Background, Aims, and Scope. The genetic structure and the functionality of soil microbes are both important when studying the role of soil in the C cycle in elevated CO, scenarios. The aim of this work was to investigate the genetic composition of the fungal community by means of PCR-DGGE and the functional diversity of soil micro-organisms in general with MicroResp-based community level physiological profiling (CLPP) in a poplar plantation (POPFACE) grown under elevated [CO2] with and without nitrogen fertilization.
Materials and Methods. The POPFACE experimental plantation and FACE facility are located in central Italy, Tuscania (VT). Clones of Populus alba, Populus nigra and Populus x euramericana were grown, from 1999 to 2004, in six 314 m(2) plots treated either with atmospheric (control) or enriched (550 mu mol mol(-1)) CO2 with FACE (Free Air CO2 Enrichment) technology in each growing season. Each plot is divided into six triangular sectors, with two sectors per poplar genotype: three species x two nitrogen levels. After removal of the litter layer one soil core per genotype (10 cm wide, 20 cm depth) was taken inside each of the three sectors in each plot, for a total of 36 soil cores (3 replicates x 2 [CO2] x 2 fertilization x 3 species) in October 2004 and in July 2005. DNA was extracted with a bead beating procedure. 18S rDNA gene fragments were amplified with PCR using fungal primers (FR1 GC and FF390). Analysis of CLPP was performed using the MicroResp method. Carbon substrates were selected depending on their ecological relevance to soil and their solubility in water. In particular rhizospheric C sources (carboxylic acids and carbohydrates) were chosen considering the importance of root inputs for microbial metabolism.
Results. he fertilization treatment differentiated the fungal community composition regardless of elevated [CO2] or the poplar species; moreover the number of fungal species was lower in fertilized soil. The effect of elevated [CO2] on the fungal community composition was evident only as interaction with the fertilization treatment as, in N-sufficient soils, the elevated [CO2] selected a different microbial community. For CLPP, the different poplar species were the main factors of variation. The FACE treatment, on average, resulted in lower C utilization rates in un-fertilized soils and higher in fertilized soils.
Discussion. Fungal biomass and fungal composition depend on different factors: from previous studies we know that the greater quantity and the higher C/N ratio of organic inputs under elevated [CO2] influenced positively the fungal biomass both in fertilized and in un-fertilized soil, whereas nitrogen availability resulted to be the main determinant of fungal community composition in this work. Whole active microbial community was directly influenced by the soil nutrient availability and the poplar species. Under elevated CO, the competition for N with plants strongly affected the microbial communities, which were not able to benefit from added rhizospheric substrates. Under N-sufficient conditions, the increase of microbial activity due to [CO2] enrichment was related to a more active microbial community, favoured by the current availability of C and N. Conclusions. Different factors influenced the microbial community at different levels: poplar species and root exudates affected the functional properties of the microbial community, while the fungal specific composition (as seen with DGGE) remained unaffected. On the other hand, factors such as N and C availability had a strong impact on the community functionality and composition. Fungal community structure reflected the availability of N in soils and the effect of elevated [CO2] on community structure and function was evident only in N-sufficient soils. The simultaneous availability of C and N was therefore the main driving force for microbial structure and function in this plantation.
Recommendations and Perspectives. Using the soil instead of soil extracts for CLPP determination provides a direct measurement of substrate catabolism by microbial communities and reflects activity rather than growth because more immediate responses to substrates are measured. Further applications of this approach could include selective inhibition of different microbial functional groups to investigate specific CLPPs. To combine the structural analysis and the catabolic responses of specific microbial communities (i.e. fungi or bacteria) could provide new outlooks on the role of microbes on SOM decomposition.