Nitrogen fixation, recycling and leaching

The symbiotic fixation of nitrogen by legumes constitutes a key source of this element in organic farming systems. On organic animal farms, the perennial legumes are very important in the production of roughage feeds, and for the transfer of nitrogen to the soil / plant system. They build up fertiliser reserves for subse-quent crops and improve soil fertility. Consequently, there is a need for information on the significance of important plant growth factors for nitrogen-fixation, and for better methods of monitoring nitrogen - fixation under field conditions to determine its long-term effects on soil fertility.

The release of nutrients from green manures and crop residues (for example, after ploughing in clover-grass pastures) constitutes an important source of these elements for subsequent crops in organic systems. The breakdown of crop residues and / or animal manure is also important for maintaining the fertility of arable land in the long term. A more detailed knowledge of C and N dynamics in the soil after the degradation of plant material is necessary to predict the amount of and the timing of nutrient release.

Balance studies on organic and conventional farms suggest that nitrogen losses are smaller and nitrogen utilisation is better on organic units. It is, however, important to improve tillage methods for organic farms to optimise nitrogen-fixation, to better synchronise the release of nutrients with crop demands, to maintain or improve soil fertility, and ensure that nutrient losses to the surroundings are minimised.

Against this background the project has the following main objectives:

• to establish the effects of selected plant growth factors on the symbiotic fixation and utilisation of inorganic nitrogen in mixed crops of legumes and non-legume plants
  
  
• to determine the effect of alternative green manures and post-cropping strategies on nitrogen loss and nitrogen utilisation in production systems for commercial crops; taking special regard to (i) the ability of green manures and crop residues to retain nitrogen in the system, (ii) nitrogen immobilisation / mineralisation in the soil after breakdown of the green manures and crop residues, and (iii) the significance of degraded plant material to nitrogen supply in subsequent crops
  
  
• to determine and model the significance of pasture history (i.e. age, sward composition and nitrogen-level of applied animal manures) with respect to the effects of residual nitrogen on subsequent crops and the leaching of nitrogen.
  
  

Summary
Publications

Project title
I.2 Symbiotic fixation of nitrogen, recycling and leaching of nitrogen in organic production systems.

Project leader
Erik Steen Jensen, The Royal Veterinary and Agricultural University, Section of Agroecology, Institute of Agricultural Sciences, Agrovej 10, DK 2630 Taastrup.
Tel: +45 35283517, Fax: +45 35282175
E-mail: ESJ@KVL.dk.

Project's home page: http://www.agsci.kvl.dk/planteer/nicleos/index.htm


Project participants:
Finn P. Vinther, Jørgen Eriksen, Danish Institute for Agricultural Science,
Jakob Magid, Jan K. Schjørring, Lars Stoumann Jensen, Torsten Müller, Allan Kwabiah and Henning Høgh-Jensen, The Royal Veterinary and Agricultural University
Henrik Hauggaard-Nielsen, Per Ambus, Risø National Laboratory