Root responses to elevated CO2, warming and irrigation in a semi-arid grassland: Integrating biomass, length and life span in a 5-year field experiment

1. Plant roots mediate the impacts of environmental change on ecosystems, yet knowledge of root responses to environmental change is limited because few experiments evaluate multiple environmental factors and their interactions. Inferences about root functions are also limited because root length dynamics are rarely measured. 2. Using a 5-year experiment in a mixed-grass prairie, we report the responses of root biomass, length and life span to elevated carbon dioxide (CO2), warming, elevated CO2 and warming combined, and irrigation. Root biomass was quantified using soil cores and root length dynamics were assessed using minirhizotrons. By comparing root dynamics with published results for soil resources and above-ground productivity, we provide mechanistic insights into how climate change might impact grassland ecosystems. 3. In the upper soil layer, 0-15 cm depth, both irrigation and elevated CO2 alone increased total root length by twofold, but irrigation decreased root biomass and elevated CO2 had only small positive effects on root biomass. The large positive effects of irrigation and elevated CO2 alone on total root length were due to increases in both root length production and root life span. The increased total root length and life span under irrigation and elevated CO2 coincided with apparent shifts from water limitation of plant growth to nitrogen limitation. Warming alone had minimal effects on root biomass, length and life span in this shallow soil layer. Warming and elevated CO2 combined increased root biomass and total root length by c. 25%, but total root length in this treatment was lower than expected if the effects of CO2 and warming alone were additive. Treatment effects on total root length and root life span varied with soil depth and root diameter. 4. Synthesis. Sub-additive effects of CO2 and warming suggest studies of elevated CO2 alone might overestimate the future capacity of grassland root systems to acquire resources. In this mixed-grass prairie, elevated CO2 with warming stimulated total root length and root life span in deeper soils, likely enhancing plant access to more stable pools of growth-limiting resources, including water and phosphorus. Thus, these root responses help explain previous observations of higher, and more stable, above-ground productivity in these projected climate conditions.