Understanding leaching
But the nitrogen cycle is leaky by nature, and some nitrogen will always be lost. Losses, however, are both expensive for the farmer and harmful for the environment. Farmers therefore strive to reduce such leakage. European agriculture in general is highly efficient. What can we do to further reduce nitrogen losses and especially leaching? Which processes contribute to leaching and how can they be managed?
Nitrogen in the soil
The displacement of excess mineral nitrogen, mostly nitrate, out of the root system into the groundwater is called leaching. Where does the excess nitrogen come from?
Nitrogen in the soil occurs in two main forms:
- Soil organic matter which contains huge amounts of nitrogen as organic compounds. These are not immediately available to plants.
- Mineral nitrogen (ammonium, nitrate) is formed by mineralization of soil organic matter or directly added as mineral fertilizer. These are immediately available to the plants.
Most of the mineral fertilizer applied is taken-up by plants, but part of it is also bound by soil microorganisms and transformed into soil organic matter. Very little actually joins the mineral nitrogen pool in the soil.
Manure contains mainly organic nitrogen compounds, except for slurry, which also contains some mineral nitrogen.Manure therefore basically increases soil organic matter. It needs to be mineralized first before it can be taken-up by plants.
Leaching occurs when mobile nitrate from the mineral nitrogen pool is washed out of the root zone by heavy rainfall. This is more likely to happen on sandy soils than on heavy clay soils. In the latter, nitrate is denitrified before it can be leached.
Nitrogen and agriculture
The European Nitrogen Assessment [7] has revealed generally stabilizing or decreasing nitrogen surpluses from agriculture in most European countries. However, due to the slow response time of aquifers, high nitrate levels will not be absorbed quickly. Further efforts are required, especially in some areas where nitrate levels remain high. Nitrate in watercourses and groundwater is undesirable. Elevated concentrations of nitrate contribute to the eutrophication of surface and coastal waters.
High nitrogen surpluses are generally related to high livestock concentrations, whereas nitrogen surpluses in agricultural regions are comparatively low. In agricultural production, nitrate leaching basically depends on the amount of unused mineral nitrogen in the soil at risk of loss and the amount of drainage that carries that nitrogen away to waters. The more rainfall there is, the more nitrate can be leached, but the more it will be diluted as well.
Dilution by rainfall is an important aspect since the limit of 50 mg NO3/l set by the European Union is a concentration, not the amount leached. In wet regions with heavy rainfall, higher amounts of nitrate might be leached without breaching the limits. In dry regions with less rain, however, even small natural leaching losses might exceed authorized limits.
Seasonal aspects
Mineral nitrogen fertilizer applied in spring is mainly absorbed by plants during the growing season. Capillary rise, caused by high levels of soil evaporation during spring and summer and lower levels of precipitation, prevent rainfall from reaching the water table. After the growing season, however, soil microbes continue to metabolize organic matter, producing mineral nitrogen. This is no longer taken-up by plants and builds up in the soil as residual nitrate.
It can be leached in winter, when percolating rainfall recharges the water table. Leaching during the growing season is an exceptional situation. The risk of nitrate leaching increase with:
- High nitrate concentration in the soil after harvest
- Low water retention by the soil
- High rain fall
- Long fallow periods
