Warming and elevated CO2 interact to alter seasonality and reduce variability of soil water in a semiarid grassland

Authors: Blumenthal, Dana; MUELLER, KEVIN - Cleveland State University; Kray, Julie; Lecain, Daniel; PENDALL, ELISE - Western Sydney University; Duke, Sara; ZELIKOVA, T - University Of Wyoming; DIJKSTRA, FEIKE - University Of Sydney; WILLIAMS, D - University Of Wyoming; MORGAN, J - Retired ARS Employee

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2018
Publication Date: 3/8/2018
Citation: Blumenthal, D.M., Mueller, K., Kray, J.A., Lecain, D.R., Pendall, E., Duke, S.E., Zelikova, T.J., Dijkstra, F., Williams, D.G., Morgan, J.A. 2018. Warming and elevated CO2 interact to alter seasonality and reduce variability of soil water in a semiarid grassland. Ecosystems. 21:1533-1544. https://doi.org/10.1007/s10021-018-0237-1.
DOI: https://doi.org/10.1007/s10021-018-0237-1

Interpretive Summary: Where climate changes alters the amount and timing of water that is available to plants, it can have profound effects on dryland ecosystems. In addition to predicted changes in precipitation, both elevated CO2 and warming can alter water availability, often in opposite ways. Few studies, however, have measured the combined influence of CO2 and warming on the amount, timing and temporal variability of soil water. Here, we report that experimental CO2 enrichment and warming effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. In the spring, negative effects of warming largely offset positive effects of elevated CO2. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with elevated CO2. Together, elevated CO2 and warming may lead to minor increases in soil water later under drier and later-season conditions. These results explain previously observed plant responses, including reduced leaf area with warming in summer, and extended growing seasons with elevated CO2. They also suggest that future increases in CO2 and warming in mixed-grass prairie may favor plant species that germinate or grow in autumn, including winter annuals and C3 graminoids, and species able to take advantage of CO2-driven increases in water availability under dry conditions.

Technical Abstract: Global changes that alter the amount and timing of plant-available water may have profound effects on arid and semi-arid ecosystems. In addition to predicted changes in precipitation, both elevated CO2 and warming can alter water availability, often in opposite ways. Few studies, however, have measured the combined influence of CO2 and warming on the amount, timing and temporal variability of soil water. Here, we ask how free air CO2 enrichment (to 600 ppmv) and infrared warming (+1.5 oC day, +3 oC night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. Volumetric soil water content was measured for six years in the upper 25 cm of the soil profile, where most root growth and biogeochemical cycling take place. We found that elevated CO2 and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming largely offset positive effects of elevated CO2. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with elevated CO2. These interactions constrained the combined effect of CO2 and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, both CO2 and warming effects depended upon ambient soil moisture availability, with elevated CO2 increasing soil water under dry and moderately dry conditions, and warming decreasing soil water under moderately wet conditions. By increasing soil water under dry conditions, elevated CO2 also reduced temporal variability in VWC. These temporal patterns in CO2 and warming effects explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with both CO2 alone and CO2 plus warming in autumn. They also suggest that future increases in CO2 and warming in mixed-grass prairie may favor plant species that germinate or grow in autumn, including winter annuals and C3 graminoids, and species able to take advantage of CO2-driven increases in water availability under dry conditions.