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

Abstract

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 (C₃ grass) and Cleistogenes songorica (C₄ 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. C₄ grass generally maintained stronger competitiveness and gained relative advantage under mixed sowing with N addition. N addition enhanced the competitive advantage of C₄ grass in mixed-sowing treatment. However, priority effects resulting from sowing order dominated the competition between C₃ and C₄ 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 C₄ 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.