Soil phosphorus availability affects niche characteristics of dominant C₃ perennial and sub-dominant C₄ annual species in a typical temperate grassland of northern China

Aims: Phosphorus (P) addition can help restore degraded Chinese grasslands. Soil P-availability affects the plant niche dynamics. However, the dynamics of niche characteristics are not yet understood, particularly for above- and belowground differences between species and plant–microbe interactions that generate these dynamics. Methods: We conducted a long-term field P-fertilization experiment (0 to 12.5 g P m⁻² year⁻¹) to explore the impacts of P addition on the niche dynamics of a competitive forb (Chenopodium aristatum, a non-mycorrhizal C₄ plant) and a dominant grass (Leymus chinensis, a mycorrhizal C₃ plant) in a temperate grassland in Inner Mongolia, northern China. Results: Phosphorus addition greatly changed the niche and exacerbated aboveground competition between C. aristatum and L. chinensis. Competitive exclusion of L. chinensis occurred at all levels, except P_2.5. Photosynthesis and above- and belowground morphology of C. aristatum responded more to P₁ due to its high photosynthetic plasticity and nutrient resorption, which was associated with its competitive advantage. Although NO peaked at P_2.5, carbon assimilation and rhizosheath microbial biomass of L. chinensis were optimal. Alleviated NO at P₅ and P_12.5 was associated with segregation of root morphologies and rhizosheath microbial composition. However, surplus niches at P₅ and P_12.5 were occupied by invasive sub-shrubs, associating with the mismatched plant–microbe feedbacks of C. aristatum and L. chinensis. Conclusions: Our findings suggest that rhizosheath microbes mediate trade-offs between a host plant’s P-conservation and acquisition and highlight the importance of above- and belowground co-responses to community productivity and stability under P addition.