A tractor spreading fertilizer on a field Click to enlarge
A tractor spreading fertilizer on a field
Source: Emjay Smith / Fotolia

In as much as nitrogen is an indispensable nutrient for all living things, the use of mineral and organic nitrogen fertilizers to increase crop yields is a common practice. It is essential that such fertilizer be used judiciously and on a needs oriented basis. Nitrogen surpluses can damage terrestrial and aquatic ecosystems and can have harmful climatic, air quality, and biodiversity effects.


Nitrogen is one of the building blocks of life, and its compounds occur in our soil, water and air. But anthropogenic overload and disturbance of the natural nitrogen cycle and of vulnerable systems can provoke health and environmental hazards. More than 50 per cent of Germany’s reactive nitrogen compounds are released into the environment owing to farming activities. Other nitrogen inputs are attributable, in roughly equal amounts, to industrial activities, transportation and private households. Nitrogen is used as an agricultural fertilizer in order to achieve high yields of quality crops, provide sufficient plant nutrition and maintain soil fertility. The still unduly high nitrogen surpluses from Germany’s farming sector result from more fertilizer being used than the crops actually need. Germany’s national sustainability strategy target of limiting nitrogen surpluses to a three-year average of 80 kilograms of nitrogen (N) per hectare (ha) has yet to be reached. In 2010 Germany’s aggregate nitrogen surplus was still 96 kg/N/ha, with in some cases considerably higher figures in northwestern Germany, where livestock farming is particularly intensive. Agricultural sector nitrogen surpluses are attributable to the following: crop cultivation (62 per cent); livestock production (33 per cent); nitrogen compound air emissions attributable to transportation, industrial activities, and households (5 per cent).

Environmental impact

Nitrogen surpluses and nitrogen inputs into fragile ecosystems can engender environmental damage.
Nitrogen fertilizer that is not absorbed by crops can, after being converted into nitrates, end up in neighbouring waterbodies or in the air, where it is hazardous to groundwater and drinking water and contributes to the eutrophication of surface waterbodies and terrestrial ecosystems. Nitrate air emissions provoke the eutrophication and acidification of fragile ecosystems and contribute to greenhouse gas formation. This in turn has a negative impact on landscape quality and biodiversity.

Soil effects

Improper and excessive use of fertilizer, particularly acid fertilizer such as ammonium sulfate fertilizer, can speed up soil acidification processes. As the nitrate engendered by nitrification is highly mobile, if it leaches into groundwater instead of being uptaken by crop plants, it expedites the soil acidification process. This also results in changes in soil structure and in soil micro-organism habitat conditions. This in turn can reduce soil fertility and crop yield and quality. Inorganic fertilizers, and particularly phosphate fertilizers but also organic-residue fertilizers such as sewage sludge, also contain heavy metals (mainly uranium and cadmium).

Impact on groundwater

Nitrogen can occur in the soil in various forms. As ammonium (NH4+) it is incorporated into soil particles and is ultimately converted by microorganisms into nitrate (NO3-), a substance that is extremely mobile in the soil and that, particularly after the fall harvest and/or heavy rain, can be transferred to groundwater along with percolation water. In groundwater, and subsequently in drinking water, under certain circumstances nitrate can be converted into nitrite, a harmful substance. Hence in 1991 the EU set the limit value for nitrate in drinking water at 50 mg/l. Groundwater quality testing in 2009 under the Water Framework Directive revealed that 26.5 per cent of Germany’s groundwater exhibits a poor chemical status solely by virtue of its high nitrate load, which is largely attributable to nitrogen surpluses from fertilizers, animal feed concentrates containing protein, and organic substance mineralization. In addition, soon after grassland is converted to arable land, the mineralization of organic soil substances engenders substantial nitrate emissions. In some regions of Germany, and particularly in cattle farming-intensive northwest Germany with its large quantities of livestock manure, this 50 mg/l limit value for nitrate in drinking water is greatly exceeded in some cases. This phenomenon is often attributable to growers spreading livestock manure at periods and in quantities that are incompatible with crop plant needs, and is exacerbated by the growth, in recent years, of the number of biogas facilities under the Renewable Energy Act (EEG). These facilities mainly use corn as a fermentation substrate for electricity generation, along with minor amounts of livestock manure. The biomass resulting from this process is rich in nutrients and is often used as fertilizer along with livestock manure. This overfertilization can lead to groundwater nitrate pollution, as the statutory limit value for nitrogen under the fertilizer regulation (170 kg/N/ha) solely applies to the nitrogen components of livestock manure.

Impact on surface waterbodies

Large amounts of nitrogen compounds from farming are input into surface waterbodies with groundwater and as a result of farmland run-off. In rivers, lakes and oceans this results in nutrient surpluses that in turn lead to waterbody eutrophication. Waterbody nitrogen surpluses increase primary production of plants such as algae, resulting in severe waterbody oxygen deprivation and to life threatening conditions for both plants and animals. However, the impact of excessive nitrogen surpluses on surface waterbodies is also determined by other growth limiting nutrients such as phosphorous, in that the ratio between nitrogen and phosphorous is a key factor when it comes to plant growth conditions. The natural 16:1 biological ratio of nitrogen and phosphorous in waterbodies has shifted toward nitrogen owing to elevated nitrogen inputs.

Whereas nitrogen is the growth limiting factor in oceans and thus determines the impact of nutrients there, in most rivers, lakes and coastal waters phosphorous (most of which is attributable to farming) is responsible for excessive plant growth. Oxygen deprivation and the displacement of native flora and fauna that adapt poorly to habitat changes results in waterbody biodiversity loss.

Impact on biodiversity

In quasi-natural terrestrial forests and other ecosystems, eutrophication can have a long term negative impact on vegetation and species composition. Plants and animals that have adapted to low nutrient habitats can be driven out by species that propagate more readily because they thrive on nitrogen. This can lead to uniformity in the ecological structure of vegetation, and to biodiversity loss.
Nitrogen overfertilization of crop plants and trees also provokes excessive lengthwise growth and to soft and spongy shoots, cells and tissues that are more vulnerable to frost and heat. This makes harvested crops less suitable for storage and promotes the propagation of pests, as well as bacterial and fungal diseases, thus reducing farm crop earnings and making forests more susceptible to storm damage.

Climate and air quality impact

In livestock manure, ammonium content relative to total nitrogen amounts to 15 per cent for barn manure, 50 per cent for slurry and 95 per cent for liquid manure. When livestock manure is stored and spread, the ammonium in it is converted to ammonia that can escape into the atmosphere. Among the mineral fertilizers, urea fertilizer contains a particularly high amount of ammonia (15 per cent relative to total nitrogen content).
The scope of ammonia loss hinges on a host of soil and weather conditions. High pH values, low buffer capacity, low soil moisture, high temperatures and windy conditions promote ammonia gas emissions. In the interest of reducing or avoiding such emissions, the fertilizer regulation stipulates a series of requirements concerning compliant livestock manure spreading and storage. For example, livestock manure must be worked into the soil immediately after being spread and may not be spread on snow-covered or frozen fields. The use of low-drift spray systems such as drag hoses also reduces ammonia loss.
The mineralization of fertilizer after it is spread on fields results in the formation of nitrous oxide, which is a highly potent greenhouse gas.

Large livestock facilities emit the nitrogen compound ammonia. The use of livestock manure and mineral fertilizers can also give rise to avoidable ammonia emissions. Ammonia is the precursor substance to secondary particulate matter, which is a health hazard. Ammonia also has a detrimental effect on nearby ecosystems. For example, moss and lichen species composition can be altered by even minute atmospheric concentrations of ammonia, whose accumulation in fragile ecosystems can contribute to biodiversity loss.

Nitrogen input reduction measures

In the interest of improving ecological statuses and reducing inputs of reactive nitrogen compounds, directives and strategies containing specific environmental quality objectives and measures have been enshrined in both German and EU environmental policy.

The EU Nitrate Directive was enacted in 1991 with the goal of protecting groundwater and surface waterbodies against nitrate pollution. The directive (a) calls for monitoring of groundwater and surface waterbodies, the designation of vulnerable zones, and the establishment of a code of good agricultural practice; (b) stipulates measures that the member states are required to implement via strategies and action programs; (c) requires the member states to assess the success of their nitrogen reduction measures by assessing the statuses of their groundwater and surface waters every four years and to submit a nitrate report to the European Commission. The Nitrate Directive was transposed into German law by the Düngeverordnung regulation. Germany and a number of other EU member states apply the Nitrate Directive’s measures to their entire territory, thus obviating the need to designate vulnerable zones. Hence the code of good agricultural practice under the Düngeverordnung regulation applies to Germany as a whole and must be adhered to by all growers.

The most recent (2012) nitrate report shows that, while the measures implemented by growers have worked to some extent, nitrogen related environmental objectives are being failed. Hence the nitrogen surplus limit of 80 kg/N/ha and the stagnating reduction in agricultural nitrous oxide emissions show that further efforts and integrated abatement measures are needed in order to adequately protect waterbodies.
In 2009 the UBA elaborated an Integrierte Strategie zur Minderung von Stickstoffemissionen (Integrated nitrogen emissions reduction strategy) containing recommendations for the relevant sectors. The greatest need for action – but also the greatest potential for minimization – lies in the agricultural sector, where the greatest synergy could be achieved as well if water and climate protection and air quality objectives were met.