Rachel E. Gallery, Nicole Trahan, Emily Dynes, Dawson Fairbanks

Short-term responses of soil microbe communities to fire:
functional shifts in high elevation mixed conifer forests
Rachel E. Gallery, Nicole Trahan, Emily Dynes, Dawson Fairbanks, David J.P. Moore
Summary
• Fires and insect outbreaks are increasing in frequency and severity across the western
U.S. with enormous and variable impacts on regional carbon, nitrogen, and water cycling.
• Wildfires heat surface soils >200°C, changing soil physical, chemical, and biological traits.
• In the short term, fire causes a wholesale transformations of organic compound substrate
available for microbial metabolism; disturbance severity can alter succession trajectories.
• In mixed conifer forests, shifts from carbon to nitrogen dominated enzyme activities in
burned forests with alkaline soils suggest loss of microbial taxa and function.
• Together, microbial biomass, substrate quality, and enzyme activity influence coupled
biogeochemical cycles in ecosystems; responses to disturbance
• Issue: Regional carbon models without explicit microbial processes may work well in
equilibrium conditions but cannot replicate observed biogeochemical changes after
disturbance. Approach: Quantify the post-fire response of microbial function in forest
ecosystems to enable testing of microbial processes models.
2012 High Park Fire, CO
A
Enzymes link microbial community structure to biogeochemical process:
Soil CO2 efflux, microbial activity, and organic substrates change post-fire
UNBURNED
BURNED
Fig 3a. Fire alters soil respiration (Rs; soil efflux) rates
that correspond with changes in soil chemistry such as
pH. pH of unburned soils, which include MPB killed
forests, were more acidic due to organic matter inputs,
which correlates with higher average soil efflux rates.
Soils from burned forests were more alkaline but also
had higher range in pH likely due to differences in burn
severity. Soil efflux rates were lower in burned forests
reflecting low substrate and microbial populations.
Fig 3b. The weak correlation between microbial
biomass carbon (C) and soil efflux rates could be
explained by the relative importance of other
constituents of respiration (e.g., autotrophic), which are
also affected by fire. Microbial biomass C is lower in
burned soils though shifts in microbial community
function (Fig 3c) may be more important drivers of Rs.
2013 Thompson Ridge Fire, NM
B
Fig 3c. β-glucosidase (cellulose decomposition;
black) and leucine amino-peptidase (protein
degradation; red) are the most widely measured
glycosidase and aminopeptidase activities
because they hydrolyze the most abundant C
and N substrates respectively. β-glucosidase
activity is lower in burned soils and correlates
with microbial biomass C patterns (Fig 3b).
Leucine amino-peptidase activity is higher in
burned soils corresponding with higher protein
relative abundance (Fig 4).
Year of infestation
2000
2006
2001
2007
2002
2008
2003
2009
2004
2010
New and integrated data needed to test heterotrophic respiration models
FEF
Extended Area
0.6
High Park Fire
Fig 1. (A) In the high elevation conifer forests of the Colorado Rocky Mountains, soil samples from 56
plots (3m radius) were sampled across 5 locations along a chronosequence of fire and mountain pine
beetle (MPB) killed forests after the 2012 High Park Fire (black perimeter). (B) Soil samples from 22 soil
pits across a gradient of burn severities in a mixed-conifer zero order basin from the Thompson Ridge Fire
in the Jemez River Basin CZO were collected 18 days following the June 2013 fire.
Soil CO2 efflux is correlated with microbial biomass in unburned forests
Relative abundance (%)
2005
lignin
phenol
unknown
protein
0.5
0.4
lipid
Fig 4. Changes in microbial substrates can be
N-bearing
quantified using pyrolysis gas chromatographic
polysaccharide mass spectroscopy (py-GC-MS). The relative
abundance of different organic compounds by
disturbance type is shown. Our preliminary
estimates of labile polysaccharide pools and
relative protein abundance appears to track
observed β-glucosidase and leucine aminopeptidase activities (Fig 3c).
0.3
0.2
0.1
0
Fig 2. Across a gradient of forest plots containing
live trees and those killed by either mountain pine
beetle or by mechanical girdling, microbial biomass
was a strong predictor of respiration (soil efflux; A)
however spatial variation in soil efflux rates across
the disturbance gradient was not well explained by
coincident measurements of soil temperature or
,moisture (B and Table). In current Earth System
Models (ESMs), heterotrophic respiration is
typically modeled as a function of available carbon
substrate, temperature and moisture (Shao et al.
2013). However when an ecological disturbance
causes changes to the availability of substrate and
the success of the microbial community, these
simple predictors are unlikely to be sufficient.
live
beetle-kill
Autotrophic
turnover
Dissolved
Organic
Carbon
Uptake
Vmax, Km
Vmax, Km
Catalyses
Enzyme
decay
Microbial
Biomass
Carbon
Soil
Organic
Carbon
Enzymes
Enzyme
production
Microbial
turnover
Modified from Allison et al. (2010).
Methods: Soil efflux was measured using a LICOR LI-6400 infrared gas analyzer. Chloroform fumigated
K2SO4 soil extracts from the chronosequences were quantified for microbial biomass carbon using the
non-purge able-organic-C protocol on a Shimadzu TOC 5000 total organic carbon analyzer. Soil microbial
biomass C was estimated by subtracting concentrations of Dissolved Organic Carbon in paired unfumigated samples and soil free controls. Potential activity of hydrolytic enzymes was measured using
established fluorometric techniques with labeled substrate. β-glucosidase [BG] is a cellulase involved in
cellulose decomposition to glucose. Leucine aminopeptidase [LAP] is involved in protein degradation.
Fig 5b. Links between organic
matter (C, N, P), microbial
biomass C and N production,
and exoenzyme activities,
which are in part influenced
by environmental cues.
burn, beetle-kill
After fire, changes in both microbial
community function and organic substrate
quality are evident. Both influence coupled
biogeochemical cycles in ecosystems.
Fig 5a. Soil microbes excrete exoenzymes to decompose
polymers and acquire nutrients. These activities respond to
disturbance and may indicate changing microbial function or soil
quality. Rh estimates could be improved by considering
microbial enzyme activity responses to effects of: temperature,
moisture, microbial biomass C, and population dynamics.
Polyphenols
& hydrocarbons
Lignin
Tannin
Cutin
Suberin
Fatty acids
Humic substrate
Organic
nitrogen
Protein
Chitin
Peptidoglycan
Exoenzyme
processing
Polysaccharides
Cellulose
Hemicellulose
Pectin
Starch
Chitin
Peptidoglycan
Organic
phosphorus
Nucleic acids
phospholipids
Inositol phosphates Exopolysaccharides
CO2
Microbial
Biomass C
Production
Assimilation
ENVIRONMENT
ENZYMES
CELL
Redrawn from Sinsabaugh and Shah (2012).
X-CZO activities and CZ services: These results highlight the importance of incorporating a standard suite
of microbial activity and community assays to enable comparisons across the CZO network and with other
observatories, providing regional-scale microbial biodiversity and activity data in a standardized
framework that can be used to consider CZ services, thus enabling more effective management and
valuation of ecosystem services and improving ecosystem projections under global change scenarios.
Acknowledgements: This research was funded by the U.S. National Science Foundation grants EAR-0724958 and EAR1331408 provided in support of the Catalina-Jemez Critical Zone Observatory. Funding to DJPM was provided by the U.S.
Department Of Energy Terrestrial Ecosystem Science Program DES-C2365077 and by NSF RAPID 1262012.
References: Shao et al. (2013); doi:10.1088/1748-9326/8/3/034034 Sinsabaugh and Shah (2012) DOI: 10.1146/annurevecolsys-071112-124414; Allison et al. (2010) doi:10.1038/ngeo846