Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment
Soil fungi couple plant and ecosystem resource demands to pools of soil resources. Research on these organisms is needed to predict how rising atmospheric CO2 will influence forest ecosystem processes and soil carbon (C) sequestration potential. We examined the influence of free-air-CO2-enrichment (FACE) on mycorrhizal and extraradical rhizomorph dynamics over a 5-year period in a loblolly pine forest using minirhizotrons. Standing crop of mycorrhizal root tips varied greatly spatially and through time. Summed across all years, CO2 enrichment increased mycorrhizal root tip production by 194% in deep soil (15-30 cm) but did not influence mycorrhizal production in shallow soil (0-15 cm). Production and mortality of soil rhizomorph length was 27% and 25% greater in CO2-enriched plots compared with controls over a 5-year period beginning in January of 2000 and running through autumn 2004. Effects of atmospheric CO2 enrichment on longevity of mycorrhizal root tips and rhizomorphs varied with soil depth (mycorrhizae and rhizomorphs) and with diameter (rhizomorphs). For instance, survival of mycorrhizal tips was reduced in CO2-enriched plots in deep soil (15-30 cm depth) but was increased in shallower soil (0-15 cm). Rhizomorph turnover was accelerated in shallow soil but effects on survivorship in deep soil varied according to diameter. A drought in 2002 coupled with loss of leaf area to an ice storm late in 2002 were followed by reductions in rhizomorph and mycorrhizal production, increases in mortality, and decreases in standing crop during 2003 and 2004. These effects tended to be more severe in CO2-enriched plots. Positive effects of atmospheric CO2 enrichment on mycorrhizal fungi, primarily observed in deeper soil, are probably contributing to the prolonged stimulation of NPP by CO2 enrichment at the Duke FACE study site.