Responses of the soil microbiota to elevated CO2 in an artificial tropicalecosystem

Plants in artificial tropical ecosystems were grown under ambient (340 mu l l(-1)) and elevated (610 mu l l(-1)) atmospheric CO2 for 530 d under low-n utrient conditions on a substrate free of organic C. At the end of the expe riment a number of soil chemical and microbiological variables were determi ned. Although we found no changes in total soil organic matter under elevat ed CO2, we did find that after physical fractionation the amount of organic C in the supernatant (< 0.2 mu m) and the amount of water extractable orga nic C (WEOC) was lower under elevated CO2. The extractable optical density (OD) indicated a higher degree of humification for the elevated than for th e ambient CO2 samples (P = 0.032). Microbial biomass C was not significantl y altered under high CO2, but total bacterial counts were significantly hig her. The microbial biomass C-to-N ratio was also higher at elevated (15.0) than at ambient CO2 (10.0). The number of mycorrhizal spores was lower at h igh CO2, but ergosterol contents and fungal hyphal lengths were not signifi cantly affected. Changes were found neither in community level physiologica l profiles (CLPPs) nor in the structural attributes (phospholipid fatty aci ds, PLFAs) of the microbial community. Overall, the effects on the soil mic robiota were small, perhaps as a result of the low nutrient supply and low organic matter content of the soil used in our study. The few significant r esults showing changes in specific, though relatively minor, organic matter pools may point to possible long-term changes of the more major pools. Fur thermore, the data suggest increased competition between plants and microbe s for N at high CO2. (C) 1999 Elsevier Science B.V. All rights reserved.