of total removals in Denmark, 18% in France and Germany, 13% in Italy and 16% United Kingdom). Barley accounts for 19% of total in Denmark and Finland, 12% in Sweden and 11% in Germany. (Figure 5)
Agricultural nitrogen balance: an indicator to highlight areas at risk from nitrogen pollution
With the information on nitrogen inputs to agricultural soils and estimates of removals of nitrogen by crops and grass, an estimate of the amount of excess nitrogen applied to agricultural soils can be made. This method of calculating the excess nitrogen is known as a soil surface nitrogen balance (Scheme 1). The soil surface balance can be used as an indicator to highlight areas potentially at risk from nitrogen pollution. Moreover, tracking the changes in these surpluses over a number of years can be used to assess the effectiveness of the agri-environmental measures or the measures of the Nitrates Directive.
However the calculation of the surplus cannot be immediately interpreted as an indicator of loss of nitrogen to water. The balance between inputs and outputs for a system contains all potential losses, plus any change in the store of nitrogen, principally within the soil. The potential loss pathways for nitrogen are:
Moreover, the relationship between nutrient balance and losses to water differs between agricultural systems, and is affected by intensity of land use, farm management practices, soil type, and climate conditions.
A first attempt has been made to construct a nitrogen soil surface balance for EU countries at national level (Figure 6). Some simple assumptions have been used for certain positions in the balance where data is not available. These assumptions are intended to approximate the real situation as closely as possible. However, given that the objective is to identify structural excesses, a general rule has been applied: in the case of major uncertainty concerning certain positions, the solution which minimises the surplus is adopted. Thus surpluses which appear as result of the calculations should be an underestimate, so should carry more weight (Box 5).
A nitrogen balance at regional level would provide better information about nitrogen surpluses and the regions where the surpluses are located. However, the current state of data availability does not yet allow a convincing regional balance to be calculated.
The farm gate balance
Another method of calculating a nitrogen surplus, known as the farm-gate balance, has been developed in some countries. The farm gate balance involves tracking the amounts of nitrogen entering a farm in fertilizer, feedstuffs, etc. and the amount of nitrogen leaving the farm in products, e.g. crops, milk, meat, eggs, etc. The remainder is assumed to have remained on the land. This method is relatively simple to apply at farm level, where the farmer has a good knowledge of what comes into and leaves his farm.
Calculating a 'farm gate' type balance at national level is more complicated. It involves recording the amounts of nitrogen entering and leaving the country, if possible distinguishing the imports and exports for the agriculture sector. Scheme 2 provides a schematic overview of the flows involved.
One of the biggest drawbacks of this method at national level is the large amount of data and coefficients needed to calculate the amount of nitrogen contained in imports and exports, especially of composite products. While foreign trade statistics provide information on tonnes of product imported and exported, converting this to tonnes of nitrogen has proved extremely difficult. A further complication is that it is often difficult to distinguish whether a product e.g. cereals, is intended for human or animal consumption.
Several countries, Denmark, Netherlands, Sweden and UK, have calculated national 'farm-gate' balances, though each has used different sources of data. A summarised balance for the Netherlands, which has developed a very detailed farm gate balance over the past ten years, is given in Box 6.
In general the advantage of national balances of this type is to give an indication of the sources of nitrogen which contribute to the nitrogen surplus.
The principles of the different types of nitrogen balances can also be applied to other nutrients, especially phosphorus. Phosphorus balances are calculated in several countries, notably Germany and the Netherlands. (Box 7)
Calculation of a phosphorus balance is simpler than for nitrogen, because phosphorus is not fixed biologically from the atmosphere, it is not lost in gaseous form to the atmosphere, and it is not very soluble in water. Therefore it tends to be stored in the soil, unless washed away in run-off to rivers and lakes.