Nitrogen enrichment alters plant N: P stoichiometry and intensifies phosphorus limitation in a steppe ecosystem

Although many studies have demonstrated that N deposition decreases biodiversity and alters ecosystem functioning, fewer studies have tested how N enrichment affect plant N and P limitation, N: P stoichiometry and ecosystem functioning. We examined the independent and interactive effects of N and P enrichment on plant N: P stoichiometry, nutrient limitation, and thereby ecosystem functioning based on two N, P, and N + P addition experiments in a typical steppe. At the species level, N enrichment increased leaf N: P ratio and P limitation of dominant species. The responses of plant tissue N: P ratio tend to saturate at soil available N: P supply ratio of approximately 20 for leaf N: P and 10 for root N: P ratio. At the community level, patterns of N and P limitation shifted from N limitation in a normal year to N and P co-limitation in a wet year, triggered mainly by inter-annual changes in soil N and P availability. The homoeostasis of N: P stoichiometry increases from plant leaves to roots and to microbes although the available N: P supply ratio varied by 56-fold in soil. Given that N deposition rates are projected to increase in upcoming decades, N deposition may further alter the stoichiometric balance of N and P and intensify P limitation of steppe ecosystems in future.