Contributions of Ectomycorrhizal Fungal Mats to Forest Soil Respiration

1
Contributions of Ectomycorrhizal Fungal Mats to
Forest Soil Respiration Claire L Phillips1,
Laurel Kluber2, Julia Pedersen2, Bruce Caldwell2,
Barbara J. Bond1 1Dept of Forest Ecosystems and
Society, 2Dept of Crop and Soil Science, 3Dept of
Botany and Plant Pathology Oregon State
University, Corvallis, OR 97330
Environmental sensitivity of mat contributions
Introduction
EcM mats covered 44 of forest floor
The common operational definition of
autotrophic soil respiration pools together
respiration from roots and microbial symbionts,
and does not capture differences in activity
between these two types of biota that have
potentially critical impacts for carbon and
associated nutrient cycles. The distinction
between roots and symbionts is particularly
important in forest ecosystems, where
ectomycorrhizal (EcM) fungi are ubiquitous and
are thought to have large influences on both soil
respiration and on plant nutrition. In an early
seral forest, respiration by EcM fungi has been
calculated to be as much as 25 of soil flux
(Heinemeyer et al. 2007). A paucity of field
measurements in different forest ecosystems has
limited more widespread consideration of the
impacts of EcM fungi on soil fluxes. The
purpose of the current study is to constrain EcM
fungal respiration in a late-seral Douglas-fir
forest using the natural observatory provided by
the mat-forming EcM genus, Piloderma
Previous molecular surveys at HJA identified two
dominant mat-forming genera that are easily
distinguished by morphology and habitat (Dunham
et al. 2007). Piloderma is a rhizomorophic mat
that colonizes the organic horizon, and Ramaria
is a mat with ashey appearance the colonizes the
mineral soil. We mapped the 20m x 50m study area
into 1m2 numbered quadrats, and randomly chose 50
quadrats to search intensively for mats.
Surface flux from both mat and non-mat soils
correlated strongly with soil temperature, the
difference between these values did not show a
relationship with soil temperature
In contrast, moisture content of the O-horizon
did correlate significantly with mat
contributions (P 0.0001). For every 10
increase in soil moisture, mat contributions
increase by 8 (95 CI 3.6-13.9).
Ramaria mats were uncommon at this site, but
rhizomorphic Piloderma mats were abundant. Only
Piloderma mats were examined for the remainder of
the study.
What is an ectomycorrhizal mat?
Several genera of EcM fungi form dense, perennial
aggregations of hyphae and rhizomorphs (thick
hyphal cords) in the litter and upper soil
layers. In mat colonized organic soil, fungal
hyphae can constitute up to 50 of dry
weight Soils dominated by EcM mats are foci of
EcM activity. In lab incubations they have
elevated enzymatic activity, higher levels of
DOC, higher respiration rates, and faster rates
of litter decomposition
EcM mats averaged 18 higher
respiration than non-mats
Study average, 95 CI0.7-39 accounting for
repeated measures
Probing deeper to explain mat/non-mat differences
A) Average respiration from mat (?) and non-mat
soils (?). Error bars are standard error. B)
Percent difference between mat and neighboring
non-mat surface flux. C) Soil temperature at
10cm depth (black) and precipitation (grey) at
headquarters weather station (430m above sea
level).
We found no significant differences between mat
and non-mat soil in root biomass, C, N, CN, or
moisture, indicating differences in respiration
are NOT due to roots or other soil properties. We
found a significant correlation between O-horizon
enzyme activity and soil respiration, indicating
respiration is strongly influenced by
near-surface microbial activity
We determined the potential for chitin
degradation using N-acetylglucosamini-dase
(NAGase) activity. Chitin is a component of
fungal cell walls, and is an important source of
N and C to soil microbial communities.
Chitinase is produced by both bacteria and
fungi, and we speculated it would be a good
indicator of microbial activity in this soil with
abundant chitin substrate. Previous work has
shown elevated NAGase activity in mat soils. In
our study NAGase activity was higher in mat soils
compared with neighboring non-mats (paired t-test
p0.024)
Multiplying the average difference between mat
and non-mat respiration by Piloderma cover
indicates mat contributions were 10.2 of total
soil respiration
Methods
Study Site This study was conducted at the HJ
Andrews LTER in the Western Cascades in central
Oregon . To maximize the likelihood of finding
both mat and non-mat areas, a 0.1ha plot was
established in a old-growth stand (450yrs)
dominated by Douglas-fir and Western Hemlock
(both EcM hosts) and Western Red Cedar (a host
for AM fungi). Soils were basalt-derived with
strong Andic properties. The O-horizon was well
developed, averaging 6cm. Quantifying mat
respiration Only mats belonging to the Piloderma
genus, which dominated the site, were examined
for this study. Piloderma mats were identified
initially based on morphology, and later
confirmed using tRFLP molecular analysis. Soil
respiration was measured monthly on twelve
co-located mat and non-mat soil pairs, and mat
contributions were quantified as the average
difference in respiration between neighboring mat
and non-mat soils. To scale up mat contributions
to the plot, the average mat contribution was
multiplied by the percent of soil surface area
occupied by mats, which was determined in an
initial cover survey.
R20.68
EcM fungi may contribute 1/3 of autotrophic soil
respiration
O-horizon had large influence on surface flux
We compared Piloderma mat respiration to
estimates of autotrophic respiration made in an
area of similar-aged forest less than 1km from
our study area, part of the Detritus Input and
Removal Treatments (DIRT) experiment (Sulzman et
al. 2005) They estimated 32 of respiration was
autotrophic, indicating that EcM mats may produce
1/3 of rhizospheric respiration in this
old-growth system.
Surface flux measurements incorporate CO2
produced throughout the soil profile, whereas EcM
mat activity is localized in the O-horizon. We
wanted to assess the sensitivity of surface flux
measurements to mat activity by calculating the
proportion of surface flux contributed by the
O-horizon over time. We vertically partitioned
soil flux by measuring the soil CO2 profile and
calculating CO2 flux at the interface of each
genetic horizon using Ficks First Law
Production of each horizon was calculated as the
difference between incoming and outgoing fluxes
from that horizon. Soil effective diffusivity
(DS) was calculated from soil moisture following
Moldrup 1999
Conclusions

• EcM mats accounted for as much as 10.2 of total
  soil respiration and one-third of rhizosphere
  respiration in this old-growth Douglas-fir stand.
• Elevated respiration on mat soils is not likely a
  result of root respiration or environmental
  characteristics, but of activity from EcM fungi
  and associated microbes.
• Both biological and physical processes impact EcM
  mat contributions to soil respiration. Higher
  respiration rates on mat soils are associated
  with increased enzyme activity. But the spatial
  location of EcM mats close to the soil surface
  also increases their contribution to surface flux
  when soils are wet.
• Soil respiration models may need to be adapted to
  better account for the carbon demands and the
  physical growth habits of plant symbionts.

• is total porosity
• is air-filled pore space
• (total porosity - water content)
• S is non-clay-sized particle fraction
• T is temperature
• Da is CO2 diffusivity in free air

Across all sampling dates, O-horizon
contributions averaged 73 (95 CI 61-85).
Soil respiration measures with Licor-6400
Contributions from the O-horizon varied over time
and seem to relate with soil moisture. O-horizon
contributions ranged from 93 in May, when snow
had just melted and the ground was saturated, to
37 in August, when the soil was extremely dry
(4-6 water content by volume at the O/A
interface).

• Accounting for differences in mat and non-mat
  respiration
• We examined a suite of soil environmental,
  chemical, and biological variables to assess
  whether differences in respiration between mat
  and non-mat locations could be influenced by
  factors other then microbial activity. These
  variables included
• Chitinase (NAGase) enzyme activity (indicator of
  microbial activity)
• Root biomass
• Soil C, N, CN
• Moisture
• pH
• Contributions of mineral soil CO2 production to
  surface flux

References Davidson, E, Savage, K et al. (2006)
Vertical partitioning of CO2 production within a
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(2007). "Diversity and community structure of
mat-forming ectomycorrhizal fungi in old growth
and rotation age Douglas-fir forests fo the HJ
Andrews Experimental Forest, Oregon, USA."
Mycorrhiza 17 633-645. Heinemeyer, A., I. P.
Hartley, et al. (2007). "Forest soil CO2 flux
uncovering the contribution and environmental
responses of ectomycorrhizas." Global Change
Biology 13 1786-1797. Moldrup, P, Olesen T,
Schjrnning P et al. (2000) Predicting the gas
diffusion coefficient in undisturbed soil from
soil water characteristics. Soil Science Society
of America Journal, 64, 94-100. Sulzman, E. W.,
J. B. Brant, et al. (2005). "Contribution of
aboveground litter, belowground litter, and
rhizosphere respiration to total soil CO2 efflux
in an old growth coniferous forest."
Biogeochemistry 73 231-256.
Changes in mat contributions may be related to
changes in CO2 production in deeper mineral soil
horizons