Mixtures of legume and grass are used worldwide to gain advantages in forage production and ecological maintenance. However, competition for nutrients by legumes in mixtures has not been fully explored. The aim was to determine how the forage proportion affected nutrient competition in legume and grass mixtures. Treatments included two species combinations and five sowing ratios. Competitive ratios (CR) of lucerne for nitrogen (N) and phosphorus (P) over two grasses were assessed to analyze how the lucerne proportion in mixtures affected the competition. Total N and P uptake were mostly lower under timothy-containing mixtures (MPs) than under smooth bromegrass-containing mixtures (MBs). Proportions of both N (NM%) and P uptake (PM%) of lucerne were higher under MPs than under MBs. Higher total N and P uptake were found under half-lucerne mixtures (M5P5 or M5B5) than under other grasslands. The NM% and PM% tended to be higher under half-lucerne mixtures, although they showed little difference among mixtures. Lucerne CR was greater under MPs than under MBs, and was greater than grass CR when lucerne was in lower proportion in the mixtures. There was little difference in soil N density among grasslands of the same cut, whereas soil P density was variable. Competitiveness of lucerne depends largely on the initial sowing ratio. High ratios of lucerne significantly reduce soil P density, leading to P limitation and reduced N and P uptake. On the Loess Plateau of China, mixing lucerne with smooth bromegrass is recommended to increase the uptake and harvest of N and P, specifically at the sowing ratio of 5:5.
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