Plant Species Mediate Changes in Soil Microbial N in Response to Elevated CO 2

The effect of elevated $\mathrm{CO}_2$ on plant—microbial interactions and nitrogen (N) cycling is critical to predicting plant growth responses to elevated $\mathrm{CO}_2$, because plant growth is often N—limited. We investigated whether the effects of elevated $\mathrm{CO}_2$ 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 $\mathrm{CO}_2$ altered plant N pools and $ { 15}\mathrm{NH}_4 + $ uptake, but the direction and magnitude of the changes were species dependent. The introduced grasses showed increased plant N pools and $ { 15}\mathrm{NH}_4 + $ uptake, whereas the native species showed smaller increases or even decreases in plant N pools and $ 1 5{\mathrm{NH}_4 + $ uptake. Under nutrient enrichment, soil microbial N and 1 5\mathrm{NH}_4 + $ uptake differed among soils with different plant species, but they were not affected by elevated $\mathrm{CO}_2$. At low nutrients, elevated $\mathrm{CO}_2$ altered soil microbrial N and 1 5\mathrm{NH}_4 + $ 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. The results also suggest that plant species composition will partly determine the direction of changes in soil N cycling in response to elevated $\mathrm{CO}_2$.