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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.