Priority effects outweigh nitrogen deposition in shaping C3–C4 grass competition in desert steppe ecosystems

Nitrogen (N) deposition and altered sowing patterns are key drivers of grassland community dynamics, influencing species interactions, resource allocation, and ecosystem functioning. However, the mechanisms through which N availability and sowing order shape competitive outcomes among co-occurring species remain poorly understood. Two dominant desert steppe grasses, Stipa breviflora (C 3 grass) and Cleistogenes songorica (C 4 grass) were sowed under three sowing treatments—mixed-sowing, S. breviflora -first sowing, and C. songorica -first sowing—combined with two N addition levels (0 and 20 kg N ha -1 yr -1 ). We found that N addition significantly increased plant height, tiller number, biomass, and N uptake in both species, but the magnitude of response varied with sowing pattern. C 4 grass generally maintained stronger competitiveness and gained relative advantage under mixed sowing with N addition. N addition enhanced the competitive advantage of C 4 grass in mixed-sowing treatment. However, priority effects resulting from sowing order dominated the competition between C 3 and C 4 grasses rather than N deposition and species-specific biological characteristics. δ¹⁵N analysis revealed that interspecific differences in N uptake were most pronounced at short competitive distances, with N addition amplifying dominance of C 4 grass. These findings demonstrate that both resource availability and initial establishment order shape the trajectory of interspecific competition in grassland systems. This work provides new insights into grassland management under scenarios of increasing atmospheric N inputs and altered disturbance regimes. • N addition increased competitive advantage of C 4 grass in desert steppe. • Priority effect has a stronger impact on competitiveness than N addition. • C 3 and C 4 grasses in desert steppe adopt conservative utilization strategies. • Investigated plant competition by pot experiments and in-situ 15 N labelling. • New insights into grassland management under scenarios of altered disturbance regimes.