DINITROGEN FIXATION, NITROGEN TRANSFER, AND PRODUCTIVITY OF FORAGE LEGUME-GRASS COMMUNITIES

The extent of N2 fixation in legume-grass mixtures grown in the northcentral USA and the importance of legume to grass N transfer are not well understood. The objective of this 4-yr field experiment was to determine seasonal N2 fixation and N transfer by stable isotope methods in swards of alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), red clover (Trifolium pratense L.), and ladino white clover (T. repens L.) with reed canarygrass (Phalaris arundinacea L.). Alfalfa in mixture with grass fixed the most N2 per season (82 to 254 kg N ha-1) and ladino white clover the least (1 to 20 kg N ha-1). For all mixtures except birdsfoot trefoilgrass, N2 fixation was greatest during Year 2 of the stands. The proportion of legume N from symbiotic fixation increased progressively from Year 1 through 4 and averaged > 93% for all legumes in Year 4. Legume to grass N transfer was greatest in Year 2 of the stands and averaged almost-equal-to 36% of grass N across all mixtures. On a land area basis, N transfer in Year 2 ranged from 29 to 53 kg N ha-1 and was greatest for the ladino white clover-grass mixture. The decline in N transfer in Years 3 and 4 was associated with a loss in sward legume content, reduced whole-plant dry matter accumulation in legume and grass, and large losses in root-plus-crown dry matter accumulation by grass in Year 4. The results indicate that forage legumes in mixture with grass are virtually self-sufficient for N and can concurrently transfer appreciable N to the companion grass.