Archived at http://orgprints.org/7906 

Biodiversity as a tool in nutrient supply
Kristian Thorup-Kristensen
DIAS, Department of Horticulture

A high biodiversity is an environmental goal, but it is also important for the efficiency and stability of the agricultural systems themselves. For environmental as well as for agricultural purposes, biodiversity is therefore seen as an important goal in organic farming.
Biodiversity is often regarded broadly as an unspecific but desirable quality, but for many purposes, it is possible to work with biodiversity in a more systematic way. The plants which we grow as main crops, green manure crops or cover crops have different life strategies and characteristics. The plants have developed these differences through their adaptation in nature, and we can use them to develop better farming systems.
In our work on root growth of cereals and vegetable crops, we have found a large diversity in root characteristics. The rooting depth among vegetables ranges from 25 cm for onion to more than 250 cm for long season cabbage crops. 
Vegetable rotations including e.g. onion, lettuce, potato and seleriac may be diverse, but all of these crops are relatively shallow rooted. In our experiments we have shown that we can improve the use of nitrogen by introducing deep rooted crops, which can take nitrogen from the subsoil layers unavailable for the shallow rooted species. In this case it is not the diversity as such, but diversity in one specific characteristic of the plants which can help to improve the system.
Much of our work has focused on green manures and cover crops. When growing green manures we have a much larger diversity to choose from, as we are not limited to species which can be sold profitably. Among these many species there are lots of differences to work with, and our studies have shown that new species can be introduced with important “new” characteristics. 
As an example, crucifer crops such as white mustard and fodder radish have deep root growth, and can be very efficient cover crops for preventing nitrogen leaching losses. However, they are not winter hardy and they are not suited for establishment by undersowing. Other types of crucifer crops are winter hardy, and we have even identified a crucifer crop (dyers voad) which is well suited for undersowing in cereals. In vegetable rotations with crucifers as main crops, crucifer cover crops may be undesirable due to disease problems, and we may need deep rooted species from other plant families for such rotations. While the experimental results show that inclusion of deep rooted cover crops may be a very powerful tool to improve crop nutrition in organic rotations, the example shows that we need a number of different species to allow us to use this efficiently.
There are many other differences among plant species which we can use to improve cropping systems. Regarding crop nutrition directly, significant differences in factors such as C/N ratio of plant material, sulphur uptake, phosphorus utilization from strongly bound phosphorus fractions in the soil, or plants whith or without mycorrhiza on their roots. To be able to use these differences in practical farming, the plants must fit into the cropping system as main crops or green manure crops. To fit into the system, their general growth habit, whether they are pioneer plants, or are adapted to later stages of succession, and whether they are annual, biannual or perennial are important characters. E.g. cover crops grown only during a short period in the autumn must show fast establishment and growth, whereas cover crops for intercropping or undersowing must not compete too strongly with the main crop, but still be able to develop efficiently after main crop harvest.