Reduction of Nitrate Content in Subsoil Lucerne Fields S1
Kwalis GmbH, Fuldaer Str. 21, D-36160 Dipperz
|Over-fertilization with nitrogen and a heavy
overall surplus of nitrogen in agriculture have been named as the main factor influencing
the nitrate pollution of ground water. The cultivation of legumes has been discussed as
being a secondary cause. On the other hand, however, the question of the influence, that
deep-rooted plants such as lucerne have on nitrate reduction and the build-up of
denitrification potential, as a result of root exudates and residue, has not yet been
considered. In dry areas, in which the ground water renewal rate is below 50 mm per annum,
an NO3-N eluviation of 30 kg per hectare is sufficient to give a nitrate concentration of
over 240 mg per litre. It is certain that this quantity was exceeded with the fertile
Loess soils from the autumn mineralization back in the 1930s; all the same, however, the
nitrate levels in the drinking water wells remained relatively low. Following an
observation made by W. Maier - a farmer - to the effect that his lucerne grows better in
areas where the subsoil has been provided with more nutrients as a result of intensive
fertilization in previous years, W. Meier and E. Scheller have put forward the following
hypotheses:1) perennial lucerne develops its main root system in deeper subsoil. At the
same time as the lucerne takes up water, it also absorbs nitrate from that area, and in
doing so, dilutes the nitrate content; 2)denitrification potential is increased by root
exudates and by the root system as it dies.
In 1993, with the assistance of Dr. Moritz (University of Halle) and his work group, and again in 1994 with the help of a well-drilling company, soil samples were taken up to 10 metres depth in different old lucerne fields of Mr. Maier. Extraction of the soil samples was carried out at once in a field laboratory, set up on the farm. In the area influenced by the lucerne's roots, the NO3-N content was reduced by as much as 90%. In the first year of utilisation, this depletion zone extends to a depth of 1.5 to 2 metres, in the third year of utilisation 3-4 metres, and in the 5th or 6th year of utilisation, it increases to over 6 metres. The low nitrate-N content arises as a result of the uptake of nitrate by the lucerne on the one hand, and due to the development of drainage water with a low nitrate content throughout the cultivation of the lucerne on the other.
The tests have provided the first evidence to back up the hypothesis, that the increase in the nitrate content of spring water in areas with a low rate of ground water renewal, coupled with over-fertilization, has been influenced, at least in part, by the decline in the cultivation of lucerne since the mid-1960s.