LAUSR

AimsEvaluate biological nitrogen fixation (BNF) and its contribution to total N yield in different grass-clover mixtures under a boreal coastal climate and assess how winter conditions affect the survival of clover and its capability to fix nitrogen (N).MethodsGrass-clover mixtures sown with 0, 15 and 30% red and white clover were subjected to two N rates and three levels of tractor trafficking over four years. BNF was estimated by N-difference for each of the two harvests per year, whereas the first harvest was used to estimate the fraction of N derived from the atmosphere (NdfA) and its transfer to grasses by 15N natural abundance.ResultsBiological N fixation, mainly by red clover, contributed substantially to total N yield, which was up to 75% greater in grass-clover mixtures than in pure grass stands in the second production year. However, the number of red clover plants and associated BNF decreased dramatically in the third and fourth production year, with N fertilization rate having a more detrimental effect than soil trafficking. Conversely, the amount of biologically fixed N transferred to grass, increased with time, evidencing that some of the biologically fixed N is retained in the system. In the first harvests, NdfA values were generally between 80 and 100%, irrespective of preceding winter conditions.ConclusionsBNF in high latitude grass-clover mixtures can be substantial but is limited by the poor survival of red clover. Variable winter conditions had no measurable effect on winter survival nor on the sward’s capacity to fix nitrogen in spring.