Analysis of Soil Organic Carbon

Soils contain carbon in two forms: organic and inorganic. Organic C is the main constituent of SOM. Inorganic C appears largely in carbonate minerals. Soil organic C is very dynamic, intensively reflects management influences, and exhibits turnover times that range from tens to hundreds of years (Six and Jastrow, 2002). Soil inorganic C, instead, is less responsive to management, and has greater turnover times than SOC. Thus, the emphasis in this section is to summarize methodologies to determine SOC concentration and to provide an update on emerging methodologies for quick and in situ determinations of soil C.

Detailed methodologies of organic C, inorganic C, and total C are provided by Nelson and Sommers (1996) and Tiessen and Moir (1993). Basically, SOC concentration can be determined by either wet or dry combustion. In the wet











(A C


















o Predicted • Measured




Figure 19.2 Predicted vs. measured soil bulk density using soil taxonomy data (Typic Haplustert, Typic Haplustalf, Typic Kandi-udult, Pachic Argiustoll, Aeric Haplaquox, Typic Dystrudept, Typic Molliorthel, Eutric Fulvudand).

combustion procedure, a soil sample is treated with acid dichromate solution in a heated vessel, and then the CO2 generated due to the oxidation of organic matter is evaluated either by titrimetric (indirect) or gravimetric (direct) methods. In titrimetric methods, the amount of organic C present in a soil sample is obtained by back titration of the unused dichro-mate with ferrous ammonium sulfate solution. This method is relatively easy to implement and has been used worldwide for many years. However, in this method, the digestion of organic matter is usually incomplete due to insufficient heating. Correction factors have been reported to correct for this incomplete oxidation, but these factors are soil dependent. Nelson and Sommers (1996) reported correction factors for 15 studies that averaged 1.24 ±0.11, or a mean recovery of 81%. Improvements in recoveries are obtained with the wet oxidation method, with determination of CO2 due to the higher digestion temperatures achieved.

In dry combustion, all forms of C (organic and inorganic, if present) are converted to CO2 at high temperatures achieved in resistance (~1000°C) or induction furnaces (>1500°C) (Nelson and Sommers, 1996). Once generated, the CO2 can be then assessed with a variety of spectrophotometric, volumetric, tit-rimetric, gravimetric, or conductimetric techniques. Dry combustion methods, instrumented in automated systems capable or performing multiple elemental analysis (C, H, N, or S), have become the standard in many laboratories worldwide. They are very accurate and exhibit minimal variability and low operational errors. Dry combustion instruments have a detection limit of about 10 mg C kg-1, and their relative deviation (accuracy) decreases as soil carbon concentration increases. Figure 19.3 presents a comparison of total soil C (%), as measured by two dry combustion instruments. Because dry combustion determines total carbon, extra steps are required for reporting organic C concentration when carbonates are present in the soil sample. If this is the case, then the fraction of total C that is inorganic can be estimated with either an independent measurement of carbonate C or the total C analysis conducted on a carbonate-free soil sample previously treated with an acid solution. Dry combustion should be the preferred methodology for measuring SOC concentration in SCS projects. However, due to its relatively high initial cost (>$20,000), the dry combustion methodology may be difficult to implement in developing countries participating in SCS projects and programs.

Assessment of SCS due to the implementation of alternative practices worldwide will require a technical effort to ensure that the results obtained are accurate and comparable, and include an estimation of the uncertainty associated with the measurements. Assessment of these changes will occur under many environmental conditions, and will have to be provided at a relatively low cost and may have to include numerous measurements within a field in order to detect more continuously the response of SOC to changes in management. With this in mind, various research groups in the United




= 1.0216x + 0.0342

R2 = 0.9453**, n = 171

Figure 19.3 Total soil C as measured by two dry combustion instruments. (Data from Izaurralde et al. 2001b. Soil Sci. Soc. Am. J. 65:431-441.)

States have been advancing and developing instrumentation for fast, in situ measurements of soil C. Three methodologies have been advanced (adapted) so far to measure soil C: (1) laser-induced breakdown spectroscopy (LIBS) (Cremers et al., 2001); (2) mid-infrared (MIRS) and near-infrared (NIRS) spectroscopy (McCarty et al., 2002); and (3) inelastic neutron scattering (INS) (Wielopolski, 2002).

The LIBS method is based on atomic emission spectros-copy (Cremers et al., 2001). In this method, a laser is applied to a (soil) sample, converting it into plasma that emits light whose colors are spectrally resolved. Cremers et al. (2001) calibrated a LIBS instrument that measured total C in soils from east-central Colorado against measurements with a dry combustion apparatus, and used the calibration curve obtained to predict the total C of additional soil samples. Their results indicated that LIBS has a detection limit of 300 mg C kg-1 with a precision of 4% to 5%, and an accuracy ranging from 3% to 14%. The laboratory version of LIBS tested was capable of analyzing samples in less than a minute, with a daily throughput of more than 200 samples. The authors also reported the development of a field version of the LIBS instrument capable of analyzing soil C over large areas and also in depth. Martin et al. (2003) also used LIBS to measure total C and N in samples of soils that had or had not received acid washing to destroy carbonates. Like Cremers et al. (2001), Martin et al. (2003) obtained high correlations between total C measured by LIBS and dry combustion (r2 = 0.962). The latter team, however, reported increased variability in C determinations in soils low in organic matter content due to spectral interference with iron whose peak (248.4 nm) appears very close to that of carbon (247.9 nm).

NIRS is a widely used technique used to characterize organic and inorganic compounds in the chemical, pharmaceutical, agricultural, semiconductor, and other industries. Dalal and Henry (1986) pioneered the use of near-infrared reflectance spectroscopy to determine water content, organic C, and total N in soils. Ben-Dor and Banin (1994) used NIRS to characterize the spectral reflectance of 91 Israeli soils for several soil properties, including carbonate and organic matter content. Because the NIRS approach is empirical, it requires the availability of calibration sets to match the spectral characteristics of the sample. They used 39 soils to calibrate the method, and 52 to validate it. Although predicted and measured SOM values were significantly correlated (r2 = 0.51) within a range of 0% to 12%, NIRS underpredicted SOM concentration at the high end. More recently, McCarty and Reeves (2001) used NIRS and pyrolysis analysis to quantify SOC content from soils under conventional and no-tillage management in central Maryland. One objective of the study was to test whether these methods could be used to understand the spatial structure of SOC distribution in agricultural fields by sacrificing some accuracy in the point measurements. Their findings confirmed that NIRS offers a simple and rapid way for assessing SOC content, but at the expense of some loss in accuracy. McCarty et al. (2002) compared two infrared spectroscopic techniques (mid-infrared and near-infrared) for determining SOC content of 273 soils from the U.S. Great Plains. Overall, the MIRS method was a better predictor of total C (r2 = 0.97) and carbonate C (r2 = 0.99) concentration than the NIRS method (r2 = 0.9 and r2 = 0.96, respectively). The total C in the samples tested ranged from 0% to 10%, while that of carbonate C ranged from 0% to 7%. McCarty et al. (2002) concluded that the MIRS method yields better spectral information than the NIRS method. Because spectral analysis for soil C is nondestructive, requires no reagents, and is easily adaptable to automated and in situ measurements, it could become a key component of methodologies for assessing the spatial distribution of SOC at landscape and regional levels.

With the same objective of developing accurate, in situ, and practical methods to measure soil C, Wielopolski et al. (2002) adapted and tested a method based on neutron scattering principles. In the inelastic neutron-scattering (INS) method, 14 MeV neutrons are applied to the C nuclei present in soil, while recording on a NaI detector the 4.4 MeV gamma rays subsequently generated after the impacts. The C concentration in the soil sample is directly proportional to the intensity of the C peak detected in the gamma spectra. A calibration curve is obtained by running the INS on a sand vessel containing known amounts of C. Preliminary results indicate the INS instrument to be able to detect SOC changes of 100 g C m-2 with a 5% precision. Several field tests of this instrument were underway at the end of this writing.

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