Aridity modifies the responses of plant stoichiometry to global warming and nitrogen deposition in semi-arid steppes

Global warming and nitrogen (N) deposition arc known to unbalance the stoichiometry of carbon (C), N, and phosphorus (P) in terrestrial plants, but it is unclear how water availability regulates their effects along a natural aridity gradient. Here, we conducted manipulative experiments to determine the effects of experimental warming (WT) and N addition (NT) on plant stoichiometry in desert, typical, and meadow steppes with decreasing aridity. WP elevated air temperatures by 1.2-2.9 degrees C using open-top chambers. WT increased forb C:N ratio and thus its N use efficiency and competitiveness in desert steppes, whereas WT reduced forb C:N and C:P ratios in typical and meadow steppes. Plant N:P ratio, which reflects nutrient limitation, was reduced by WI in desert steppes but not for typical or meadow steppes. NT reduced plant C:N ratios and increased N:P ratios in all three steppes. NT reduced forb C:P ratios in desert and typical steppes, but it enhanced grass C:P ratio in meadow steppes, indicating an enhancement of P use efficiency and competitiveness of grasses in wet steppes. WT and NT had synergetic effects on grass C:N and C:P ratios in all three steppes, which helps to increase grasses' productivity. Under WI' or NT, the changes in community C:N ratio were positively correlated with increasing aridity, indicating that aridity increases plants N use efficiency. However, aridity negatively affected the changes in N:P ratios under NT but not WT, which suggests that aridity mitigates P limitation induced by N deposition. Our results imply that warming could shift the dominant functional group into forbs in dry steppes due to altered stoichiometry, whereas grasses become dominated plants in wet steppes under increasing N deposition. We suggest that global changes might break the stoichiometric balance of plants and water availability could strongly modify such processes in semi-arid steppes.

Automated summary

Global warming and nitrogen deposition can unbalance the stoichiometry of carbon, nitrogen, and phosphorus in terrestrial plants. Water availability can regulate these effects. Experimental results from desert, typical, and meadow steppes show that warming can increase nitrogen use efficiency and competitiveness of forb plants in desert steppes, but reduce carbon:nitrogen and carbon:phosphorus ratios in typical and meadow steppes. Nitrogen addition decreases the carbon:nitrogen ratios in all three steppes and increases nitrogen:phosphorus ratios. The effects of warming and nitrogen addition have synergistic effects on carbon:nitrogen and carbon:phosphorus ratios in all three steppes, which enhance the productivity of grass plants. Water availability also modulates the effects of stoichiometry, and its influence is related to aridity and nitrogen deposition.