John Kimble

Mid-Infrared and Near-Infrared Diffuse Reflectance Spectroscopy for Soil Carbon Measurement

Diffuse reflectance spectroscopy offers a nondestructive means for measurement of C in soils based on the The ability to inventory soil C on landscapes is limited by the reflectance spectra of illuminated soil. Both the NIR ability to rapidly measure soil C. Diffuse reflectance spectroscopic (400–2500 nm) and MIR (2500–25 000 nm) region have analysis in the near-infrared (NIR, 400–2500 nm) and mid-infrared been investigated for utility in quantifying soil C (Dalal (MIR, 2500–25 000 nm) regions provides means for measurement of soil C. To assess the utility of spectroscopy for soil C analysis, we and Henry, 1986; Meyer, 1989; Janik et al., 1998; Reeves compared the ability to obtain information from these spectral regions et al., 1999; McCarty and Reeves, 2000; Reeves et al., to quantify total, organic, and inorganic C in samples representing 14 2001). The characteristics of spectra obtained in these soil series collected over a large region in the west central United regions varies markedly, with the MIR region domiStates. The soils temperature regimes ranged from thermic to frigid nated by intense vibration fundamentals, whereas the and the soil moisture regimes from udic to aridic. The soils ranged NIR region is dominated by much weaker and broader considerably in organic (0.23–98 g C kg 1 ) and inorganic C content signals from vibration overtones and combination bands. (0.0–65.4 g CO3-C kg 1 ). These soil samples were analyzed with and These divergent spectral characteristics may be exwithout an acid treatment for removal of CO3. Both spectral regions pected to have substantial influence on the ability to contained substantial information on organic and inorganic C in soils obtain quantitative information from spectral data. studied and MIR analysis substantially outperformed NIR. The supeOver the last two decades, NIR spectroscopy (NIRS) rior performance of the MIR region likely reflects higher quality of has developed as a major tool for quantitative determiinformation for soil C in this region. The spectral signature of inorganic nations of components within often complex organic C was very strong relative to soil organic C. The presence of CO3 matrices whereas MIR spectroscopy (MIRS) has been reduced ability to quantify organic C using MIR as indicated by improved ability to measure organic C in acidified soil samples. The used mainly in research for qualitative analysis involving ability of MIR spectroscopy to quantify C in diverse soils collected spectral interpretation of chemical structures. The main over a large geographic region indicated that regional calibrations reason for the exclusion of MIRS in quantitative analysis are feasible. has been the belief that quantitative analysis using the MIR region required KBr dilution because of the strong absorptions present (Perkins, 1993; Olinger and Griffiths, 1993a, 1993b). The strength of these absorptions I CO2 content of the atmosphere from ancan lead to spectral distortions and nonlinearities (Culthropogenic sources has stimulated research to assess ler,1993), and could make quantitative analysis difficult the role of terrestrial ecosystems in the global C cycle. or impossible in undiluted samples. Recent work, howThe terrestrial biosphere is an important component of ever, with a number of sample matrices including food the global C budget, but estimates of C sequestration (Downey et al., 1997; Kemsley et al., 1996; Reeves and in terrestrial ecosystems are partly constrained by the Zapf, 1998), forage (Reeves, 1994), and soil (Janik and limited ability to assess dynamics in soil C storage. AgSkjemstand, 1995; Janik et al., 1998; Reeves et al., 2001) ricultural croplands have a great potential for sequesterhas demonstrated that good quantitative measurements ing atmospheric C (Lal et al., 1998), but current technolare possible in the MIR region. These reports have ogies for monitoring soil C sequestration in terrestrial demonstrated that quantitative MIRS analysis can be ecosystems are not cost effective, or they depend on performed on neat (as is) samples with good accuracy. intensive methods. Recent work has demonstrated good ability to establish local (within-field) NIRS and MIRS calibrations for G.W. McCarty and J.B. Reeves, Environmental Quality Laboratory, soil C (Reeves et al., 1999; McCarty and Reeves, 2000; Building 007 Room 201, BARC-West, Beltsville, MD 20705; V.B. Reeves et al., 2001). The diversity of samples included Reeves III, FDA, Rockville, MD; R.F. Follett, USDA-ARS Fort Collins, CO; and J.M. Kimble, USDA-NRCS Lincoln, NE. Received Abbreviations: MIR, mid-infrared; MIRS, MIR spectroscopy; NIR, 4 Jan. 2001. *Corresponding author (mccartyg@ba.ars.usda.gov). near-infrared; NIRS, NIR spectroscopy; PLS, partial least squares; RMSD, root mean squared deviation; SD, standard deviation. Published in Soil Sci. Soc. Am. J. 66:640–646 (2002). MCCARTY ET AL.: INFRARED DIFFUSE REFLECTANCE SPECTROSCOPY 641 Fig. 1. Geographic location of the 14 sampling sites within the west central United States. of soil carbonates involved addition of 100 mL of 0.33 M in these evaluations was limited to a few agricultural H3PO4 to 5 to 6 g of soil and shaking for 1 h. The procedure fields, and a question remained concerning the ability was repeated until the pH of the soil solution remained within to establish broader calibrations across diverse soil types. 0.2 pH unit of that of the original acid solution (Follett et al., The purpose of this study was to compare the abilities 1997; Follett and Pruessner, 2000). These acidified soil samples of MIRS and NIRS to measure total, organic, and inorwere oven dried at 60 C, ground to pass a 180m screen ganic C in a highly diverse set of soils and to assess opening, and analyzed for C by dry combustion. Follett and feasibility of establishing regional diffuse reflectance Pruessner (2000) reported that acidification removed soil inorcalibrations for soil C. ganic C (carbonates), but little or no organic C. However, they did caution that for some soils, acidification may remove neutral sugars and possibly other soluble organic compounds MATERIALS AND METHODS and the significance of this influence needs further investigation. Soil Collection and Conventional Analyses The 273 samples used in this study were soil profile samples Infrared Spectroscopy collected as described by Follett et al. (2001) from 14 geographically diverse locations in the central United States (Fig. Samples were scanned in the MIR from 4000 to 400 cm 1 1). Soil temperature regimes ranged from thermic to frigid (2500–25 000 nm) at 4 cm 1 resolution with 64 coadded scans and soil moisture regimes from udic to aridic. From each per spectra, on a DigiLab FTS-60 Fourier transform spectromlocation, the soil samples were collected from adjacent parcels eter (Bio-Rad, Randolph, MA) equipped with standard DRIFT of land under crop production, native vegetation (never cultioptics under purge and with a custom fabricated sample transvated), and conservation reserve program (CRP) manageport which allowed a 50 by 2 mm sample to be scanned ment. The soils were sampled to a depth of 200 cm by genetic (Reeves, 1996). Samples of ground soil were placed in the horizons with the surface layer sampled at 0 to 5, 5 to 10, and sample cell without sample dilution and no precautions were 10 to 25 cm (bottom of the Ap for cultivated soils). Before used to avoid specular reflection. Log-transformed reflectance analyses, soil samples were air dried, mixed, sieved, and data was used in analysis. Near infrared spectra were obtained ground by a roller mill (180m mesh size). Soil C analyses using a NIRSystems model 6500 scanning monochromator were performed by dry combustion (1500 C) on a Carlo Erba (Foss-NIRSystems, Silver Spring, MD). Samples were scanned C/N analyzer (Haake Buchler Instruments Inc., Saddle Brook, from 1100 to 2498 nm (PbS detector) using a rotating cup. NJ ). Total soil C was determined on unamended soil samples Data were collected every 2 nm (700 data points per spectra) and organic soil C was determined on acidified soil samples. at a resolution of 10 nm. Inorganic soil C was determined by difference between total and organic soil C. The acidification procedure for removal Statistical Analysis Descriptive statistics on soil properties were performed us1 Trade and company names are included for the benefit of the ing SAS data analysis software (SAS, 1988),and analyses of reader and do not imply endorsement or preferential treatment of the product by the authors or the USDA. NIRS and MIRS spectral were performed by Partial least 642 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002 Table 1. Location, soil series, texture, and classification of soils studied. Location Map symbol† Soil series Texture Taxonomic classification Akron, CO COS Weld silt loam Fine-loamy, smectitic, mesic Aridic Argiustolls Indianola, IA IAS Macksburg silty clay loam Fine, smectitic, mesic, Aquic Argiudolls Dorothy, MN DOS Radium loamy sand Sandy, mixed, frigid, Oxyaquic Hapludolls Glencoe, MN GCS Nicollet clay loam Fine-loamy, mixed, superactive, mesic Aquic Hapludolls Roseau, MN ROS Percy loam Coarse-loamy, mixed, superactive, frigid Typic Calciaquolls Columbia, MO MOS Mexico silt loam Fine, smectitic, mesic Aeric Vertic Epiaqualfs Sidney, MT MTS Bryant loam Fine-silty, mixed, superactive, frigid Typic Haplustolls Lincoln, NE NES Crete silt loam Fine, smectitic, mesic Pachic Argiustolls Mandan, ND MDS Farnuf loam Fine-loamy, mixed superactive, frigid Typic Argiustolls Medina, ND MES Barnes loam Fine-loamy, mixed, superactive, frigid Calcic Hapludolls Boley, OK BOS Stephenville loamy fine sand Fine-loamy, siliceous, active, thermic Ultic Haplustalfs Vinson, OK VIS Madge loam Fine-loamy, mixed, superactive, thermic Typic Argiustolls Bushland, TX BLS Pullman clay loam Fine, mixed, superactive, thermic Torrertic Paleustolls Dalhart, TX DHS Dallam fine sandy loam Fine-loamy, mixed, mesic Ari

Conservation tillage for carbon sequestration

World soils represent the largest terrestrial pool of organic carbon (C), about 1550 Pg compared with about 700 Pg in the atmosphere and 600 Pg in land biota. Agricultural activities (e.g., deforestation, burning, plowing, intensive grazing) contribute considerably to the atmospheric pool. Expansion of agriculture may have contributed substantially to the atmospheric carbon pool. However, the exact magnitude of carbon fluxes from soil to the atmosphere and from land biota to the soil are not known. An important objective of the sustainable management of soil resources is to increase soil organic carbon (SOC) pool by increasing passive or non-labile fraction. Soil surface management, soil water conservation and management, and soil fertility regulation are all important aspects of carbon sequestration in soil. Conservation tillage, a generic term implying all tillage methods that reduce runoff and soil erosion in comparison with plow-based tillage, is known to increase SOC content of the surface layer. Principal mechanisms of carbon sequestration with conservation tillage are increase in micro-aggregation and deep placement of SOC in the sub-soil horizons. Other useful agricultural practices associated with conservation tillage are those that increase biomass production (e.g., soil fertility enhancement, improved crops and species, cover crops and fallowing, improved pastures and deep-rooted crops). It is also relevant to adopt soil and crop management systems that accentuate humification and increase the passive fraction of SOC. Because of the importance of C sequestration, soil quality should be evaluated in terms of its SOC content.

SOIL CARBON SEQUESTRATION TO MITIGATE CLIMATE CHANGE AND ADVANCE FOOD SECURITY

World soils have been a source of atmospheric carbon dioxide since the dawn of settled agriculture, which began about 10 millennia ago. Most agricultural soils have lost 30% to 75% of their antecedent soil organic carbon (SOC) pool or 30 to 40 t C ha−1. The magnitude of loss is often more in soils prone to accelerated erosion and other degradative processes. On a global scale, CO2-C emissions since 1850 are estimated at 270 ± 30 giga ton (billion ton or Gt) from fossil fuel combustion compared with 78 ± 12 Gt from soils. Consequently, the SOC pool in agricultural soils is much lower than their potential capacity. Furthermore, depletion of the SOC pool also leads to degradation in soil quality and declining agronomic/biomass productivity. Therefore, conversion to restorative land uses (e.g., afforestation, improved pastures) and adoption of recommended management practices (RMP) can enhance SOC and improve soil quality. Important RMP for enhancing SOC include conservation tillage, mulch farming, cover crops, integrated nutrient management including use of manure and compost, and agroforestry. Restoration of degraded/desertified soils and ecosystems is an important strategy. The rate of SOC sequestration, ranging from 100 to 1000 kg ha−1 year−1, depends on climate, soil type, and site-specific management. Total potential of SOC sequestration in the United States of 144 to 432 Mt year−1 (288 Mt year−1) comprises 45 to 98 Mt in cropland, 13 to 70 Mt in grazing land, and 25 to 102 Mt in forestland. The global potential of SOC sequestration is estimated at 0.6 to 1.2 Gt C year−1, comprising 0.4 to 0.8 Gt C year−1 through adoption of RMP on cropland (1350 Mha), and 0.01 to 0.03 Gt C year−1 on irrigated soils (275 Mha), and 0.01 to 0.3 Gt C year−1 through improvements of rangelands and grasslands (3700 Mha). In addition, there is a large potential of C sequestration in biomass in forest plantations, short rotation woody perennials, and so on. The attendant improvement in soil quality with increase in SOC pool size has a strong positive impact on agronomic productivity and world food security. An increase in the SOC pool within the root zone by 1 t C ha−1 year−1 can enhance food production in developing countries by 30 to 50 Mt year−1 including 24 to 40 Mt year−1 of cereal and legumes, and 6 to 10 Mt year−1 of roots and tubers. Despite the enormous challenge of SOC sequestration, especially in regions of warm and arid climates and predominantly resource-poor farmers, it is a truly a win-win strategy. While improving ecosystem services and ensuring sustainable use of soil resources, SOC sequestration also mitigates global warming by offsetting fossil fuel emissions and improving water quality by reducing nonpoint source pollution.

ACHIEVING SOIL CARBON SEQUESTRATION IN THE UNITED STATES: A CHALLENGE TO THE POLICY MAKERS

Carbon (C) sequestration in soil implies enhancing the concentrations/pools of soil organic matter and secondary carbonates. It is achieved through adoption of recommended management practices (RMPs) on soils of agricultural, grazing, and forestry ecosystems, and conversion of degraded soils and drastically disturbed lands to restorative land use. Of the 916 million hectares (Mha) comprising the total land area in the continental United States and Alaska, 157 Mha (17.1%) are under cropland, 336 Mha (36.7%) under grazing land, 236 Mha (25.8%) under forest, 14 Mha (1.5%) under Conservation Reserve Programs (CRP), and 20 Mha (2.2%) are under urban land use. Land areas affected by different soil degradative processes include 52 Mha affected by water erosion, 48 Mha by wind erosion, 0.2 Mha by secondary salinization, and more than 4 Mha affected by mining. Adoption of RMPs can lead to sequestration of soil organic carbon (SOC) at an annual rate of 45 to 98 Tg (teragram = 1 × 1012 g = 1 million metric tons or MMT) in cropland, 13 to 70 Tg in grazing land, and 25 to 102 Tg in forestlands. In addition, there is an annual soil C sequestration potential of 21 to 77 Tg by land conversion, 25 to 60 Tg by land restoration, and 15 to 25 Tg by management of other land uses. Thus, the total potential of C sequestration in soils of the United States is 144 to 432 Tg/y or an average of 288 Tg C/y. With the implementation of suitable policy initiatives, this potential is realizable for up to 30 years or when the soil C sink capacity is filled. In comparison, emission by agricultural activities is estimated at 43 Tg C/y, and the current rate of SOC sequestration is reported as 17 Tg C/y. The challenge the policy makers face is to be able to develop and implement policies that are conducive to realization of this potential.

Characteristics of cryogenic soils along a latitudinal transect in arctic Alaska

The morphological, chemical, and physical properties of arctic tundra soils were examined along a 200-km latitudinal gradient in northern Alaska which includes two major physiographic provinces; the Arctic Coastal Plain and the Arctic Foothills. Annual air temperature and precipitation increase along the gradient from north to south. Soils on the Arctic Coastal Plain support wet, nonacidic tundra vegetation and have high carbonate contents. Soil on the Arctic Foothills support moist, nonacidic tundra in the northern part and moist acidic tundra in the southern part. Most arctic tundra soils are characterized by medium texture, poor drainage, and high organic matter content. From north to south along the transect, the base saturation of the active layer decreases and exchangeable aluminum increases from north to south. Most soils have strongly developed cryogenic features, including warped and broken horizons, ice lenses, thin platy structure, and organic matter frost-churned into the ice-rich upper permafrost horizons.

Carbon storage along a latitudinal transect in Alaska

Global warming is anticipated to have a significant impact on high-latitude ecosystems which store large amounts of C in their soils and have a predominance of permafrost. The purpose of this study was to estimate the total C storage of different ecosystems along a north-south transect in Alaska. Soil pedons from three Alaska climate zones were studied. These zones were the arctic slope with continuous permafrost and vegetation predominantly tussock tundra and coastal marsh, Interior Alaska with discontinuous permafrost and vegetation predominantly spruce forest on the upland and tundra or bog in the lowland, and Southern Alaska free of permafrost with the vegetation predominantly mixed hardwood and conifers with moss bogs.Soil samples were taken from the representative ecosystems of these zones for carbon storage analysis. In the Arctic and Interior Alaska zones, many soils are cryoturbated and as a result the horizons are warped and often broken. These conditions made it impractical to use the common method for estimating C storage that is used for soils with roughly parallel horizons. For this study the linear proportion of each horizon in the cryoturbated pedon was digitized by using a Geographic Information System (GIS) and the irregular horizons were collapsed to form a simulated profile with parallel horizons. The carbon content of each pedon was then calculated based on the linear proportions. These carbon stores based on the whole soil (1 m deep) approach were compared to other available estimates from the literature.Calculations for pedons from selected ecosystems in Alaska ranged from 169 MgC/ha to 1292 MgC/ha. The organic carbon storage of the arctic coastal marsh pedon amounted to 692 MgC/ha, and that of the arctic tundra pedon amounted to 314 and 599 MgC/ha. The carbon storage of interior forest pedons was 169 and 787 MgC/ha, and the associated organic soil stored nearly 1300 MgC/ha. The carbon storage in the mixed forest and coastal forest pedons was 240 and 437 MgC/ha, respectively. The bog associated with the mixed and coastal forest stored 1260 MgC/ha. Soils with the thickest organic layers were bogs associated with the tundra and boreal forest. These soils had the largest carbon storage. Carbon stores estimated from the whole pedon approach are 30 to 100% higher than those from the literature from the same zones. These data suggest that the global carbon storage estimates based in part on literature values from the N. latitudes, may be underestimated.

Can Near or Mid‐Infrared Diffuse Reflectance Spectroscopy Be Used to Determine Soil Carbon Pools?

Abstract The objective of this study was to compare mid‐infrared (MIR) an near‐infrared (NIR) spectroscopy (MIRS and NIRS, respectively) not only to measure soil carbon content, but also to measure key soil organic C (SOC) fractions and the δ13C in a highly diverse set of soils while also assessing the feasibility of establishing regional diffuse reflectance calibrations for these fractions. Two hundred and thirty‐seven soil samples were collected from 14 sites in 10 western states (CO, IA, MN, MO, MT, ND, NE, NM, OK, TX). Two subsets of these were examined for a variety of C measures by conventional assays and NIRS and MIRS. Biomass C and N, soil inorganic C (SIC), SOC, total C, identifiable plant material (IPM) (20× magnifying glass), the ratio of SOC to the silt+clay content, and total N were available for 185 samples. Mineral‐associated C fraction, δ13C of the mineral associated C, δ13C of SOC, percentage C in the mineral‐associated C fraction, particulate organic matter, and percentage C in the particulate organic matter were available for 114 samples. NIR spectra (64 co‐added scans) from 400 to 2498 nm (10‐nm resolution with data collected every 2 nm) were obtained using a rotating sample cup and an NIRSystems model 6500 scanning monochromator. MIR diffuse reflectance spectra from 4000 to 400 cm−1 (2500 to 25,000 nm) were obtained on non‐KBr diluted samples using a custom‐made sample transport and a Digilab FTS‐60 Fourier transform spectrometer (4‐cm−1 resolution with 64 co‐added scans). Partial least squares regression was used with a one‐out cross validation to develop calibrations for the various analytes using NIR and MIR spectra. Results demonstrated that accurate calibrations for a wide variety of soil C measures, including measures of δ13C, are feasible using MIR spectra. Similar efforts using NIR spectra indicated that although NIR spectrometers may be capable of scanning larger amounts of samples, the results are generally not as good as achieved using MIR spectra.

Soil degradation in the United States : extent, severity, and trends

Soil Degradation in the United States: Extent, Severity, and Trends examines the magnitude and severity of soil degradation by different processes in the U.S., including water erosion, wind erosion, C depletion, soil compaction, salt build-up, and soil contamination. In addition, it addresses policy issues with regard to economic and environmental impact, land use change, and global trends. It covers past trends and future projections regarding soil degradation. The book provides a ready reference and data source for soil scientists and researchers, agronomists, environmentalists, land use planners, land managers, and policy makers.

Crop Residues: The Rest of the Story

Sinking agricultural botanical and soil residues to the deep seafloor may not be a viable option for long-term carbon sequestration.

Effects of climate change on soil carbon and nitrogen storage in the US Great Plains

Soils of the US Great Plains contain enormous stocks of soil organic carbon (SOC) and soil organic nitrogen (SON) that are vulnerable to predicted climate and land use change. Climate change scenarios predict a 2.2°C to 3.6°C (4°F to 6.5°F) increase and more variability in precipitation across most of the United States. This study quantifies management effects (native grassland, Conservation Reserve Program [CRP], and cropped) on SOC and SON stocks across the region and assessed soil variables (soil texture, cation exchange capacity, and others) and climatic drivers (precipitation and temperature) to predict future changes in carbon (C) and nitrogen (N) stocks. Across all sites, cropped land had significantly lower C and N stocks in the 0 to 5 cm (0 to 2 in) and 0 to 10 cm (0 to 3.9 in) depths than native sites, while CRP sites were intermediate. Mean annual temperature (MAT), the ratio of mean annual precipitation to potential evapotranspiration (MAP:PET), soil bulk density (BD), and clay content were important covariates for SOC and SON stocks within land use. Soil C and N stocks under all three land uses were strongly negatively related to MAT and positively related to MAP:PET, suggesting that they are equally vulnerable to increased temperature and decreasing water availability. Based on these empirical relationships, a 1°C (1.8°F) increase in MAT could cause a loss of 486 Tg SOC (536 million tn) and a loss of 180 kg SON ha−1 (160 lb SON ac−1) from the top 10 cm (3.9 in) of soil over 30 years, but the decrease will be mediated by water availability (MAP:PET). Combined, increased temperature and conversion from CRP to cropland could decrease the existing SOC sink, but improved soil management and increased water availability may help offset these losses in the US Great Plains.