● Nutrient constraints in low-fertility soil were modified by different species combinations.

To investigate the interplay of competition and facilitation between plants in low-fertility pasture grasslands of New Zealand, we compared nutrient uptake and acquisition of key nutrients of three species from different functional groups. Combinations of Pilosella officinarum (mouse-eared hawkweed, an invasive weed), Trifolium repens (white clover, a nitrogen fixer) and Dactylis glomerata (cocksfoot, a pasture grass) were planted into a soil with low-to-deficient concentrations of key nutrients. Highest yields were achieved by the grass growing alone but, when the clover and grass had grown together, there were complementary benefits in terms of procurement of a wide range of nutrients from soil despite lower root biomass. The invasive weed negated these benefits, and soil nutrients were exploited less efficiently when Pilosella had grown alone or in a mixture with the other species. Competition from the weed removed the benefits of grass-legume coexistence. These findings are interpreted to suggest that requirements for legumes to be the main source of nitrogen in pasture grasslands may be compromised unless competitive weeds are controlled to avoid disrupted procurement of key nutrients. It is likely these constraints to nutrient procurement would similarly impact conservation grasslands.