Responses of aboveground C and N pools to rainfall variability and nitrogen deposition are mediated by seasonal precipitation and plant community dynamics

Plant productivity and tissue chemistry in temperate ecosystems are largely driven by water and nitrogen (N) availability. Although changes in rainfall patterns may influence nutrient limitation, few studies have considered how these two global change factors could interact to influence terrestrial ecosystem productivity and stoichiometry. Here, we examined the influence of experimentally-increased intra-annual rainfall variability and low-level nitrogen addition on aboveground productivity, C and N pools, and C:N ratios in a restored tallgrass prairie across two growing seasons. In the drier first year of the experiment, increased rainfall variability boosted productivity and C pools. In the wetter second year, aboveground productivity and C pools increased with N addition, suggesting a switch in primary resource limitation from water to N. Increased rainfall variability also reduced aboveground N pools in the second year. Community-level C:N increased under increased rainfall variability in the wetter second year and N addition slightly reduced community C:N in both years. These changes in element pools and stoichiometry were mostly a result of increased forb dominance in response to both treatments. Overall, our findings from a restored prairie indicate that increased rainfall variability and N addition can enhance aboveground productivity and C pools, but that N pools may not have a consistent response to either global change factor. Our study also suggests that these effects are dependent on growing season precipitation patterns and are mediated by shifts in plant community composition.