Response of soil biota to elevated atmospheric CO2 in poplar model systems

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO2 would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus x euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO2 in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO2. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO2. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO2. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO2 treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO2, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO2 was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose.