The effect of elevated CO2 on plant-microbial interactions and nitrogen (N) cycling is critical to predicting plant growth responses to elevated CO2, because plant growth is often N-limited. We investigated whether the effects of elevated CO2 on plant-microbial N dynamics differed among six annual plant species: three European grasses that have invaded California grasslands, and one grass and two forbs native to California serpentine grassland. Elevated CO2 altered plant N pools and '5NH4+ uptake, but the direction and magnitude of the changes were species dependent. The introduced grasses showed increased plant N pools and '5NH4+ uptake, whereas the native species showed smaller increases or even decreases in plant N pools and '5NH4+ uptake. Under nutrient enrichment, soil microbial N and '5NH4+ uptake differed among soils with different plant species, but they were not affected by elevated CO2. At low nutrients, elevated CO2 altered soil microbial N and '5NH4+ uptake, but the direction and magnitude of the changes were species dependent. The changes in soil microbial N were positively correlated with changes in the plant N pool, suggesting that there was no trade-off in N uptake between plants and microbes. These results also suggest that plant species composition will partly determine the direction of changes in soil N cycling in response to elevated CO2.
Nitrogen (N) limits plant growth in many terrestrial ecosystems, potentially constraining terrestrial ecosystem response to elevated CO<sub>2</sub>. In this study, elevated CO<sub>2 </sub>stimulated gross N mineralization and plant N uptake in two annual grasslands. In contrast to other studies that have invoked increased C input to soil as the mechanism altering soil N cycling in response to elevated CO<sub>2</sub>, increased soil moisture, due to decreased plant transpiration in elevated CO<sub>2</sub>, best explains the changes we observed. This study suggests that atmospheric CO<sub>2 </sub>concentration may influence ecosystem biogeochemistry through plant control of soil moisture.