Precipitation and Plant Functional Composition Mediate Desert Canopy Nutrient Responses to Water and Nitrogen Addition

Background and aims Plant nutrients play a fundamental role in regulating ecosystem functioning and are sensitive to global changes such as precipitation change and nitrogen (N) deposition. However, the mechanisms by which resource availability and community composition mediate canopy nutrients remain unclear. Methods We investigated soil resource availability, plant productivity, community composition and canopy nutrient status in a 6-year water and N addition experiment in a desert shrubland of northern China. Results Water addition overall negatively affected canopy N content but had no effect on canopy phosphorus (P) content, and these effects were mediated by ambient precipitation. In drier years, water addition reduced canopy N and P contents via decreasing shrub-herb ratio and increasing community productivity. In wetter years, higher precipitation levels weakened water effects on plant community, causing a neutral water effect on canopy N content. Furthermore, this increased precipitation intensified soil calcium leaching, which, in turn, had a positive water effect on canopy P content. Accordingly, canopy N and P contents showed negative logarithmic and concave-shaped relationships with precipitation, respectively. Nitrogen enrichment increased canopy N but reduced P content through increasing soil N availability and reducing shrub-herb ratio. Conclusions Our results suggest that both ambient precipitation levels and plant functional composition play pivotal roles in mediating desert canopy nutrient responses to precipitation change and N deposition. Our findings also highlight the fundamental role of soil calcium in regulating plant P responses to precipitation change, which provide new insights into our understanding of biogeochemical coupling under global change.