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Soil Science Society of America Journal 63:1429-1435 (1999)
© 1999 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Separation of Root Respiration from Total Soil Respiration Using Carbon-13 Labeling during Free-Air Carbon Dioxide Enrichment (FACE)

Jeffrey A. Andrewsa, Kevin G. Harrisonb, Roser Matamalac and William H. Schlesingerd

a Dep. of Ecology & Evolutionary Biology—MS 170, Rice Univ., 6100 Main Street, Houston, TX 77005-1892 USA
b Dep. of Geology and Geophysics, Boston College, Chestnut Hill, MA 02167 USA
c Dep. of Botany, Duke Univ., Box 90340, Durham, NC 27708 USA
d Dep. of Botany and Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke Univ., Box 90340, Durham, NC 27708 USA

jandrews{at}ruf.rice.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Methods
 Results and discussion
 Conclusions
 REFERENCES
 
Soil respiration constitutes a major component of the global carbon cycle and is likely to be altered by climatic change. However, there is an incomplete understanding of the extent to which various processes contribute to total soil respiration, especially the contributions of root and rhizosphere respiration. Here, using a stable carbon isotope tracer, we separate the relative contributions of root and soil heterotrophic respiration to total soil respiration in situ. The Free-Air Carbon dioxide Enrichment (FACE) facility in the Duke University Forest (NC) fumigates plots of an undisturbed loblolly pine (Pinus taeda L.) forest with CO2 that is strongly depleted in 13C. This labeled CO2 is found in the soil pore space through live root and mycorrhizal respiration and soil heterotroph respiration of labile root exudates. By measuring the depletion of 13CO2 in the soil system, we found that the rhizosphere contribution to soil CO2 reflected the distribution of fine roots in the soil and that late in the growing season roots contributed 55% of total soil respiration at the surface. This estimate may represent an upper limit on the contribution of roots to soil respiration because high atmospheric CO2 often increases in root density and/or root activity in the soil.

Abbreviations: FACE, free air carbon dioxide enrichment • SOM, soil organic matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Methods
 Results and discussion
 Conclusions
 REFERENCES
 
SOILS contain the largest active terrestrial carbon pool on Earth, and through soil respiration, contribute an annual flux of CO2 to the atmosphere that is 10 times greater than that from fossil fuel combustion (Schlesinger, 1997). Because of the size of this flux, even small changes in the rate of soil respiration could have large effects on the atmospheric CO2 concentration. Current studies demonstrate that increased rates of soil respiration may result from increases in atmospheric CO2 (Johnson et al., 1994; Vose et al., 1995; Hungate et al., 1997; Ball and Drake, 1998) and/or soil temperature (Jenkinson et al., 1991; Kirschbaum, 1995; Winkler et al., 1996; Christensen et al., 1997).

Soil respiration is derived from both root respiration and the respiration of soil organic matter (SOM) by heterotrophs. For this study, we define root respiration as the sum of live root respiration, mycorrhizal respiration, and microbial respiration of labile carbon derived from live roots. Sometimes known collectively as rhizosphere respiration, these processes are tightly coupled with plant photosynthesis (Robinson and Scrimgeour, 1995; Horwath et al., 1994). Increases in root respiration may indicate increased carbon inputs to the soil through greater photosynthesis, specific root activity, or root biomass (Rouhier et al., 1996; Hungate et al., 1997). Increases in respiration of SOM by soil heterotrophs, in contrast, reduce the potential for carbon storage in the soil. If we are to predict feedbacks between global change and soil processes, we must first understand the relative contributions of root respiration and respiration by soil heterotrophs to total soil respiration.

The relative contribution of roots to soil respiration is difficult to determine, as reflected by the wide range of published estimates for soils under trees and tree seedlings—from 5% (Philipson et al., 1975) to 90% (Rouhier et al., 1996; Thierron and Laudelout, 1996). Much of the variability in these estimates might originate from the variety of measurement techniques used, each with a unique set of limitations. Estimates that rely on data obtained from greenhouse (e.g., Silvola et al., 1996) or laboratory studies (e.g., Thierron and Laudelout, 1996) may not reflect conditions found in natural environments. Other estimates use highly destructive techniques, such as plot trenching (e.g., Ewel et al., 1987; Bowden et al., 1993) or clear cutting (e.g., Nakane et al., 1983), that create a pulse of dead roots for decomposition by soil heterotrophs while eliminating the continuous input of root exudates and other labile carbon compounds to the soil. Estimates of total root respiration based on direct measurements of excised roots (e.g., Hendrickson and Robinson, 1984; Edwards and Harris, 1977) involve soil disturbance and an extrapolation of the rates from individual roots to a whole system. Root respiration rates may be modified when the soil environment is disturbed because of concomitant changes in environmental CO2 (Burton et al., 1997) and O2 (Palta and Nobel, 1989). Ideally, estimates of rhizosphere respiration are made with minimal alteration of natural environments.

Isotopic tracing techniques have been used to estimate root respiration with a minimum of soil and root disturbance. Previous studies using 13C to estimate root respiration have relied on the natural abundance of 13C, most often capitalizing on the different isotopic composition of photosynthate derived from C3 and C4 plants (e.g., Robinson and Scrimgeour, 1995; Rochette and Flanagan, 1997). Other studies of root respiration used a pulse label (e.g., Howarth et al., 1994) or continuous label (e.g., Liljeroth et al., 1994) of 14C. However, these techniques are usually applied in artificial growth environments, thereby potentially altering the root-soil relationship from the natural condition.

The objective of this study was to estimate the relative contribution of root (i.e., rhizosphere) respiration to total soil respiration with 13C as an isotopic tracer. 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 stable isotopic label in an entire undisturbed forest plot. Here, we use 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.


   Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
Results and discussion
 Conclusions
 REFERENCES
 
The FACE experiment is composed of six 30-m-diam. plots in a 15-yr-old loblolly pine plantation in the Duke Forest, Orange County, NC. Each plot is surrounded by a plenum that delivers gas to an array of vertical vent pipes that extend to the top of the forest canopy. In the three treatment plots, the vent pipes fumigate the plot with CO2 to maintain an atmospheric concentration of CO2 that is 200 µL L-1 above ambient (575 µL L-1 average, May–October 1997); the three control plots are fumigated with ambient air only. Continuous fumigation of these plots began 27 Aug. 1996. A seventh FACE plot at the site, the engineering prototype, is paired with a 30-m-diam. plot of adjacent forest. This plot has been fumigated during the growing season, from March through October, beginning 22 May 1995.

The site was cleared of a mixed forest in 1981, drum chopped and burned in 1982, and established as a loblolly pine plantation in 1983. Soils at the site are of the Enon Series, a low-fertility Ultic Alfisol that is typical of many upland areas in the Southeast. The soil is derived from igneous rock, yielding an acidic (pH = 5.75), well-developed profile with mixed clay mineralogy.

The CO2 used for fumigation is derived from natural gas, and is therefore strongly depleted in 13C . The standard expression of stable carbon isotope ratios is in differential notation (Craig 1953), where:

(1)
and is expressed in parts per thousand ({per thousand}). The accepted standard is the PDB carbonate (Craig, 1957). Using this CO2 to elevate the atmospheric concentration by 200 µL/L changes the {delta}13C of CO2 in the FACE plot from -8{per thousand} to -20{per thousand}. The photosynthetic fractionation by the loblolly pine results in new photosynthate with , as measured in young needles sampled from September 1997 (D.S. Ellsworth, unpublished data). This value agrees well with the theoretical value of -40.0{per thousand}, as calculated after Farquhar et al. (1982) by means of gas exchange measurements from loblolly pine growing in the FACE plots (D.S. Ellsworth, unpublished data).

Soil gas wells were installed in each of the replicated FACE plots at 15- and 30-cm depths and in the FACE prototype plot at 20-cm depth. Gas wells installed at the 15-cm depth were 14 cm long; all other wells were 20 cm long. Each gas well consists of a 5-cm-diam. polyvinylchloride (PVC) pipe situated vertically in the soil. Each PVC pipe was placed in a 10-cm-diam. augured hole so that the bottom rested at the depth of interest, and the space around the pipe was filled with the original soil in reverse order from removal. Each gas well was open at the bottom and closed at the top with a 2-holed rubber stopper. Two 0.6-cm-diam. Kynar plastic tubes extended from each stopper to the soil surface, and the tops of the tubes were sealed by Kynar caps attached with stainless steel Swage-loc tube connectors with nylon ferrules.

Samples of CO2 efflux from the soil surface were taken from respiration chambers inserted 3 cm into the forest floor. Chambers were installed in April 1996 and were left in place, allowing the soil to reequilibrate after any disturbance associated with the cutting of surface roots. Each chamber was a 30.5-cm-diam. PVC pipe, 15 cm in height with 6.4-mm-thick walls. The top edge was machined level and grooved for a 0.3-mm O-ring. Chambers were closed with a methylacrylate lid sealed against the O-ring with Apezion grease before sampling at 60- to 120-min intervals determined by the rate of soil respiration. Because the respiration chambers were not flushed prior to sampling, a lower rate of soil respiration required a longer time for CO2 concentrations inside the chamber to reach a concentration sufficient for analysis. A 2.5-cm2 fan attached to the inside of the lid mixed air inside the chamber. Air in the chamber was sampled through Kynar tubes extending through a rubber stopper in the lid. Gas samples from the soil surface include a fraction of the CO2 from the atmosphere which has a different isotopic signature. However, on the basis of measured soil respiration rates and the average concentration and isotopic composition of atmospheric CO2 at the soil surface, we estimate that atmospheric CO2 in the soil respiration chamber contaminated measurements of the {delta}13C of soil respired CO2 by not more than 2%, excluding one sampling date (February 1997) where the measurement was biased by 3.6%.

Soils from all plots were sampled from 5- to 25-cm depth in September 1997, 396 d after the start of fumigation. Four samples were taken from each plot, and kept on ice until return to the laboratory. Within 4 h of sampling and while at field moisture content, the soil samples were passed through a 2-mm sieve to remove stones and roots; all roots not removed by sieving were picked by hand. The four soil samples from each plot were then composited, and a subsample was removed for mass loss determination of water content after drying at 110°C for 24 h. The composited samples were stored overnight at 5°C. Field capacity for these soils had been determined previously to be 300 g kg-1. For each composited sample, approximately 250 g of root-free soil was placed in a 1-L Mason jar and adjusted to 150 g kg-1 water content with deionized water. The jars were capped with a perforated lid and incubated in the dark at 23.0°C so that we could measure {delta}13C in CO2 derived from the heterotrophic respiration of SOM.

Gas samples taken from the soil gas wells, respiration chambers and incubation jars were collected in 75-cc Whitey stainless steel gas cylinders that were sealed with Nupro bellows valves equiped with Kel-F stem tips. The cylinders were preevacuated in the laboratory to 10-5 Pa. When sampling the gas wells, the sample was first pulled through a portable stainless steel vacuum manifold that was evacuated in the field and purged with two equivalent volumes of sample gas with a hand pump. When sampling the soil respiration chambers, the sample cylinders were attached directly to the access tubes with a stainless steel Swage-loc tube connector. Gas samples were taken from the incubation jars by flushing the jar with CO2-free air for 2 min and allowing CO2 to accumulate for 1 h. Carbon dioxide was concentrated in the samples via cryogenic purification and vacuum distillation (Boutton, 1991), and {delta}13C was determined by stable isotope ratio mass spectrometry (VG ISOGAS series 2) at the Duke University Phytotron.

The fractional contribution of root respiration, f, to soil CO2 or to soil-respired CO2 was calculated following Robinson and Scrimgeour (1995) as:

(2)
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. We expected the {delta}13C of CO2 respired from the rhizosphere to be equal to the {delta}13C of total carbon in roots grown under FACE because there is no fractionation during respiration (Cheng 1996, Lin and Ehleringer 1997). Total carbon {delta}13C from fine roots was measured to be -39.5 ± 0.2{per thousand} (R. Matamala, 1997, unpublished data) consistent with new foliage values. However, for consistency with sampling dates and following Lin et al. (1999), we used the carbon isotope ratio of needles that grew under the FACE atmosphere as an estimate of the {delta}13C of newly produced roots and rhizosphere respired CO2.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results and discussion
Conclusions
 REFERENCES
 
Replicated FACE Experiment
Within 1 wk after beginning CO2 fumigation, the soil respired CO2 and soil CO2 at 15-cm depth in the treatment plots was depleted in 13C relative to the control plots (Fig. 1a, b) . The isotope signal of the FACE fumigation gas was detected in the soil CO2 at 30-cm depth within 60 d of fumigation (Fig. 1c). There was no difference in the average {delta}13C of soil CO2 at 15- and 30-cm depths between treatment and control plots at the start of the FACE experiment (Fig. 1b, c). Because of the rapid appearance of the isotopic label in the soil, we assume that this CO2 was derived from the respiration of new photosynthate by roots and rhizosphere heterotrophs. In a pulse-labeling experiment, Horwath et al. (1994) found that 14C-labeled CO2 was respired from the root systems of 2-yr-old poplar trees (Populus deltoides Bartram ex Marshall) within 12 h of labeling.



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  		Fig. 1 The {delta}13C of CO2 from (a) surface flux, (b) 15-cm depth, and (c) 30-cm depth collected from the replicated FACE experiment in treatment (closed circles, n = 3) and control (open circles, n = 3) plots. Error bars show 1 standard error from the mean. Vertical broken lines show the start of CO2 fumigation

 
The rapid depletion of the {delta}13C of soil CO2 in the FACE treatment plots cannot be attributed to diffusion of the FACE-labeled atmosphere into the soil pore space. Soil CO2 mixes with the atmospheric CO2 through molecular diffusion, and the {delta}13C of soil CO2 can be considered as the product of a two-component mixing model between pure atmospherically derived CO2 and pure biologically respired CO2 produced in the soil (Amundson et al., 1997). The FACE atmosphere contains CO2 with a {delta}13C of approximately -21{per thousand}. This CO2 is less depleted in 13C than pretreatment {delta}13C values in total SOM in the top 35 cm of soil (-26 to -24{per thousand}) (K. Harrison, 1996, unpublished data) and pretreatment {delta}13C values for loblolly photosynthate (-27{per thousand}). In contrast, the {delta}13C of rhizosphere respired CO2 in the treatment plots is estimated to be -39.3{per thousand}, equivalent to that in leaf tissue grown under the FACE atmosphere. Only carbon derived from the products of new FACE plot photosynthate is sufficiently depleted in 13C to account for the magnitude of the label measured in the soil ({delta}13C of -30 to -32{per thousand}).

After one year of fumigation, incubated root-free soil taken from the treatment plots produced CO2 with a versus a produced from root-free soil taken from the control plots. The 3.2{per thousand} shift in {delta}13C reflects the incorporation of the isotopic label into SOM during the first year of FACE. Over the course of a 32-d incubation, the {delta}13C of CO2 evolved from treatment and control plot soils converged (Fig. 2) , demonstrating that the shift in {delta}13C occurred in the labile, or rapidly cycling, SOM pool. This pool receives continuous inputs from root exudates and root detritus. We used the {delta}13C of CO2 respired from the root-free soils as d0—the isotopic signature of soil heterotroph respired CO2 in the field. Then, using Eq. [2], we calculated the percentage contribution of root-respired CO2 to soil CO2 in the soil pore space and to surface-respired CO2 for September 1997 (Table 1) . Essentially all fine roots (<1 mm diam.) at the site are found in the top 20 cm of the soil, with 60 to 70% of those in the top 5 cm of soil (R. Matamala, unpublished data). Given that most roots are found at the top of the soil profile, it is not surprising that roots contribute 55% of the surface flux, but only 26% of CO2 at 30 cm. However, because root respiration is more sensitive to temperature than is bulk soil heterotroph respiration (Boone et al., 1998), the relative contribution of root respiration is likely to shift during the year.



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  		Fig. 2 The {delta}13C of CO2 evolved from root free soil incubations over time in days from sample collection. Soils were collected from FACE treatment (closed circles, n = 3) and control (open circles, n = 3) plots. Error bars show 1 standard error from the mean

 

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  		Table 1 Percent contribution from roots to soil CO2 at various depths and years in the FACE prototype (n = 1) and replicated (n = 3) experiments

 
It is important to note that this technique for the separation of root respiration from soil respiration was applied to data taken from the FACE plots before October 1997. The seasonal maximum in litterfall at the site begins in October, adding plant materials that have assimilated the FACE isotope to the forest floor. Decomposition of this litter leads to further depletion of 13C in soil respired CO2, which would result in an overestimation of root respiration. A three-component mixing model (root derived CO2, aboveground litter derived CO2, and SOM derived CO2) using a two-isotope analysis (13C/12C and 18O/16O) (Lin et al., 1999) would be required to estimate root respiration under these conditions.

FACE Prototype
As a result of root respiration, the isotope signal of the fumigation gas was detected in the soil CO2 at 20-cm depth after 1 wk of fumigation (Fig. 3a) in the FACE prototype plot. The {delta}13C of soil CO2 became increasingly negative throughout the growing season. This pattern contrasts with the relatively constant {delta}13C of soil CO2 in the reference plot, averaging -23.6 ± 0.1{per thousand} (±SE) over 29 sampling dates. During 1995 and 1996, the isotopic composition of soil CO2 increased sharply during brief mid-summer periods in the FACE plot. These periods corresponded to droughts (Fig. 3b) when we would expect a reduced input of the isotopic label as a result of decreased net photosynthesis (Ellsworth, 1999) and/or decreased root respiration (Kosola and Eissenstat, 1994; Burton et al., 1998).



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  		Fig. 3 (a) The {delta}13C of soil CO2 collected from 20-cm depth in the FACE prototype treatment (solid circles) and reference (open circles) plots. The vertical broken lines mark the dates that CO2 fumigation started for the growing season (on) and stopped for the winter (off). Arrows mark the points used for the calculation of percent live root contribution to soil CO2. (b) Daily rainfall (mm) measured at Chapel Hill, Orange County, NC (Office of the State Climatologist, North Carolina State University, Raleigh)

 
At the end of each growing season, when fumigation ended, the 13C-depleted CO2 diffused out of the soil, and the {delta}13C of the soil CO2 in the FACE prototype plot approached that in the reference plot during mid-winter (Fig. 3a). During each successive winter, the {delta}13C of soil CO2 generally equilibrated at increasingly negative values, changing from the pretreatment value of -23.5{per thousand} in the winter of 1995, to -24.7{per thousand} in 1996, to -26.6{per thousand} in 1997, and to -26.0{per thousand} in 1998. We believe this decline in the {delta}13C of soil CO2 results from the incorporation of new, isotopically labeled organic C inputs to the soil from litter, root exudates, and leachates that carry the FACE isotopic label. Measurements of bulk forest floor samples indicate the carbon {delta}13C had shifted from -28.6{per thousand} in the reference plot to -33.4{per thousand} in the treatment plot by April 1997. The winter {delta}13C of soil CO2 may therefore reflect the isotopic signature of SOM mineralized by soil heterotrophs.

Using Eq. [1], we calculated the proportional contribution, f, of root respiration to total soil CO2 in 1997 (Table 1). The {delta}13C of soil heterotrophs, d0, was assumed to be -25.2{per thousand}—the {delta}13C of CO2 evolved from the incubation of root free soils collected from the prototype plot in September 1997. The {delta}13C of bulk soil CO2, d, was measured directly in the field in September 1997. Estimating the {delta}13C of root respired CO2, d1, to be -39.3{per thousand} from foliar data, we calculated the rhizosphere contribution of soil CO2 at 20 cm to be 33%. We repeated this calculation, replacing the measured {delta}13C of soil heterotrophs, d0, with the 1998 wintertime {delta}13C of soil CO2 in the treatment plot . We used the 1998 wintertime value to account for the new, labeled carbon that was added to the soil during the growing season. Here, we found the root contribution to soil CO2 to be 29%. The similarity between these two results suggests that the winter offset between the {delta}13C of treatment and reference plot soil CO2 is a reasonable basis from which to estimate the {delta}13C of soil heterotroph CO2.

We also used this approach to estimate d0 and calculated the root contribution to soil CO2 in the 1995 and 1996 growing seasons. By the end of the growing season, after several months of fumigation, the new 13C label from the roots is equilibrated with the soil atmosphere. Thus, to represent the 13C of bulk soil CO2, d, we used the field measurement taken nearest the time that fumigation ended (Fig. 3). Our estimate of the13C of soil heterotroph CO2, d0, in each growing season was based on the final offset between the 13C of treatment and reference plot CO2 in the following winter. In using this value, we accounted for the labeled carbon added to SOM during the growing season. The results of these calculations are shown in Table 1. For the three seasons of CO2 fumigation, the mean contribution of root respiration to soil CO2 at 20-cm depth is 30.2 ± 4.1% (SE).


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results and discussion
 Conclusions
REFERENCES
 
Using the unique isotopic signature of the FACE fumigation gas and a simple linear-mixing model, we calculated the proportion of rhizosphere respired CO2 that contributes to CO2 in the soil pore space and CO2 respired at the surface. We found that late in the growing season in a 15-yr-old loblolly pine forest in central North Carolina, roots contribute 55% of total soil respiration, 28% of soil CO2 at 15 cm and 26% of soil CO2 at 30 cm. This relative contribution may shift during the year with changing soil temperature. It is also important to note that the high CO2 treatment of the FACE experiment may be increasing the root contribution to soil CO2, either by increasing the specific rate of root respiration, the total fine root biomass, or both. Much of the response of plants to elevated atmospheric CO2 is found in root growth and activity (Rogers et al., 1994). However, as of September 1997 there was no significant effect of FACE on root growth as measured by sequential soil coring (R. Matamala, unpublished data), although the total carbon flux from the soil surface was 11% greater in FACE plots (J. Andrews, unpublished data). Thus our estimates of the root contribution to soil CO2 may be high because of enhanced total belowground respiration with CO2 fumigation. Currently, estimates in the literature of root contributions to soil respiration range from 5 to 90%. This study helps to narrow that range considerably with an estimate that roots contribute 55% of soil respiration, as measured in a forest exposed to high CO2.


   ACKNOWLEDGMENTS
 
We thank Lawrence Giles for his assistance with carbon isotope measurements. This paper was prepared with support from the National Aeronautics and Space Administration, the National Science Foundation, The Electric Power Research Institute, and the U.S. Department of Energy as a contribution to the Duke Forest FACE project.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Methods
 Results and discussion
 Conclusions
 REFERENCES
 
This work was completed while the senior author was at the Dep. of Botany, Duke Univ., Durham, NC.

Received for publication March 4, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results and discussion
 Conclusions
 REFERENCES
 




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