Elevated CO2 and O3 effects on Carbon demand of the Extramatrical Mycorrhizal Fungal Network
Contact: R. Michael Miller (rmmiller@anl.gov)
We are evaluating the interactive effects of elevated CO2 and O3 on the sequential growth and allocation of both ectomycorrhizal fungi (EMF) and arbuscular mycorrhizal fungi (AMF) associated with quaking aspen (Populus tremuloides), paper birch (Betula papyrifera), and sugar maple (Acer saccharum) at the Aspen FACE site. The Aspen FACE approach consists of 30 m diameter rings of gas-dispensing pipes that allow us to fumigate intact forest canopies with atmospheric pollutants and study the interaction of plants, soils and atmosphere (http://aspenface.mtu.edu/index.html). We have used several different approaches to quantifying treatment effects on the mycorrhizal fungal network, especially how host responses influence root associated colonization and extramatrical hyphal (EMH) production and symbiotic benefit. Over the last six years we have been developing and improving upon methods to better quantify root associated mycorrhizal fungal biomass and EMH production and standing crop. Because both AMF and EMF play a significant role in the system of study we also have had to develop a means of separating the production of these different mycorrhizae, especially quantification of the EMH.
Net EMH production has been determined directly by using hyphal in-growth bags. EMH are isolated from the bags by elutriation followed by determining C and N content, and isotopic 13C and the 15N on the recovered hyphae. The proportion of EMH consisting of AMF from EMF was determined using the ratio of phospholipid markers 16:1ω5c and 18:2ω6,9. Also, by using published allometric models for tree growth (King et al., New Phytologist 168:623, 2005) we have been able to determine incremental C gain for trees associated with bag placement. Additionally, measures of foliar N content of associated trees was also determined. Host C flux to mycorrhizal fungi is calculated from the in-growth bag hyphal mass (Mh) × % C in fungus and corrected for metabolic efficiency of fungi. We find the average C demand of the new fungal growth for host photosynthate to be around 15.6 g C m-2 y-1 for the ambient treatment. When current production fungal C demand is expressed as a response ratio [(treatment – ambient)/ambient] we find a 15% increase in fungal C with CO2 fumigation and a 53% reduction with O3 fumigation. The combined CO2 + O3 fumigation treatment demonstrated a 16% reduction in C transfer to the fungus from the host. Although not expressed in the above flux values, preliminary measures indicate that correcting for winter mortality of hyphae will likely account for an additional 30% increase in host C flux to EMH.
If we express fungal C demand on a host tree’s yearly growth increment (i.e., fungus C demand per yearly C gain of the host) we find a different relationship. In this case, when C demand of the fungus associated with ambient grown trees is expressed on incrementally accrued host C, fungal demand was 51 mg C transferred per g host tree C gained per m2 per year. When expressed as a response ratio the C flux values represent an 80.4% increase in C being allocated to the fungus when trees are grown under elevated CO2, whereas fungal C demand is reduced by 56.8% with O3 fumigation and 5.8% with the combined CO2 + O3 treatment. The relationship between incremental fungal C demand and tree biomass gain for ambient, elevated CO2, and the combined fumigation treatment are similar with slopes being parallel. The slope is significantly different for the O3 fumigation treatment. The observed parallel slopes for ambient and the combined CO2 + O3 treatment suggest the above observed fungal C demand differences are most likely explained by tree size or growth differences imposed by the treatments, with a different mechanism existing for fungal C demand from hosts grown with O3.
Because of the use of an enriched 13C source for CO2 fumigation we are able to get a glimpse into what may be controlling differential mycorrhizal fungal C demand by using the specific activity of 13C transfer from the plant host to the fungus. Specific activity is calculated as δ13C expressed as atomic proportion × Mh. If we assume all C comes from the host (not necessarily true) differences in Atom % 13C should indicate potential metabolic and functional differences. We find that a significant increase in specific activity of the extramatrical hyphae is found with the ozone treatment compared to the ambient growing fungi, whereas a slight nonsigificant decrease in specific activity is found for 13C transfer with CO2 fumigation.
This research was supported by the U.S. Department of Energy’s Office of Biological and Environmental Research, Program for Ecosystem Research, under Contract No. DE-AC02-06CH11357 to UChicago Argonne, LLC, Operator of Argonne National Laboratory.
