Root responses to elevated CO2, warming, and irrigation in a semiarid grassland: integrating biomass, length, and lifespan in a 5-year field experiment

Authors: MUELLER, K - Cleveland State University; Lecain, Daniel; MCCORMACK, L - University Of Minnesota; PENDALL, E - Western Sydney University; Carlson, Mary; Blumenthal, Dana

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2018
Publication Date: 4/12/2018
Citation: Mueller, K.A., Lecain, D.R., McCormack, L., Pendall, E., Carlson, M.J., Blumenthal, D.M. 2018. Root responses to elevated CO2, warming, and irrigation in a semiarid grassland: integrating biomass, length, and lifespan in a 5-year field experiment. Journal of Ecology. doi:10.1111/1365-2745-12993.
DOI: https://doi.org/10.1111/1365-2745-12993

Interpretive Summary: Using five years of observations from an experiment in an intact, mixed-grass prairie, we report the responses of root biomass, length, and lifespan to elevated carbon dioxide (CO2), warming, elevated CO2 and warming combined, and irrigation. For roots in the upper 15 cm of soil, both irrigation and elevated CO2 alone approximately doubled standing root length, but strikingly, irrigation decreased root biomass and elevated CO2 had small positive effects on root biomass. Increased root production and lifespan contributed to the large positive effects of irrigation and CO2 on standing root length, and coincided with apparent shifts from water-limitation of plant growth to nutrient limitation. Warming alone had few effects on roots, but warming and elevated CO2 combined increased root biomass and length by ~25%. Treatment effects on standing root length and root lifespan varied with soil depth and root diameter. Stimulation of standing root length and root lifespan in deeper soils by elevated CO2 and warming combined likely contributed to the higher aboveground productivity in these conditions.

Technical Abstract: Plant roots mediate the impacts of environmental change on ecosystems, yet knowledge of root responses to environmental change is limited because few experiments manipulate multiple environmental factors and root dynamics are rarely measured thoroughly. Using five years of observations from an experiment in an intact, mixed-grass prairie, we report the responses of root biomass, length, and lifespan to elevated carbon dioxide (CO2), warming, elevated CO2 and warming combined, and irrigation. By comparing root dynamics with published results for soil resources and aboveground productivity, we also provide mechanistic insights into how climate change might impact ecosystem functions in semiarid grasslands. For roots in the upper 15 cm of soil, both irrigation and elevated CO2 alone increased standing root length by two-fold, but strikingly, irrigation decreased root biomass and elevated CO2 had small positive effects on root biomass. Our minirhizotron-based observations also showed that increased root production and lifespan contributed to the large positive effects of irrigation and CO2 alone on standing root length. The increased root length and lifespan under irrigation and elevated CO2 coincided with apparent shifts from water-limitation of plant growth to nutrient limitation. Warming alone had minimal effects on root biomass, length, and lifespan in this shallow soil layer. Warming and elevated CO2 combined increased root biomass and length by ~25%, but standing root length in this treatment was substantially lower than expected if the effects of CO2 and warming alone were additive. Thus, studies of elevated CO2 alone might overestimate the future capacity of grassland root systems to acquire resources. Finally, treatment effects on standing root length and root lifespan varied with soil depth and root diameter. Stimulation of standing root length and root lifespan in deeper soils by elevated CO2 and warming combined likely contributed to the higher aboveground productivity in these futuristic conditions.