Nitrogen fixation in perennial forage legumes in the field

Nitrogen acquisition is one of the most important factors for plant production, and N contribution from biological N-2 fixation can reduce the need for industrial N fertilizers. Perennial forages are widespread in temperate and boreal areas, where much of the agriculture is based on livestock production. Due to the symbiosis with N-2-fixing rhizobia, perennial forage legumes have great potential to increase sustainability in such grassland farming systems. The present work is a summary of a large number of studies investigating N-2 fixation in three perennial forage legumes primarily relating to ungrazed northern temperate/boreal areas. Reported rates of N-2 fixation in above-ground plant tissues were in the range of up to 373 kg N ha(-1) year(-1) in red clover (Trifolium pratense L.), 545 kg N ha(-1) year(-1) in white clover (T. repens L.) and 350 kg N ha(-1) year(-1) in alfalfa (Medicago sativa L.). When grown in mixtures with grasses, these species took a large fraction of their nitrogen from N-2 fixation (average around 80%), regardless of management, dry matter yield and location. There was a large variation in N-2 fixation data and part of this variation was ascribed to differences in plant production between years. Studies with experiments at more than one site showed that also geographic location was an important source of variation. On the other hand, when all data were plotted against latitude, there was no simple correlation. Climatic conditions seem therefore to give as high N-2 fixation per ha and year in northern areas (around 60degreesN) as in areas with a milder climate (around 40degreesN). Analyzing whole plants or just above-ground plant parts influenced the estimate of N-2 fixation, and most reported values were underestimated since roots were not included. Despite large differences in environmental conditions, such as N fertilization and geographic location, N-2 fixation (Nfix; kg N per ha and year) was significantly (P < 0.001) correlated to legume dry matter yield (DM; kg per ha and year). Very rough, but nevertheless valuable estimations of Nfix in legume/grass mixtures (roots not considered) are given by Nfix = 0.026 center dot DM + 7 for T. pratense, Nfix = 0.031 center dot DM + 24 for T. repens, and Nfix = 0.021 center dot DM + 17 for M. sativa.