1. Boller BC, Nösberger J (1987) Symbiotically fixed nitrogen from field-grown white and red clover mixed with ryegrasses at low levels of 15N-fertilization. Plant Soil 104:219–226
2. Carlsson G (2005) Input of Nitrogen from N2 Fixation to Northern Grasslands. Doctoral Thesis No. 2005:76. Swedish University of Agricultural Sciences. Acta Universitatis Agriculturae Sueciae, Umeå. pp 50
3. Carlsson G, Huss-Danell K (2003) Nitrogen fixation in perennial forage legumes in the field. Plant Soil 253:353–372
4. Carlsson G, Palmborg C, Huss-Danell K (2006) Discrimination against 15N in three N2-fixing Trifolium species as influenced by Rhizobium strain and plant age. Acta Agric Scand B Soil Plant Sci 56:31–38
5. Ericson L (2005) Norrländsk växtodling. Swedish University of Agricultural Sciences, Umeå. http://www.njv.slu.se/nvo/NVO2005.pdf Accessed 2 October 2006. (In Swedish)