Phosphorus application changes the competitive status between legume and grass species in a desert steppe

Global environmental changes significantly impact nitrogen (N) and phosphorus (P) availability in desert steppes, thereby reshaping plant species interactions and ultimately influencing ecosystem structure and functioning. This study investigated how these nutrients affect the competitive interactions between legumes and grasses by altering their adaptive strategies.

Pot experiments were conducted using the dominant grass species Stipa breviflora and the leguminous species Melissitus ruthenicus under different treatments involving control without nutrient inputs, N input alone, P input alone and combined N and P inputs. Plant growth, nutrient uptake and root traits were evaluated in monocultures and mixed plantings.

In the relatively N-enriched desert steppes, P addition increased grass biomass by 76 % in monocultures; however, this effect was not observed when the grass were planted alongside the leguminous species. Legume exhibited a more pronounced response to P supplementation, with biomass increasing by up to 106 %. The relative total biomass (RBT) remained below 1 across all treatments, indicating the presence of interspecific competition. In simultaneous mixed planting, grass species were dominant under N-only treatments, whereas the legume exhibited a competitive advantage under P-only treatment. The concentrations of N and P in shoots of the grass remained unchanged following nutrient inputs and coexistence with the legume. In contrast, the N and P concentrations in legume shoots demonstrated the contrary trends, and were negatively and positively correlated with biomass, respectively. Both the grass and the legume increased total root length and reduced root diameter when coexisting. The priority effect (i.e. first seeding) enhanced the secretion of acid phosphatase and carboxylates by roots of the legume, and coexistence stimulated these root exudates in the grass.

Nitrogen inputs moderately enhance grass growth, whereas P inputs benefited legumes by modifying rhizosphere processes, thereby mitigating their competitive disadvantage under N enrichment. These findings highlight the potential for global change-induced reduced P availability to shift plant dominance in grasslands.