| Summary: | In a multisite field experiment conducted over three years, the amount of total nitrogen yield (Ntot) and yield from symbiotic N2 fixation (Nsym) was quantified from grass-legume stands with greatly varying legume proportions. The five experimental sites spanned a climatic gradient from Atlantic to continental, and from temperate to arctic. Ntot of mixtures was, on average, greater than that of monocultures at all sites, the effect being most pronounced when legume proportions in mixtures were around 60%. Ntot was 256 kg ha-1 yr-1 when calculated across all years and sites from mixtures with 50% legume proportion, while the corresponding Nsym was 77 kg ha-1 yr-1. Thus, 30% of Ntot originated from symbiotic N2 fixation. Average Nsym in mixtures was positively related to mean site productivity (P=0.096), while percent Nsym (the ratio of Nsym to Ntot) was not (P=0.909). This indicates that the relative contribution of N2 fixation to Ntot remained, on average, unchanged across a large productivity gradient. We conclude that the use of grass-legume mixtures can contribute to resource efficient, yet productive, agricultural grassland systems under a wide range of environmental conditions.
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