Investigators:
Alf Ekblad
Rytas Vilgalys
Grants:
"Effects of Elevated CO2 on the Production and Turnover of Mycorrhizal Mycelium and the Formation and Priming of Soil Organic Matter", Swedish Research Council for Environment, Agricultural Sciences and Spatial planning, , 2009 - 2011
Description:
Questions addressed:
How will elevated CO2 affect,1. The production and tunrover of of mycorrhizal external mycelium? Hypothesis-increased production, but might be affected negatively by the nitrogen availability.
2. The contribution of external mycelium to the formation of soil organic matter? Hypothesis-the turnover rate will not change.
3. The decomposition of old soil organic matter? Hypothesis- The increased allocation of carbohydrates belowground will result in a priming effect but this will be less pronounced at high nitrogen conditions.
Experimental protocol:
To test the hypotheses listed above, we will make two series of experiments:In the first set of experiments we will test the above hypotheses (1) and (2). We will use a modified method of Wallander et al. (2001) to harvest mycorrhizal mycelium several times over two growing seasons (I know that the Duke FACE might not run for more than one year). We will use in-growth tubes (diam. 2 cm, height 10 cm) constructed of nylon mesh (50 ¼m mesh size, with washed silica sand), allowing in-growth of fungal hyphae, but not of roots. In a recent pilot-study we found mycelium in-growth into the tubes to be very good and we estimated a fungal biomass in the cores of 470 kg ha-1 81 days after installation into the soil of a Norway spruce forest. Before the tubes are inserted, a core of soil with the same size will be removed. Tubes in harvest regime (A) will be harvested frequently to give production from season to season (I can send a Figure if you like). Harvest regime (C) in combination with (B) and (D) will give the ageing and turnover of mycelium (see calculations below). An unknown amount of saprotrophic fungi could contribute to the biomass in the in-growth tubes. However, EM fungi make up the very most of the biomass in the bags in these soils as shown by root trenching (Wallander et al. 2001), and as confirmed with molecular techniques (Wallander, pers. comm.). In the second set of experiments we will test hypotheses (3), a possible priming effect, by using mycelium ingrowth tubes filled with soil from a C4-ecosystem. This soil has a slightly higher 13C-content than the C3-plants found at the site, making it possible to detect a priming. It will also be possible to detect if litter from hyphae and/or roots has contributed to the formation of SOM and if elevated CO2 has caused any changes in this respect (see calculations below).
Measurements on mycelium and soils The following measurements will be made on each solid sample: 1. Mycelium dry weight. 2. Ergosterol concentrations. Ergosterol is a fungus specific lipid used as a marker for living fungal biomass (Nylund & Wallander, 1992; Ekblad et al. 1998). Ergosterol give in the pure sand a measure of EM biomass since the amount of heterotropic fungi should be low and AM fungi do not contain ergosterol (Olsson et al., 2003). 2. Analysis of fatty acids to give a measure of mycelial biomass of AM fungi and a second measure of EM fungi (Olsson, 1999). The neutral lipid fatty acid 16:1?â5 will be used as a marker for AM fungi and the phospholipid fatty acid 18:2w6,9 will be used as a marker for EM fungi. 3 Molecular identification of fungal species by rDNA PCR, cloning and sequencing (Landeweert et al. 2003) in combination with terminal restriction fragment length polymorphism (T-RFLP) analysis (Dicke et al. 2002). This analysis is relevant since there may be species differences in the response to elevated CO2 (Fransson et al. 2001; Gorissen & Kuyper, 2000). We will also detect colonization of heterotrophic fungi. 4 d13C of SOM mycelium and roots to calculate changes in new and old C (see equations below).
Theory
Calculations of changes in SOM content and contribution from old and new C
To calculate the change in old (Cold) and new C (Cnew) of total C (Ctot) in the mesh tubes filled with C4-soil, a simple two-component mixing model similar to the models used by Ekblad & H??gberg (2000) will be used:
Ctot = Cnew + Cold (3)
and Ctot * d13Ctot = d13Cnew * Cnew + d13Cold * Cold. (4)
Where the d13Cnew is determined from the mesh tubes with pure sand and the d13Cold is the value for the bulk C4-soil.
As mentioned above, in previous studies the d13C of the root material has been used as a proxy for all new C allocated below ground. However, it is known that the external mycelium of EM fungi is 13C-enriched compared to plant biomass (eg. Wallander et al. 2001, 2004). Furthermore, we have found that the C used in rhizosphere respiration is enriched compared to root biomass (Ekblad, pers. comm.). This is possibly explaining the inconsistent results of previous studies concerning the effect of elevated [CO2] on the formation of SOM and priming effects (see above)!
Calculations of production and turnover of mycelium Equations used to estimate the fractional loss per day k and the MRT of the biomass. We assume an exponential decay of material and that the mycelium is not turned over several times over one season [That the mycelium can exist and grow slowly for several months has been shown in the field (Coutts & Nicoll, 1990), and been observed in microcosms at the laboratory (Wallander, own obs.)].
Then the fraction left y from year 1 at last harvest is given by:
y = (C5 - B5)/(C3+B5) = e-kt, (5)
and k is calculated as, k = - (ln y)/t, (6)
then the MRT = 1/k. (7)
Similar calculations will be made for the other harvests (harvests 3 and 4).
In the first calculations above we use dry weight as a measure of the biomass. However, this measure will include both AM and EM hyphae. The ergosterol concentration of the harvested mycelium will be used as a measure of % living EM mycelium (Ekblad et al. 1998). This living biomass will be calculated as follows:
Living fungal biomass of EM mycelium (% of total) = 100(s-b)/(a-b) (8)
where s is the ergosterol concentration of the sample,a is the ergosterol concentration in young mycelium (from (A)-tubes), and b is the ergosterol concentration in inactive fungal cells (from (D) tubes). If inactive fungal cells contain no ergosterol (b = 0), the function is condensed to 100(s/a). Similar calculations as for ergosterol will be applied for fatty acids which will give estimates for the AM mycelium.