Atmospheric CO2 and the composition and function of soil microbial communities

Elevated atmospheric CO2 has the potential to increase the production and a lter the chemistry of organic substrates entering soil from plant productio n, the magnitude of which is constrained by soil-N availability. Because mi crobial growth in soil is limited by substrate inputs from plant production , we reasoned that changes in the amount and chemistry of these organic sub strates could affect the composition of soil microbial communities and the cycling of N in soil. We studied microbial community composition and soil-N transformations beneath Populus tremuloides Michx. growing under experimen tal atmospheric CO2 (35.7 and 70.7 Pa) and soil-N-availability (low N = 61 ng N.g(-1).d(-1) and high N = 319 ng N.g(-1).d(-1)) treatments. Atmospheric CO2 concentration was modified in large, open-top chambers, and we altered soil-N availability in open-bottom root boxes by mixing different proporti ons of A and C horizon material. We used phospholipid fatty-acid analysis t o gain insight into microbial community composition and coupled this analys is to measurements of soil-N transformations using N-15-pool dilution techn iques. The information presented here is part of an integrated experiment d esigned to elucidate the physiological mechanisms controlling the flow of C and N in the plant-soil system. Our objectives were (1) to determine wheth er changes in plant growth and tissue chemistry alter microbial community c omposition and soil-N cycling in response to increasing atmospheric CO2 and soil-N availability and (2) to integrate the results of our experiment int o a synthesis of elevated atmospheric CO2 and the cycling of C and N in ter restrial ecosystems. After 2.5 growing seasons, microbial biomass, gross N mineralization, micro bial immobilization, and nitrification (gross and net) were equivalent at a mbient and elevated CO2, suggesting that increases in fine-root production and declines in fine-root N concentration were insufficient to alter the in fluence of native soil organic matter on microbial physiology; this was the case in both low- and high-N soil. Similarly, elevated CO2 did not alter t he proportion of bacterial, actinomycetal, or fungal phospholipid fatty aci ds in low-N or high-N soil, indicating that changes in substrate input from greater plant growth under elevated CO2 did not alter microbial community composition. Our results differ from a substantial number of studies report ing increases and decreases in soil-N cycling under elevated CO2. From our analysis, it appears that soil-N cycling responds to elevated atmospheric C O2 in experimental situations where plant roots have fully colonized the so il and root-associated C inputs are sufficient to modify the influence of n ative soil organic matter on microbial physiology. In young developing ecos ystems where plant roots have not fully exploited the soil, microbial metab olism appears to be regulated by relatively large pools of soil organic mat ter, rather than by the additional input of organic substrates under elevat ed CO2.