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Published online 16 May 2007
Published in Soil Sci Soc Am J 71:1003-1009 (2007)
DOI: 10.2136/sssaj2006.0193
© 2007 Soil Science Society of America
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SOIL & WATER MANAGEMENT & CONSERVATION

Carbon-13 Fractionation of Relic Soil Organic Carbon during Mineralization Effects Calculated Half-Lives

D. E. Claya,*, C. E. Clappb, C. Reesea, Z. Liuc, C. G. Carlsona, H. Woodarda and A. Blya

a Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
b USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108
c Former Research Associate at South Dakota State Univ.

* Corresponding author (david.clay{at}sdstate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The 13C natural abundance approach for determining soil organic C (SOC) stability and turnover has been used to determine SOC mineralization kinetics. These calculations generally assume that 13C fractionation during relic SOC and unharvested biomass mineralization is insignificant. The objective of this study was to determine the impact of this assumption on calculated relic SOC half-lives. Study sites were located in Minnesota and South Dakota. At the Minnesota site, SOC contained in the surface 30 cm of soil in a fallowed area decreased from 90.8 to 73.2 Mg ha–1 during a 22-yr period. Associated with this decrease was a 0.72{per thousand} increase in the soil {delta}13C values (from –18.97 to –18.25{per thousand}). Based on these values, the Rayleigh fractionation constant ({varepsilon}) of relic SOC was –3.45{per thousand}. At the South Dakota site, SOC decreased 10% (2.8 ± 1.8 g kg–1) and {delta}13C increased 3.2% (0.548 ± 0.332{per thousand}) during a 5-yr period. The Rayleigh fractionation constant for this experiment was –6.94{per thousand} (±4.74{per thousand}). In a separate experiment, the {delta}13C value of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] residue remained unchanged after 4 mo. The impact of 13C enrichment during relic C mineralization on calculated C budgets depends on the type of residue returned to the soil. A simulation study showed that for systems where C4 residues are returned to soil derived from C3 and C4 plants, not considering 13C enrichment during relic SOC mineralization will result in underestimating relic SOC half-lives and overestimating the contribution of fresh C4 biomass in the SOC. The effect of 13C enrichment during relic SOC and unharvested biomass mineralization had cumulative impacts on C budgets and did not cancel each other out. The reverse was true for C3 biomass. To minimize these errors, SOC maintenance rate experiments should measure 13C enrichment during relic SOC and unharvested biomass mineralization.

Abbreviations: NHCa, the amount of non-harvested C applied • PCR, plant biomass C returned to soil • PCRincorp, new biomass C incorporated into SOC • SOC, soil organic carbon • SOCretained, the amount of soil organic carbon retained in soil after mineralization • SOCfinal, soil organic carbon contained in soil at the end of the experiment • SOCinitial, soil organic carbon at the beginning of the experiment • SOClost, the amount of organic C lost • {delta}13Csoil final, {delta}13C value of soil at the end of the experiment • {delta}13CPCR, {delta}13C value of plant material remaining in soil after mineralization • {delta}13CSOC retained, {delta}13C of soil organic carbon at the beginning of the experiment that is retained in the soil after mineralization • {varepsilon}, Rayleigh fractionation coefficient


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
To minimize erosion and improve long-term sustainability, SOC must be maintained. Soil organic C maintenance experiments typically rely on measuring nonisotopic changes in SOC with time and using differences in 13C isotopic discrimination in C3 and C4 plants to quantify C transfers across trophic levels. The nonisotopic approach to estimate SOC maintenance requirements is based on the equation

Formula 1[1]
where NHCa is unharvested biomass C returned to the soil, SOCe is soil organic C at the equilibrium point, kSOC is the mineralization rate of soil organic C, and kNHC is the mineralization rate of unharvested C (Clay et al., 2006). Maintenance rates using Eq. [1] are calculated by: (i) defining NHC/(SOC at the beginning of the experiment) as y and (dSOC/dt) as x; (ii) fitting the data to a zero-order rate equation; and (iii) multiplying SOC times the y intercept. Sensitivity analysis of the nonisotopic approach showed that to accurately determine C budgets and maintenance rates, accurate estimates of belowground biomass were needed (Clay et al., 2006).

The 13C natural abundance method for estimating C budgets has been used in systems where plants growing in the soil have different {delta}13C values than the soil. This approach is based on several premises. First, plants with different photosynthetic pathways have different 13C/12C isotopic ratios. Second, biological discrimination during the mineralization process is insignificant when compared with the amount of discrimination that occurs during photosynthesis (Balesdent et al., 1988). This study investigated the ramifications associated with the second assumption. The objectives of this study were to: (i) quantify the amount of 13C isotopic discrimination that occurs during relic SOC and fresh biomass mineralization; (ii) propose a technique for considering 13C isotopic discrimination during relic and fresh biomass mineralization on C budgets and calculated half-lives; and (iii) determine the potential impacts of 13C fractionation during relic SOC and unharvested biomass mineralization on calculated soil C budgets and SOC half-lives.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Experiment
Carbon-13 Isotopic Fractionation during Mineralization
Research sites were located in Minnesota and South Dakota. These sites were routinely cultivated to prevent plant growth. The Minnesota data were previously reported in Clapp et al. (2000) and Dolan et al. (2006). The soil was a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed, superactive, mesic Typic Hapludoll), and the parent materials were a silt loam loess cap (50–80 cm thick) over neutral to calcareous glacial outwash sand and gravel. Initial soil samples (0–15- and 15–30-cm depths) were collected in 1980 (Clay et al., 1989). Duirng the 22 yr of the study, clean tillage was used to prevent plant growth in the alleyways that were approximately 6 m wide. The north and south end of the alleyways were sampled in 1993 (Clapp et al., 2000) and 2002 (Dolan et al., 2006). Bulk densities were determined as reported by Clapp et al. (2000) and Dolan et al. (2006) when soil samples were collected. Composite soil samples were passed through a 2-mm sieve, stones were removed, and roots and residue returned to the soil samples. Ground samples were ball-milled and duplicate samples analyzed on an elemental analyzer (Carlo Erba, Model NA 1500, Milan, Italy) and stable isotope ratio mass spectrometer (Fisons Optima Model, Fisons Middlewich, UK). The {delta}13C values of soil samples were determined using the equation

Formula 2[2]
where R is the ratio of the heavy to light isotopes in the sample and standard (PDB, R = 0.0112372) (Clapp et al., 2000). The working standards were urea ({delta}13C value of –18.2{per thousand}) and soil (–17.6{per thousand}). The CV for the {delta}13C values was 2.8%. Soil organic C on a volume basis was calculated with the measured mass fraction of C and measured bulk density.

Soil at the South Dakota site was a Divide loam (fine-loamy over sandy or sandy-skeletal, mixed, superactive, mesic, Pachic Calciaquoll). The site is located at 44°21' N, 96°49' W. The soil pH (1:1 water) was ~7.5. Soil samples from the 0- to 7.5- and 7.5- to 15-m depths were collected from six different sites in 2000 and 2005. Each sample consisted of 10 individual cores. Samples did not visibly react with 0.5 M HCl. Samples were dried, ground (2 mm), and analyzed for total C and {delta}13C on a Europa 20–20 ratio mass spectrometer (SerCon, Crewe, UK). Each sample was analyzed in duplicate. The working standard was wheat (Triticum aestivum L.) flower, which had a {delta}13C of –24.64{per thousand}. The standard deviation of the working standards were generally <1{per thousand}. Means and 90% confidence intervals for samples collected in 2000 and 2005 were determined. To determine changes in total C and {delta}13C during the 5-yr period, a paired t-test (P = 0.05) was used. In this analysis, the values of the samples collected from the same area at the two dates were subtracted from each other and the mean difference was tested to determine if it was different from zero. The 95% confidence intervals for individual means were determined.

To assess 13C discrimination during corn and soybean residue mineralization, 50 g of dry material, contained in residue bags, was placed on the soil surface. Ten bags of each residue type were used in the study. The {delta}13C value of the initial corn and soybean residue was –11.80 and –27.20{per thousand}, respectively. The C/N ratio for the initial corn and soybean residue was approximately 42:1 and 48:1, respectively. Bags were placed in the field on 17 June 2005 and removed on 25 Oct. 2005. After removing any soil sticking to the residue bag surface, the bags were dried and weighed, gently rinsed with water over a 0.152-mm (100 mesh) screen, dried, ground, and analyzed for {delta}13C, total C, and total N. Means and 95% confidence intervals for total biomass, total C, C/N ratio, and {delta}13C were determined. A paired t-test (P = 0.05) was used to assess changes in {delta}13C with time.

Determining Carbon Budgets
The relic C half-life calculations were based on the following mass balance equations:

Formula 3[3]

Formula 4[4]

Formula 5[5]
where SOCinitial is the SOC in the soil at the beginning of the experiment, SOClost is the amount of SOC mineralized, SOCfinal is SOC at the end of the study, {delta}13Csoil final is the {delta}13C value of SOC when the experiment was completed, PCRincorp is the plant C retained in the soil that was incorporated into SOC, {delta}13CPCR is the {delta}13C value of the plant material retained in the soil after mineralization, SOCretained is the amount of relic C (SOCinitial) retained in the soil at the end of the study, and {delta}13CSOC retained is the associated {delta}13C value. By simultaneously solving Eq. [3] and [4], the equations

Formula 6[6]

Formula 7[7]
were derived. If it is assumed that 13C fractionation during SOC and PCR mineralization is minimal, i.e., {delta}13CSOC retained = {delta}13Csoil initial and {delta}13CPCR = {delta}13Cplant, then Eq. [7] can be simplified into the expression

Formula 8[8]
This equation can be solved if soil and plant material collected at time zero ({delta}13Csoil initial and {delta}13Cplant) and soil collected at the end of the experiment are analyzed for total C and {delta}13C (SOCfinal and {delta}13Csoil final). Equation [8] can be reorganized into

Formula 9[9]
where the ratio between PCRincorp and SOCfinal was the relative proportion (p) of new C incorporated in SOC (p = PCRincorp/SOCfinal). By replacing {delta}13Csoil initial with {delta}C3, {delta}13Cplant with {delta}C4, and {delta}13Csoil final with {delta}, the equations

Formula 10[10]

Formula 11[11]
reported in Wolf et al. (1994) were derived. This derivation shows that Eq. [8, 9, 10, 11] are based on the assumption that 13C discrimination during SOC and unharvested biomass mineralization is minimal. Equations [8, 9, 10, 11] have been used in numerous studies (Balesdent et al.,1988; Follett et al., 1997; Huggins et al., 1998; Collins et al., 1999; Clapp et al., 2000; Allmaras et al., 2004; Clay et al., 2005; Zach et al., 2006). Equation [6] contains three values (SOCretained, {delta}13CPCR, and {delta}13CSOC retained) that are unknown and therefore to derive a solution for Eq. [6], three independent equations must be solved simultaneously. The first two are Eq. [5] and [6]. The third is the Rayleigh equation:

Formula 12[12]
where {varepsilon} is the Rayleigh fractionation constant (Balesdent and Mariotti, 1996). This equation can also be used to calculate the {delta}13C value of the unharvested biomass after mineralization ({delta}13CPCR). The Rayleigh equation has been used to explain isotopic fractionation in a variety of biological systems (Balesdent and Mariotti, 1996; Accoe et al., 2002; Fukada et al., 2003; Spence et al., 2005).

To solve Eq. [5, 6, 12], an iterative approach was used. After stability in the individual C pool sizes, first-order mineralization rate constant (k), half-lives of SOC, and residence times were determined using

Formula 13[13]

Formula 14[14]

Formula 15[15]

Potential Impacts of Carbon-13 Discrimination on Half-Life Calculations
Three different systems were used in this analysis. For Model System 1, corn (a C4 plant) was grown in soil derived from C3 and C4 plants. For this system, the values for SOCsoil initial, SOC soil final, {delta}13Csoil initial, {delta}13Csoil final, and {delta}13Cplant were 96.25 Mg ha–1, 91.4 Mg ha–1, –19.06{per thousand}, –18.754{per thousand}, and –12.0{per thousand}, respectively. For Model System 2, soybean (a C3 plant) plants were sown into soil derived from C3 and C4 plants. In this system, the values for SOCsoil initial, SOCsoil final, {delta}13Csoil initial, {delta}13Csoil final, and {delta}13Cplant were 96.25 Mg ha–1, 91.4 Mg ha–1, –19.06{per thousand}, –19.4{per thousand}, and –28.0{per thousand}, respectively. For Systems 1 and 2, the impact on C budgets and half-lives of 13C isotopic discrimination (six hypothetical Rayleigh fractionation constants, –3.52, –2.14, –1.24, 0, 1.24, and 2.40{per thousand}) during fresh organic matter and relic SOC mineralization was determined for a 13-yr period. For these calculations, it was assumed that 65% of the unharvested fresh biomass was mineralized.

In Model System 3, the potential impact on half-lives of landscape position and 13C fractionation during mineralization was determined. Data previously reported by Clay et al. (2006) were used in this assessment. Soil organic C budgets were developed for SOC relic {varepsilon} values of 0 and –2.52{per thousand}. Clay et al. (2006) reported C budgets for this field when the Rayleigh fractionation constant was assumed to be 0{per thousand}. This scenario investigates the ramification of 13C fractionation ({varepsilon} = –2.52{per thousand}) on these half-lives. As discussed in Clay et al. (2006), >600 soil samples from the 0- to 15-cm soil depth were collected from a 65-ha field located at 44°10' N and 96°37' W. The samples were collected from a 30- by 30-m offset grid in May 1995 and between May and June in 2003. Each sample was a composite that contained 15 individual 1.7-cm-diameter cores collected every 11.4 cm along a transect. Soil samples were air dried (35°C), ground, sieved (2-mm sieve), and analyzed for total N, total C, {delta}15N, and 13C discrimination ({Delta}) on a ratio mass spectrometer (Clay et al., 2003). Total C was corrected for inorganic C (Loeppert and Suarez, 1996). The soil samples from each year were aggregated to a common 40- by 40-m grid cell. The value of a grid cell was calculated as the average value of all the samples contained within a cell. A grid cell SOC value was the difference between inorganic (measured on the 1995 data set) and total C. As discussed in Clay et al. (2006), the grid cells were separated into the elevation zones <523.4, 523.4 to 527.3, 527.3 to 529.74, 529.74 to 532.2, and 532.2 to 534.30 m, which were approximately footslopes, toeslopes, backslopes, shoulder, and summit areas.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Carbon-13 Discrimination during Relic Carbon Mineralization
At the Minnesota site, relic SOC values in the surface 30 cm were 90.8, 77.9, and 73.2 Mg ha–1 in 1980, 1993, and 2002, respectively. These values show that in areas where plant growth was prevented, SOC decreased 19.4% during 22 yr. Based on changes in total C during 22 yr, the relic SOC mineralization k value was 0.0098 and the half-life was 70.7 yr. The relic {delta}13C SOC values at 0, 13, and 22 yr were –18.97, –18.37, and –18.25{per thousand}. The Rayleigh fraction coefficients ({varepsilon}) from 1980 to 1993 and from 1993 to 2002 were –3.91 and –1.92{per thousand}, respectively. During the 22 yr of the experiment, {varepsilon} averaged –3.45{per thousand}. The value from 1993 to 2002 was similar to the long-term {varepsilon} value of –1.71{per thousand} reported for the Versailles experiment (Balesdent and Mariotti, 1996).

At the South Dakota research site, total soil C in the 0- to 15-cm depth averaged 26.8 g kg–1 (±0.53) in 2000 and 24.0 g kg–1 (±0.62) in 2005. Net loss of SOC was 2.8 g kg–1 (±1.8) or 10.4% during 5 yr. Based on these values, the half-life was 31.4 yr. At this site, soil {delta}13C averaged –17.19{per thousand} (±0.95) in 2000 and –16.64 {per thousand} (±1.14) in 2005. The Rayleigh fractionation constant for this soil was –6.94{per thousand} (±4.74). Findings from South Dakota and Minnesota suggest that 13C enrichment during relic SOC mineralization occurred and that the Rayleigh equation could be used to describe this enrichment process. Nedelhoffer and Fry (1988) had similar results and reported that the {delta}13C value of bulk soil organic matter from forest mineral soils increased up to 0.5{per thousand} during a 600-d period. Balesdent and Mariotti (1996) reported that, during a 60-yr period in an experiment initiated in 1928 at Versailles, France, relic SOC decreased 60% and {delta}13C increased 1.6{per thousand} at sites kept free of vegetation. Ueda et al. (2005) reported that {delta}13C values increased with depth. The enrichment of relic C with depth and time has been attributed to respired CO2 from soil microorganisms being depleted in 13C (DeNiro and Epstein, 1978; SantrFormula 15cková et al., 2000).

Carbon-13 Fractionation during Fresh Plant Biomass Mineralization
A 4-mo in-field incubation of corn and soybean residues was conducted to determine the fraction constant associated with fresh biomass. During the 4 mo, 31.5 (±1.94) and 22.8 (±0.44)% of 50 g of corn (C4) and soybean (C3) residues placed on the soil surface were mineralized, respectively. The mineralization of this material did not result in a measurable change in the {delta}13C values of the corn and soybean residues. During the 4 mo, the corn residue C/N ratio increased from 42 to 57 and the C/N ratio in the soybean residue increased from 48 to 62. This increase was attributed to preferential mineralization of compounds with low C/N ratios. Others have also reported that 13C enrichment during fresh plant biomass mineralization is insignificant. Balesdent and Mariotti (1996) summarized the unpublished work of M. Linères (1996, INRA, Unite Agronomie, Laon, France), in which the {delta}13C value of the initial corn biomass did not change after 85% of the biomass had been mineralized. Cleveland et al. (2004) reported that the {delta}13C signatures of dissolved organic matter did not change during decomposition. Griebler et al. (2004) reported that 13C fractionation of trichlorobenzene during mineralization was not observed under aerobic conditions but was observed under anaerobic conditions. Boutton (1996), in a review of isotopic ratios of SOC as indicators of change, stated that, "Direct measurements indicate that the {delta}13CPDB of plant tissue remains relatively constant during the early stages of decomposition (1–7 yr)." Fernandez and Cadisch (2003) reported that, with time, fractionation may even out, with microbes discriminating against 13C (relative to the initial label) during early stages followed by a period of time when microbes discriminate against 12C (relative to the initial label). The apparent lack of 13C enrichment during the early stages of unharvested biomass mineralization may result from two independent processes cancelling each other out. The first factor is that many consumers of SOC tend to accumulate 13C. The second factor is that materials that are resistant to microbial degradation (waxes and lignin) tend to be depleted in 13C (Boutton, 1996; Huang et al., 1999; Conte et al., 2003).

Potential Impacts of Carbon-13 Discrimination Impacts on Half-Lives
Effects of C3 and C4 Residue
In a sensitivity analysis, the potential impacts of treating soil derived from C3 and C4 plants with C4 residue and C3 residue was assessed. For C4 residue, the {varepsilon}plant and {varepsilon}SOC values were negatively correlated to half-lives and relic C remaining in the soil after mineralization (Table 1). For C3 residue, the signs of the correlation coefficient were opposite those observed for C4 residue.


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  		Table 1. The potential influence of plant type on correlation coefficients between calculated half-life, relic C remaining in the soil after mineralization (SOCremaining), the amount of relic C lost during mineralization (SOClost), and new C incorporated into SOC (PCR) and Rayleigh fractionation constants ({varepsilon}) used to calculate 13C isotopic discrimination during unharvested biomass (plant {varepsilon}) and relic SOC (SOC {varepsilon}) mineralization.

 
Carbon-13 fractionation during SOC and unharvested biomass mineralization influenced the calculated relic C half-lives (Fig. 1, Table 2) and amount of plant carbon remaining in the soil after mineralization (PCR) (Fig. 2). When a C4 plant was grown, half-lives of relic C ranged from 35 to 149 yr. Decreasing the {varepsilon}SOC and {varepsilon}plant values increased relic SOC half-lives and reduced the calculated amount of corn biomass incorporated into SOC (Table 2). For C3 plants, different results were observed. Decreasing the {varepsilon}SOC decreased the calculated half-life, while decreasing {varepsilon}PCR increased relic C half-lives. The effect of 13C fractionation on the amount of new C incorporated into the SOC mirrored the half-life results (Fig. 2).


Figure 1
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  		Fig. 1. The influence of the type of plant [(a) C4 and (b) C3] growing at a site and 13C fractionation (e) during the mineralization of fresh unharvested biomass returned to soil and soil organic C (SOC) on the calculated half-life of relic SOC.

 

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  		Table 2. The influence of 13C fractionation during SOC and unharvested biomass mineralization on the half-life, relic SOC residence time (1/k), relic C remaining in the soil (SOCremaining), SOC lost (SOClost), and plant C incorporated into the soil (PCRincorporated) after a simulated time of 13 yr.

 

Figure 2
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  		Fig. 2. The influence of plant type [(a) C4 and (b) C3] and 13C fractionation during the mineralization of fresh unharvested biomass returned to soil and relic soil organic C (SOC) on the amount of fresh plant C retained (PCR) in the soil after mineralization.

 
Not considering 13C fractionation in the relic SOC and unharvested biomass results in a cumulative error (Table 3). To further demonstrate the impact of this error, the ratio between SOCretained and SOCfinal, [({delta}13Csoil final – {delta}13CPCR)/({delta}13CSOC retained – {delta}13CPCR)] (derived from Eq. [6]) was determined for different assumptions, i.e., 13C enrichment during mineralization occurred or did not occur. This ratio represents the relative proportion of SOC in the final sample after mineralization (Table 3). These calculations show that not considering 13C fractionation, in either the relic soil or fresh biomass added to soil, results in cumulative errors. In systems where a C4 plant was grown in soil derived from C3 and C4 plants, not considering 13C fractionation results in underestimating the C derived from the relic C and overestimating the C from the fresh biomass. For C3 plants, the reverse was true.


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  		Table 3. The hypothetical influence of 13C isotopic discrimination on the calculated percentage of soil organic C (SOC) remaining in the soil after mineralization. The percentage of SOC retained relative to the amount of SOC at the end of the study (SOCfinal) was derived from Eq. [6]. PCR initial is the of plant C returned to the soil.

 
These calculations show that 13C discrimination during mineralization had a consistent impact on calculated C budgets. Differences between the C4 and C3 plants were the direct result of 13C discrimination on the relative difference between the {delta}13C value of relic SOC after mineralization and the {delta}13C value of the added residue. For C4 plants, isotopic discrimination decreased this difference while for C3 plants isotopic discrimination increased this difference.

Landscape Effects
To demonstrate the impact of 13C discrimination on the interpretation of real data, SOC half-lives, using several {varepsilon} values, were determined for data previously reported by Clay et al. (2006). If {varepsilon}SOC was 0, then relic SOC half-lives ranged from approximately 50 yr in the tile-drained footslope area to 180 yr in the shoulder areas (Table 4). If {varepsilon} was –2.52{per thousand}, however, then the calculated half-lives almost doubled. Associated with the increase in the half-life was a decrease in the contribution of unharvested biomass to SOC. In addition to influencing half-life calculations, 13C discrimination will impact the mineralization rate constants calculated from these data, which in turn will impact any modeling effort designed to assess long-term changes. Clearly to accurately estimate SOC dynamics when using the 13C natural abundance approach, an accurate estimate of 13C enrichment during SOC mineralization is needed.


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  		Table 4. The influence of the Rayleigh fractionation coefficient ({varepsilon}) and elevation zone on the initial amount of organic C contained in the soil at the beginning of the study (SOCinitial), the amount of relic C present in the soil after mineralization (SOCretained), the amount of relic C lost during mineralization (SOCL), the amount of plant C retained in the soil during the study (PCR), net change in C, and half-life.

 
Landscape differences in SOC half-lives have clear implications for management. This is based on the assumption that relic SOC derived from C3 and C4 plants behave similarly. This assumption may not be valid. Henn and Chapela (2000) reported that 13C discrimination during the mineralization of sucrose derived from C3 plants increased with decreasing O2 tension, and that fungi were 13C enriched when cultured on C derived from C3 sucrose and were not enriched when cultured on C4 sucrose. Differences between C sources were attributed to the 13C atom not being randomly located in the sucrose molecule (Rossmann et al., 1991).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The measured SOC Rayleigh fractionation constants at sites located in South Dakota and Minnesota were negative, while the Rayleigh fractionation constant for NHC ({varepsilon}PCR) was not different from zero. The lack of 13C enrichment during fresh residue mineralization can be attributed to two independent processes (13C enrichment in consuming organisms and 13C fractionation in plant biomass, i.e., lignin tends to be 13C depleted relative to bulk soil) that tend to cancel each other out.

Others have reported isotopic enrichment during biological activity (Nedelhoffer and Fry, 1988; Balesdent and Mariotti, 1996; Boutton, 1996; Rochette et al., 1999; Clapp et al., 2000; Accoe et al., 2002; Fernandez and Cadisch, 2003; Griebler et al., 2004). The negative {varepsilon}SOC value for SOC mineralization was attributed to invertebrates and microbial biomass that feeds on SOC being 13C enriched relative to the bulk soil (DeNiro and Epstein, 1978; SantrFormula 15cková et al., 2000). The effect of 13C discrimination on the SOC {delta}13C value can be significant. For example, if {varepsilon} is equal to –3.52 and 10% of the SOC is mineralized, then the {delta}13C value of the relic C remaining after mineralization will increase 0.37{per thousand} (Eq. [12]). Carbon-13 enrichment can have a profound impact on calculated half-lives and mineralization rate constants. These results were attributed to the impact of {delta}13C discrimination on the calculated relative pool sizes of relic SOC and unharvested biomass C (Fig. 3).


Figure 3
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  		Fig. 3. Diagram showing the impact of plant type (C3 and C4 plants) and 13C fractionation (Rayleigh fractionation coefficeint {varepsilon} = 0 and {varepsilon} < 0) on the relative pool sizes of relic soil organic C (SOC) and unharvested biomass remaining in soil after mineralization. PCR = plant C retained.

 
Carbon-13 discrimination may partially explain lower mean residence time estimated using 13C natural abundance than the 14C dating approach (Paul et al., 2003). To account for 13C discrimination during mineralization, experiments should contain controls where 13C fractionation is measured. The effects of 13C enrichment during relic SOC and unharvested biomass mineralization were cumulative and did not cancel each other out. Findings from this study suggest that experiments that do not account for 13C enrichment will overestimate the contribution of C4 biomass to SOC, underestimate the contribution of C3 biomass to SOC, and underestimate SOC stability.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication May 19, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome