Testing nitrogen and water co-limitation of primary productivity in a temperate steppe

Background and aims: Primary productivity in the temperate steppe is assumed to be co-limited by nitrogen (N) and water availability, but empirical evidence is scarce. We examined the N and water limitation status of primary productivity from the species scale to community scale under the framework of resource co-limitation. Methods: We compared the responses of aboveground net primary productivity (ANPP) at different ecological levels to factorial N and water addition in two years in a temperate steppe of northern China. Results: Water addition significantly enhanced total ANPP by 46%, with stronger effects in the dry year. Total ANPP was sub-additively co-limited by N and water availability, being more sensitive to water addition than to N addition in the dry year and equally sensitive to both resources in the year with normal precipitation. The responses of total ANPP to resource additions were largely driven by the changes of grasses rather than the forbs. Species level ANPP showed conservative responses to resource additions. Conclusions: Our results highlight the hierarchical patterns of limitation status in primary productivity at different biological organization levels in this temperate steppe. The sub-additive limitation by N and water in this ecosystem deserves more attention in modelling the dynamics of ecosystem carbon cycle under global change scenarios