Isotopic Natural Abundance Techniques Plant Carbon in Soil

The 13C isotopic natural abundance C-budget approach can be used to determine the amount of NHC remaining in soil, SOC half-lives, and SOC turnover because relic SOC and new plant material additions have different isotopic values. When making these calculations, it is important to consider that above-ground and below-ground carbon inputs may have different isotopic signatures. For example, plant roots are often 13C-enriched compared to plant leaves (Badeck et al. 2005; Bowling et al. 2008). Furthermore, mycorrhizal fungi are frequently 13C-enriched compared to host plant leaves, probably because mycorrhizal fungi receive 13C-enriched carbon from host plant (Bostrom et al. 2008).

An important benefit of the isotopic approach is that below-ground biomass values are not required. The 13C natural abundance isotopic carbon budget approach is based on C3 plants having lower 813C value than C4 plants (Ehleringer 1991; Clay et al. 2006) and that the signatures can be tracked by placing C3 plant residue into a soil derived from C4 plants or vice versa. In these calculations, several definitions are needed. These definitions include

where 13C and 12C are the amounts of 13C and 12C contained in the sample and standard. By international convention, 813C is always expressed relative to PDB CaCO3 standard. This standard was a limestone fossil of Belamnitella Americana for the Cretaceous Pee Dee formation in South Carolina. It has been assigned the 813C value of 0%e and has been reported to have an R value of 0.0112372 (Craig 1957). Using mass balance relationships, the S13C values in a soil sample and total carbon in soil can be defined by the equations, d 13_ \PCRincorp (d CPCR )+SOCretained (d CSOCretained (8 13)

\ incorp retained J

SOf = PCRincorp + SOC

retained'

SOCimtml - SOCietained + SOClost (8.15)

In these equations SOCinitial is the SOC in the soil at the beginning of the experiment, SOC , is SOC at the «enid of the study, S13C .. f , is the S13C value of SOC when tinal J soil final the experiment was completed, PCR is the new plant carbon incorporated into r r incorp r r

SOC, S13C PCR is the S13C value of the plant material retained in the soil after mineralization, SOC , is the amount of relic C (SOC , ,) retained in the soil at the retained initial end of the study, and S13C SOC retained is the associated S13C value. By simultaneously solving Eqs. 8.13 and 8.14 the equations

S CSOC retained S CPCR )

SOCfinal Csoil final 0 CSOC retained )

^O cpcr o csoc retained )

are derived. If it is assumed that 13C fractionation during SOC and PCR mineralization is minimal, i.e., S13C = S13C and S13C = S13C , then the

SOC retained soil initial PCR plant

PCR equation can be simplified into the expression incorp

SOCfnal ( Csoil final 8

incorp

^soil initial

This equation can be solved if soil and plant material collected at time zero (813C , .,. , and 813C , ,) and soil collected at the end of the experiment are analyzed for soil initial plant' r J

total C and S13C (SOC , and 813C ,, ,). The PCR equation can be reorganized final soil final incorp into the equation

PCRincorp (S Csoil final S C

soil initial

(d13Cpant - S13C

soil initial where the ratio between PCR and SOC , was the relative proportion (p) of new incorp final 1 1 1 '

C incorporated in SOC (p = PCR. /SOCf ,). By replacing S13C , ..ti1 with S 3, incorp final soil initial c3

S13C , , with S and S13C . with S the equations plant c4' soil final 1

reported in Wolf et al. (1994) were derived. The p and S equations are based on the assumption that 13C discrimination during SOC and non-harvested biomass mineralization is minimal. This equation is similar to Eq. 8.10 reported above. Equation 8.20 has been used in numerous papers to calculate the percentage of C derived from C3 and C4 plants (Balesdent et al. 1988; Follett et al. 1997; Huggins et al. 1998; Collins et ail. 1999; Clapp et al. 2000; Allmaras et al. 2004; Clay et al. 2005; Zach et al. 2006). However, extreme care must be used when applying these equations because the assumption that 13C fractionation during SOC and PCR mineralization is insignificant may not be valid for many soils (Stout et al. 1981; Ehleringer et al. 2000; Clay et al. 2007).

Clay et al. (2007) tested the assumption that 13C enrichment during SOC and fresh biomass mineralization did not impact calculated carbon turnover. They showed that 13C enrichment during SOC mineralization occurred during mineralization but did not occur during fresh biomass mineralization. In this analysis, SOC contained in the surface 30 cm of fallowed soil at a Minnesota site decreased from 90.8 to 73.2 Mg ha-1 over a 22-year period. Associated with this decrease was a 0.72%e increase in the soil S13C value (from -18.97 to -18.25%c). At the South Dakota site, SOC decreased 10% (2.8 ± 1.8 g kg-1) and S13C increased 3.2% (0.548 ± 0.332 %e) over a 5-year period. Nadelhoffer and Fry (1988) had similar results and reported that S13C value of bulk soil organic matter from forest mineral soils increased up to 0.5%e over a 600-day period. Balesdent and Mariotti (1996) reported that over a 60-year period in an experiment initiated in 1928 at Versailles, France, relic SOC decreased 60% and S13C increased 1.6%c at sites kept free of vegetation. The vegetation change from the C3 plant wheat to the C4 plant maize has added naturally 13C-enriched material to the soil (Gleixner et al. 1999, 2002). Ueda et al. (2005) reported that S 13C of SOM values increased with depth in forest tropical soils. 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; Agren et al. 1996; Santruckova et al. 2000; Ekblad et al. 2002; Bostrom et al. 2007; Bowling et al. 2008). Furthermore, mycorrhizal fungi were 13C-enriched compared with plant materials (Bostrom et al. 2008).

Different results have been observed for fresh biomass. Clay et al. (2007) reported that the 813C values of corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) residues remained unchanged after 4 months. Balesdent and Mariotti (1996) had similar results and reported that the 813C value of the initial corn biomass did not change after 85% of the biomass had been mineralized. Cleveland et al. (2004) reported that the 813C signatures of dissolved organic matter (DOM) 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 S13CPDB of plant tissue remains relatively constant during the early stages of decomposition (1-7 years)." Fernandez and Cadisch (2003) reported that, over 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 non-harvested biomass mineralization may result from two independent processes cancelling each other out. The first factor is that many SOC consumers 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 (Lichtfouse et al. 1995; Boutton 1996; Huang et al. 1999; Conte et al. 2003). These data suggest that 13C fractionation during SOC mineralization occurs and therefore this assumption should not be accepted without testing. In systems where C4 residue is applied to soil derived from C4 and C3 plants, Clay et al. (2007) showed that the half-life increased when 13C fractionation during relic carbon mineralization was considered. For C3 plants, the reverse was true.

Clay et al. (2006) proposed an approach to account for isotopic discrimination that occurs during relic carbon mineralization. This approach is based on the equation d CSOC retained C soil initial + £SOC In (SOCretained/SOCmM ) (8.22)

where e was the Rayleigh fractionation constant. If fractionation occurs during fresh biomass mineralization, a similar equation can be used. The Rayleigh frac-tionation constant of the SOC (eSOC) is calculated from plots where plant growth is prevented. 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; Wynn et al. 2005). The amount of 13C frac-tionation and the selection of the model to predict fraction (Eq. 8.22) that occurs may be a function of soil texture. Wynn et al. (2005) reported that different models describing 13C accumulation may be needed in coarse- and fine-textured soils. Once the fractionation is identified, carbon budgets are determined using appropriate equations. In Clay et al. (2006) the equations were d CSOC retained Csoil initial + eSOC

ln (SOC

/SOC

retained initial

SOCimtial _ SOCretained "

SOC.

\_SOCfinal Csoil final d CpcR

CSOC retained d CPCR )

After the pool sizes are determined, the first-order rate constant (k), half-life, and MRT can be determined using the equations k = -

ln (S0Cremaining / SOCinitM ) number of years t _ In (0.5)

This approach was used to recalculate the half-lives for a field study. These calculations showed that considering 13C enrichment during SOC mineralization almost doubled the calculated half-lives of SOC when C4 material was added to a soil derived from C3 grasses. An alternative solution to direct measurement of 13C isotopic fractionation is to use a simulation model, such as CENTURY to estimate 13C fractionation. The CENTURY model was calibrated to give a slight increase in the delta 13C value for the total soil organic matter relative to the vegetation (http://www.nrel.colostate.edu/projects/century5/).

The stable isotopic approach can be used to develop carbon budgets for individual sampling points in fields and can be used to develop contour maps (Fig. 8.7). These contour maps visualize the relationships between landscape position and potential carbon storage. In this budget, less new carbon was incorporated into summit shoulder

Annual New C Incorporated (kg C ha 1yr 1)

Annual New C Incorporated (kg C ha 1yr 1)

Footslope Projects

2600 2000 1200 800 -200

Fig. 8.7 Landscape position influence on annual carbon additions from 1995 to 2003 (Clay et al. 2005)

2600 2000 1200 800 -200

Fig. 8.7 Landscape position influence on annual carbon additions from 1995 to 2003 (Clay et al. 2005)

Table 8.2 The influence of landscape position and 13C fractionation on calculated half-lives of SOC at the Moody field (Modified from Clay et al. 2007)

13 Fraction considered

Landscape position No (years) Yes (years)

Footslope 49.8 89.1

Lower backslope 56.1 87.8

Backslope 113.1 232

Upper backslope 181 341

Shoulder/summit 78.9 151

areas than footslope areas. These results were attributed to less biomass being produced in the summit than the footslope area. Similar contour maps can be developed for mineralized carbon and the amount of relic carbon remaining in the soil after mineralization. Based on these maps, the data can be aggregated into landscape positions (Table 8.2) and management recommendations can be implemented.

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