Changes in soil carbon and nitrogen properties and microbial communities in relation to growth of Pinus radiata and Nothofagus fusca trees after 6 years at ambient and elevated atmospheric CO2

Increasing atmospheric CO2 concentration can influence the growth and chemical composition of many plant species, and thereby affect soil organic matter pools and nutrient fluxes. Here, we examine the effects of ambient (initially 362 mu LL-1) and elevated (654 mu LL-1) CO2 in open-top chambers on the growth after 6 years of two temperate evergreen forest species: an exotic, Pinus radiata D. Don, and a native, Nothofagus fusca (Hook. F.) Oerst. (red beech). We also examine associated effects on selected carbon (C) and nitrogen (N) properties in litter and mineral soil, and on microbial properties in rhizosphere and hyphosphere soil. The soil was a weakly developed sand that had a low initial C concentration of about 1.0 g kg(-1) at both 0-100 and 100-300mm depths; in the N. fusca system, it was initially overlaid with about 50mm of forest floor litter (predominantly FH material) taken from a Nothofagus forest. A slow-release fertilizer was added during the early stages of plant growth; subsequent foliage analyses indicated that N was not limiting. After 6 years, stem diameters, foliage N concentrations and C/N ratios of both species were indistinguishable (P > 0.10) in the two CO2 treatments. Although total C contents in mineral soil at 0-100mm depth had increased significantly (P < 0.001) after 6 years growth of P. radiata, averaging 80 +/- 0.20g m(-2)yr(-1), they were not significantly influenced by elevated CO2. However, CO2-C production in litter, and CO2-C production, microbial C, and microbial C/N ratios in mineral soil (0-100mm depth) under P. radiata were significantly higher under elevated than ambient CO2. CO2-C production, microbial C, and numbers of bacteria (but not fungi) were also significantly higher under elevated CO2 in hyphosphere soil, but not in rhizosphere soil. Under N. fusca, some incorporation of the overlaid litter into the mineral soil had probably occurred; except for CO2-C production and microbial C in hyphosphere soil, none of the biochemical properties or microbial counts increased significantly under elevated CO2. Net mineral-N production, and generally the potential utilization of different substrates by microbial communities, were not significantly influenced by elevated CO2 under either tree species. Physiological profiles of the microbial communities did, however, differ significantly between rhizosphere and hyphosphere samples and between samples under P. radiata and N. fusca. Overall, results support the concept that a major effect on soil properties after prolonged exposure of trees to elevated CO2 is an increase in the amounts, and mineralization rate, of labile organic components.