Abstract: Nitrogen transfer between legumes and grasses in mixed grass-legume swards plays an important role in the nitrogen cycle of grassland agricultural systems. There is a nitrogen-sharing pathway in legume-cereal intercropping systems and in grass-legume mixed swards whereby the legume plant not only meets its own nitrogen requirements for growth by way of fixed nitrogen from the atmosphere, but also provides nitrogen for the associated grasses through various pathways. The pathways of nitrogen transfer in mixed grass-legume swards can be categorized as above-ground or below-ground. The above-ground pathway occurs when herbage of legume plants is eaten by grazing animals and returned to the soil via excreta, then absorbed by other plants (including grasses), or when herbage litter falls to the ground and is decomposed and mineralized by microorganisms in soil to release plant-available nitrogen. The below-ground pathway is relatively complex and involves three routes: 1) Decomposition: decomposition of legume root tissues followed by uptake of released N by neighboring plants. 2) Mycorrhizae: transfer of N mediated by plant-associated mycorrhizae. 3) Root exudation: the exudation of soluble N compounds by donors and uptake by receivers. Although the current study identifies the possible routes of nitrogen transfer, many questions remain. Which is the main transfer route? What is the contribution of each route? To what extent can particular transfer routes operate concurrently? Such questions still lack key evidence. This paper briefly describes the current research status of biological nitrogen fixation of legume forages and of legume-grass nitrogen transfer in mixed grass-legume swards. A number of possible nitrogen transfer routes are reviewed, and directions for future research are identified. Better understanding of these questions will pave the way to achieve sustainable development of agriculture and animal husbandry in China, through realization of the potential benefits offered by legumes.

Key words: biological nitrogen fixation, nitrogen transfer, legume grass, mixed grassland, N rhizodeposition