Elevated CO₂ effects on semi-arid grassland plants in relation to water availability and competition

Authors

Feike A. Dijkstra, USDA-ARSFollow
Dana Blumenthal, USDA-ARSFollow
Jack A. Morgan, USDA-ARS
Daniel R. LeCain, USDA-ARSFollow
Ronald F. Follett, USDA-ARSFollow

Date of this Version

2010

Comments

Published in Functional Ecology, 2010, 24, 1152–1161. doi: 10.1111/j.1365-2435.2010.01717.x

Abstract

1. It has been suggested that much of the elevated CO₂ effect on plant productivity and N cycling in semi-arid grasslands is related to a CO₂-induced increase in soil moisture, but the relative importance of moisture-mediated and direct effects of CO₂ remain unclear.

2. We grew five grassland species common to the semi-arid grasslands of northern Colorado, USA, as monocultures and as mixtures of all five species in pots. We examined the effects of atmospheric CO₂ concentration (ambient vs. 780 p.p.m.) and soil moisture (15 vs. 20% m⁄m) on plant biomass and plant N uptake. Our objective was to separate CO₂ effects not related to water from water-mediated CO₂ effects by frequently watering the pots, thereby eliminating most of the elevated CO₂ effects on soil moisture, and including a water treatment similar in magnitude to the water-savings effect of CO₂.

3. Biomass of the C₃ grasses Hesperostipa comata and Pascopyrum smithii increased under elevated CO₂, biomass of the C₄ grass Bouteloua gracilis increased with increased soil moisture, while biomass of the forbs Artemisia frigida and Linaria dalmatica had no or mixed responses. Increased plant N uptake contributed to the increase in plant biomass with increased soil moisture while the increase in plant biomass with CO₂ enrichment was mostly a result of increased N use efficiency (NUE). Species-specific responses to elevated CO₂ and increased soil moisture differed between monocultures and mixtures. Both under elevated CO₂ and with increased soil moisture, certain species gained N in mixtures at the expense of species that lost N, but elevated CO₂ led to a different set of winners and losers than did increased water.

4. Elevated CO₂ can directly increase plant productivity of semi-arid grasslands through increased NUE, while a CO₂-induced increase in soil moisture stimulating net N mineralization could further enhance plant productivity through increased N uptake. Our results further indicate that the largest positive and negative effects of elevated CO₂ and increased soil moisture on plant productivity occur with interspecific competition. Responses of this grassland community to elevated CO₂ and water may be both contingent upon and accentuated by competition.