New School:
Using Isotopes to Separate Components of Soil Respiration
Labeling
techniques
The use of radioactive or stable isotopes of carbon
to partition soil respiration components has an advantage over conventional
techniques because partitioning can be done in situ, thus avoiding
the disturbance of the root-soil system. Both 14C and 13C
can be used to determine CO2 evolved from rhizosphere respiration
and heterotrophic respiration of soil carbon. Distinctions can be made
between root-derived carbon (root respiration, respiration of root exudates)
and soil-derived carbon (soil organic matter) by examining the differences
in isotopic composition of evolved CO2. The isotopic signature
of root-derived carbon will reflect its carbon source, the labeled CO2
that was applied to the plant (Coleman and Fry, 1991). Therefore, the CO2
evolved from the respiration of that root-derived carbon will also have
the same isotopic signature as the labeled CO2.
Isotopic analyses of soil-respired
CO2 fall under two main categories, pulse labeling and continuous
labeling.
Pulse
labeling
Plants are exposed to a one-time
application of 14C- or 13C-labelled CO2
for the purpose of quantifying the distribution of labeled C
within a plant and the amount of above- and below-ground labeled C
during a given period of time. A variation of pulse labeling is repeated
pulse labeling, where labeled CO2 is applied to plants
at different times during the growing season. Plants are exposed to
14C- or 13C-labeled CO2 inside a
chamber in which belowground plant parts are separated from aboveground
parts (see Figure below, from Warembourg and J. Kummerow, 1991)
The time period between application of labeled CO2
and the final experimental measurements is called the "chase
period." The selection of the chase period is dependent on the
amount of time needed for allocation of the labeled C (Paterson
et al., 1997). Immediately following exposure to labeled CO2,
labeled C within the plant will be predominantly distributed within
labile C-pools. Longer chase periods will result in more of the labeled
C being incorporated into less labile pools (structural and storage).
During and after exposure, CO2 is collected from
the root container and analyzed for 14C.
Using pulse-labelling in a pasture
system, Saggar and Hedley (2001) found 14C-CO2 losses to be as high
as 66-70% during summer, autumn, and winter. Low rates (37-39%) were
found during the spring.
Disadvantages of pulse labeling
·
This approach cannot be utilized on C pools with rapid turnover
(plant metabolic and rhizosphere microbial biomass) without incorporating
further assumptions into a modeling approach (Meharg, 1994).
·
Non-homogeneous labeling of 14C throughout the
plant leads to difficulties in determining sources of carbon for respiration
studies (Coleman and Fry, 1991).
·
14C is preferentially allocated to labile
carbon pools, which could lead to overestimation of rhizosphere respiration
rates (Paterson et al., 1997)
Continuous labeling
In continuous labeling, plants are exposed to labeled
CO2 throughout the growth of the plant, which results in
uniform labeling of all plant C-pools, including labile metabolic
substances and more stable structural components. Few field studies
are available on 14C continuous labeling. However, Free Air Carbon
dioxide Exchange (FACE) experiments using continuous 13C exposure
are proving to be successful in determining carbon pools (Andrews
et al. 1999) (See FACE studies, below).
Disadvantages of continuous labeling
Continuous labeling gives only cumulative data regarding
carbon transport belowground. It is not possible to calculate transport
of carbon below ground over a given time period as this flux cannot
be partitioned from label already in the root and soil (Meharg,
1994). However, the information on total carbon inputs from the
plant into the soil that is gained from continuous labeling is an
important part of the ecosystem carbon budget (Meharg, 1994; Paterson
et al., 1997). Field studies using 14C continuous labeling are
very difficult due to the complexity of the experimental setup.
Bomb carbon
The labeling of atmospheric CO2 labeled with
14C from bomb carbon provides a unique type of continuous
labeling experiment where one can calculate rhizosphere- versus soil-respired
CO2 based on the content of 14C in the atmosphere,
soil organic matter, and soil respiration (Dorr and Munnich, 1986).
Rhizosphere-respired CO2 can be assumed to reflect the 14C
signature of atmospheric CO2, whereas CO2 evolved
from the decomposition of organic matter has a less modern 14C
content due to its longer turnover time.
Gaudinski et al. (2000) partitioned soil respiration in Harvard Forest
using measurements of 14CO2 in the soil atmosphere and in total soil
respiration combined with surface CO2 fluxes and a soil gas diffusion
model. They determined that reservoir-C (from soil organic matter that
resides in the soil for several years or longer) contributes approximately
41% of annual total soil respiration.
from Gaudinski et al., 2000
Figure 1. The time record of 14C in the atmosphere (Northern
Hemisphere) based on grapes grown in Russia (Burchuladze et al. 1989)
for 1950–1977 and direct atmospheric measurements for 1977–1996
(Levin & Kromer 1997). We express radiocarbon data here as Δ
14C, the difference in parts per thousand (per mil or ‰) between
the 14C/12C ratio in the sample compared to that of a universal standard
(oxalic acid I, decay-corrected to 1950). All samples are corrected
for mass-dependent isotopic fractionation to -25‰ in 13C. Expressed
in this way, Δ 14C values greater than zero contain bomb-produced
radiocarbon, and those with Δ 14C less than zero indicate that
carbon in the reservoir has, on average, been isolated from exchange
with atmospheric 14CO2 for at least the past several hundred years.
The 14C content of a homogeneous, steady state C reservoir with turnover
times of 10, 50 or 100 years is compared with that of the atmosphere
through time.
Free Air Carbon dioxide Exchange studies
Recently, Free Air Carbon dioxide Enrichment (FACE) technology
was developed to study the effects of high CO2 on intact
ecosystems without the use of enclosures (Hendrey et al., 1993).
When the CO2 used to fumigate these experiments is derived
from the combustion of natural gas, it contains a unique 13C
signature that can be followed through the experimental plots. This
carbon is strongly depleted in 13C and functions as a continuous
isotopic label in an entire undisturbed forest plot.
Isotopic signatures of 13C are reported in δ13C
(‰):
δ13C = (Rsample - Rstandard)/Rstandard * (1000), where
R = 13C/12C.
Andrews et al. (1999) used the 13C label applied by
a FACE system in a loblolly pine forest to calculate the relative contribution
of root respiration to total soil respiration. They calculated the fractional
contribution of root respiration, f, to soil CO2 or
to soil-respired CO2 using the following equation:
f = (d - d0)/(d1 - d0)
where d is the isotopic ratio of soil or soil respired CO2
at a given time, d0 is the isotopic ratio of CO2
of heterotrophic origin, and d1 is the isotopic ratio
of root-respired CO2. The δ13C of CO2
respired from the rhizosphere was expected to be equal to the δ13C
of total carbon in roots grown under FACE because there is no fractionation
during respiration (Cheng, 1996; Lin and Ehleringer, 1997). The
carbon isotope ratio of needles that grew under the FACE atmosphere
were used as an estimate of the δ13C of newly produced
roots and rhizosphere respired CO2.
Their results showed that root respiration contributed 55% of the surface
soil respiration, 28% at 15cm depth, and 26% at 30cm depth.
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