Traditional methods for separating
soil and root respiration have usually involved physical separation
of the two sources. Hanson et al. (2000) broadly categorized
these techniques as component integration and root exclusion.
Component
Integration
Component integration of soil
respiration involves calculating the individual contributions of each
belowground source of CO2 and summing based on weight. Roots and litter
are typically removed from soil samples and respiration rates of each
are measured.
Component integration can be
a relatively simple technique in estimating root and soil respiration.
In situations where in situ measurements are not possible, it may be
feasible to estimate or model respiration by measuring soil bulk density
and root mass by species, if species- and site-specific respiration
values are known. These results should be verifiable by comparing them
to total soil respiration over a sufficient integration time.
Vose and Ryan (2002) used
an infrared gas analyzer with custom foam-sealed Lexan cuvettes to measure
respiration from white pine (Pinus strobus) roots directly. Roots
near the soil surface were exposed by removing the litter layer and
washed free of soil with deionized water. Roots were not detached from
the tree during analysis. New sets of fine roots were exposed and measured
during each monthly measurement, while coarse roots were fitted with
permanent chambers. Root respiration rates were only compared to foliage
and woody tissue, but not soil respiration rates.
Burton et al. (1997) found
that sugar maple (Acer saccharum) root respiration rates measured
in vitro decreased with increasing CO2 concentrations. CO2 concentrations
were controlled with an open system IRGA and an O2 electrode cuvette.
Respiration rates decreased sharply from 350 ppm but leveled off at
CO2 concentrations higher than 3000 ppm. Root respiration measurements
made at near-atmospheric CO2 concentrations are likely overestimate
actual respiration by about 39%. While these results supported previous
efforts, the mechanism for root respiration inhibition is still uncertain.
To compensate for higher concentrations
of CO2 in the soil than in the cuvettes, Vose and Ryan used a correction
factor of 1/2.5 for fine root respiration measurements. Accounting for
soil CO2 concentrations is an important consideration; however, use
of correction factors could be avoided my making use of a portable IRGA
device that can control CO2 concentrations within the cuvette.
Perhaps the biggest disadvantage
of the component integration method is that separation of the roots
and litter from the soil is a serious disturbance to the natural matrix.
Removal of litter alters soil moisture, microbial species composition,
and gas diffusivity. In vitro analysis of root tissue requires cutting
the root from the tree, while in situ measurements can be made with
portable equipment. This process can also drastically alter the rhizosphere
environment. (Hanson et al. 2000) Experiments must allow for
adequate time for disturbed roots and soil to equilibrate after disturbance,
but should be analyzed before desiccation or death occurs. Rustad (2000)
also notes that spatial and temporal variations lead to potential problems
with scaling-up from chamber experiments, to the stand, ecosystem, or
global level.
Root
Exclusion Experiments
Experiments using root exclusion
compares respiration measurements taken from soil with and without living
roots. The primary method of root exclusion experiments involves trenching
and physically excluding new root growth into sample plots. In the gap
formation method, vegetation is cleared or killed and the vegetation-free
soil is compared to similar, undisturbed soil.
Trenching
Trenching with root exclusion
is a straightforward approach to measure soil respiration without roots
on relatively undisturbed soil using standard surface flux techniques.
It can be applied in a variety of ecosystems, although may require extensive
labor and root barriers in locations with deep soil.
Bowden et al. (1992) set
up a series of 3 x 3 m plots in an 80-year-old hardwood stand in Harvard
Forest. Trenches were dug 0.5 m outside of their plots to a depth of
70-100 cm (20 cm below the rooting depth) and roots were excluded with
corrugated fiberglass sheets. Root respiration was calculated by subtracting
annual CO2 fluxes from the no-root plots from control plots. Root contribution
to total soil respiration was found to be 33% or 123 g C m-2 year -1.
This study assumed that root respiration is a constant proportion of
the total respiration throughout the year. Decomposition of the severed
roots was not differentiated from soil respiration in the no-root plots.
If the rate of decay of these roots is higher than soil, these results
may underestimate the total contribution of root respiration. The authors
acknowledged this factor, but noted other studies in which analysis
of severed roots suggested that carbon in dead roots does not turnover
rapidly.
Buchanann (2000) ran a
series of root exclusion experiments with staggered start times, allowing
for the comparison between recently cut roots and roots that had been
dead for up to 6 months. Results showed that soil respiration collars
(10 cm deep, 10 cm internal diameter) for a portable IRGA system installed
24 hours prior to sampling were 20 to 30% higher than collars that had
been installed from 1 to 6 months earlier. No significant difference
in respiration rates was seen in collars installed between 1 and 6 months
earlier, suggesting that respiration rates stabilized shortly after
the initial disturbance. One possible explanation for the high initial
rates of decomposition that Buchanann does not propose is that organic
matter around the rhizosphere (including fine roots, micorrhizae, soil
microorganisms, and labile organic matter) continues to contribute to
surface flux shortly after killing the root (< 1 month).
Root exclusion experiments also
involve disturbance to the soil, although since larger plots can be
used, the severity is arguably less. Hanson et al. (2000) suggest
that when transpiration ceases after root exclusion soil moisture may
increase, ultimately leading to altered heterotrophic respiration rates
depending upon preexisting soil moisture conditions. Gap experiments
that remove a substantial portion of the canopy can change the radiative
balance of the soil. Canopy removal results in increased incoming shortwave
radiation and leads to increased soil temperature during the day but
traps less longwave radiation, leading to colder temperatures at night.
Since much of respiration occurs in the upper organic layers, temperature
changes can affect respiration rates.
Root exclusion experiments also
involve disturbance to the soil, although since larger plots can be
used, the severity is arguably less. Hanson et al. (2000) suggest
that when transpiration ceases after root exclusion, soil moisture may
increase, ultimately leading to altered heterotrophic respiration rates
depending upon preexisting soil moisture conditions. As noted in the
precious section on component integration, applying the results from
these small-scale experiments to larger scales comes with sizable uncertainty.
Gap
Formation
Gap formation experiments do
not seem to be as common as root exclusion, possibly because of the
severe disturbance typically involved. Scale of clearing is an obvious
consideration when assessing gap results. The labor required for large
scale gaps in forests can be very large (unless the study follows a
disturbance such as windthrow or logging), while small scale grassland
experiments may be ideal for this technique.
Hanson et al. (2000) describes
several experiments using 30 m gaps that set out to compare vegetation-free
soil with nearby vegetated soil. Deforested plots were shaded and moisture
controlled, to some extent. Respiration rates of roots ranged from 40%
to 51%, but may have contained a significant contribution by the dead
roots.
Smaller gap sizes were used in
a study by Ohashi et al. (2000) in an attempt to minimize alteration
of soil properties. The authors cleared four Japanese cedar (Cryptomeria
japonica) trees to create a 2.5 x 2.5 m plot and measured respiration
with an open system IRGA. Respiration rates at the cleared plot did
not differ significantly during the first year and decreased by year
two, suggesting that stored carbohydrates and fine roots decomposed
over the first year, maintaining high total soil respiration. Root respiration
accounted for 57% of total soil respiration over the year. Monthly measurements
illustrated a seasonal trend of higher proportionate rates of root respiration
in the summer and lower rates in the winter.
Gap experiments represent an
attempt to examine respiration on a larger scale than chambers or cuvettes
are able to do. However, experimentation in vegetation changes at this
scale must be carefully designed and maintained to minimize effects
on soil temperature and moisture. Gap experiments that remove a substantial
portion of the canopy can change the radiative balance of the soil.
Canopy removal results in increased incoming shortwave radiation and
leads to increased soil temperature during the day but traps less longwave
radiation, leading to colder temperatures at night. Since much of respiration
occurs in the upper organic layers, temperature changes can affect respiration
rates. Large scale gap studies are useful methods in quantifying respiration
changes after disturbance or land use change, but present many potential
problems and confounding factors. Reported respiration rates from recently
clearcut forests range from no significant change to twice that of control
plots (Raich and Schlesinger 1992).
Home
Introduction
Background
Traditional
Methods Isotopic
Methods New
Directions
References
About
the Authors
Biology
265 Duke
Biology Home
Duke
Ecology
Duke
University