Precipitation variations, rather than N deposition, determine plant ecophysiological traits in a desert steppe in Northern China

Understanding how plant ecophysiological traits of coexisting species within a community respond to environmental changes could help to predict the shift in plant community structure and function, but it remains limited in the scenarios of co-occurring precipitation variations and N deposition. A two-year field experiment was conducted to explore the effects of large precipitation changes (reduced and increased precipitation amount by 25% and 50% relative to ambient control) and high N deposition (10 g N m−2 yr−1) on a series of leaf ecophysiological traits of three dominant species (Stipa tianschanica, a C3 grass; Cleistogenes squarrosa, a C4 grass; and Artemisia capillaris, a C3 forb) in a desert steppe in Northern China. Increasing precipitation significantly linearly promoted the leaf light-saturated photosynthesis rate (Asat) and N use efficiency of the two C3 species, irrespective of N addition. The rises in Asat of both C3 species were mainly caused by increased soil moisture, which strongly induced increases in leaf stomatal conductance (gs) and declines in quantum yield of photosystem II (ΦPSII). However, the Asat of the two C3 species was weakly correlated with their specific leaf area and leaf N concentration, whereas the Asat of the C4 grass was negatively related to its leaf N. Moreover, the Asat, height, and aboveground biomass of the forbs were much more water-sensitive than those of both grass species, with a consequence of the most dominant species turning from grass to forbs as precipitation increased. Our findings highlight that water limitation, rather than N deficit, is the largest factor controlling plant growth in drylands, and plant species-specific ecophysiological responses to precipitation fluctuations will cause a substantial shift in the production and composition of plant community