Contribution of the ectomycorrhizal sheath to nitrogen pools and fluxes in two oak dominated Midwestern forest sites
It has been suggested that in forest communities dominated by ectotrophic trees a significant proportion of the belowground plant biomass may be sequestered in the ectomycorrhizal (ECM) sheath. If so, this biological compartment may have a significant role in soil C and N cycles. However, there is virtually no direct quantification of this pool particularly as it relates to belowground N, or how it responds to changes in the environmental conditions. We conducted a seven year field study at two oak dominated sites located in Cook County, IL and Porter County, IN. At each site, two plots were fertilized monthly at a rate of 3X the atmospheric N deposition. We used National Atmospheric Deposition Program data from the most recent five years to determine the application rates and the exact ratios of ammonium to nitrate. Soils from near mature oak trees were analyzed for fine roots, coarse roots, hyphae and the ECM sheath. Using acupuncture needles, Sheaths were gently removed from the root tips under dissecting microscopes. The N pool sizes of each of these belowground compartments as well as the leaf litter were determined from the product of N concentration and biomass of each tissue type.
At the Cook county site, ECM sheath accounted for over 20% of the total plant fungal N pool. This is a significant proportion of the total pool of N, but experimental N addition markedly reduced this compartment by a factor of 2. Compared to Cook, ECM sheath constituted only 10% of the total plant fungal N, and this pool did not respond to experimental N addition. We have reasons to attribute the markedly lower N pool of the ECM sheath at the Porter site to a historically higher N deposition rate relative to the Cook County forest site. Analysis of the cumulative deposition rates from 20 years preceding this study indicates that the site at Porter received 15 to 20 % more N deposition than the Cook site. The saturating or near saturating N deposition condition at the Porter site could also explain the lack of a response to the experimental N addition of this fungal compartment. Using scarce turn over data from the literature we estimate that N fluxes through the ECM sheath can range from 35 to 175 kg y-1 h-1. These fluxes are likely to be much higher in coniferous forest where ECM sheath are more prevalent.