R. F. Follett

Sustainable Biofuels Redux

Science-based policy is essential for guiding an environmentally sustainable approach to cellulosic biofuels.

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

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.

Estimation of slow- and fast-cycling soil organic carbon pools from 6N HCl hydrolysis

Acid hydrolysis is used to fractionate the soil organic carbon pool into relatively slow- and fast-cycling compartments on soils from Arizona, the Great Plains states and Michigan collected for carbon isotope tracer studies related to soil carbon sequestration, for studies of shifts in C 3 /C 4 vegetation, and for “pre-bomb” soil-carbon inventories. Prior to hydrolysis, soil samples are first treated with cold 0.5–1N HCl to remove soil carbonates if necessary. Samples are then dispersed in a concentrated NaCl solution (ρ≍1.2 g cm -3 ) and floated plant fragments are skimmed off the surface. After rinsing and drying, all remaining recognizable plant fragments are picked from the soil under 20x magnification. Plant-free soils, and hot, 6N HCl acid-hydrolysis residue and hydrolyzate fractions are analyzed for carbon content, δ 13 C and 14 C age, and the carbon distribution is verified within 1–2% by stable-carbon isotope mass balance. On average, the recalcitrant residue fraction is 1800 yr older and 2.6% more 13 C-depleted than total soil organic carbon. A test of hydrolysis with fresh plant fragments produced as much as 71–76% in the acid-hydrolysis residue pool. Thus, if plant fragments are not largely removed prior to hydrolysis, the residue fraction may date much younger than it actually is.

Use of a chlorophyll meter to evaluate the nitrogen status of dryland winter wheat

Abstract Chlorophyll meter leaf readings were compared to grain yield, leaf N concentration and soil NH4‐N plus NO3‐N levels from N rate studies for dryland winter wheat Soil N tests and wheat leaf N concentrations have been taken in the spring at the late tillering stage (Feekes 5) to document a crop N deficiency and to make fertilizer N recommendations. The chlorophyll meter offers another possible technique to estimate crop N status and determine the need for additional N fertilizer. Results with the chlorophyll meter indicate a positive association between chlorophyll meter readings and grain yield, leaf N concentration and soil NH4‐N plus NO3‐N. Additional tests are needed to evaluate other factors such as differences among locations, cultivars, soil moisture and profile N status.

Contrasting effects of elevated CO2 and warming on nitrogen cycling in a semiarid grassland

SUMMARY *Simulation models indicate that the nitrogen (N) cycle plays a key role in how other ecosystem processes such as plant productivity and carbon (C) sequestration respond to elevated CO(2) and warming. However, combined effects of elevated CO(2) and warming on N cycling have rarely been tested in the field. *Here, we studied N cycling under ambient and elevated CO(2) concentrations (600 micromol mol(-1)), and ambient and elevated temperature (1.5 : 3.0 degrees C warmer day:night) in a full factorial semiarid grassland field experiment in Wyoming, USA. We measured soil inorganic N, plant and microbial N pool sizes and NO(3)(-) uptake (using a (15)N tracer). *Soil inorganic N significantly decreased under elevated CO(2), probably because of increased microbial N immobilization, while soil inorganic N and plant N pool sizes significantly increased with warming, probably because of increased N supply. We observed no CO(2 )x warming interaction effects on soil inorganic N, N pool sizes or NO(3)(-) uptake in plants and microbes. *Our results indicate a more closed N cycle under elevated CO(2) and a more open N cycle with warming, which could affect long-term N retention, plant productivity, and C sequestration in this semiarid grassland.

Climate change alters stoichiometry of phosphorus and nitrogen in a semiarid grassland

Nitrogen (N) and phosphorus (P) are essential nutrients for primary producers and decomposers in terrestrial ecosystems. Although climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, the impacts of climate change on the relative availability of N with respect to P remain highly uncertain. In a semiarid grassland in Wyoming, USA, we studied the effects of atmospheric CO(2) enrichment (to 600 ppmv) and warming (1.5/3.0°C above ambient temperature during the day/night) on plant, microbial and available soil pools of N and P. Elevated CO(2) increased P availability to plants and microbes relative to that of N, whereas warming reduced P availability relative to N. Across years and treatments, plant N : P ratios varied between 5 and 18 and were inversely related to soil moisture. Our results indicate that soil moisture is important in controlling P supply from inorganic sources, causing reduced P relative to N availability during dry periods. Both wetter soil conditions under elevated CO(2) and drier conditions with warming can further alter N : P. Although warming may alleviate N constraints under elevated CO(2) , warming and drought can exacerbate P constraints on plant growth and microbial activity in this semiarid grassland.

Carbon sequestration in agricultural lands of the United States

Reducing concentrations of carbon dioxide (CO2) and other greenhouse gases (GHG) in Earths atmosphere is identified as one of the most pressing modern-day environmental issues (IPCC 2007). As a signatory country to the United Nations Framework Convention on Climate Change (UNFCCC), the United States is actively engaged in a critical international effort to find solutions to the problems posed by climate change. Agriculture, in addition to being affected by the climate, contributes to climate change through its exchanges of GHG with the atmosphere. Thus, the management of agricultural systems to sequester atmospheric CO2 as soil organic carbon (SOC) and to minimize GHG emissions has been proposed as a partial solution to the climate change problem. In this paper, we discuss the potential role of agriculture in the United States to mitigate climate change through sequestration of carbon (C). We also identify critical knowledge gaps where further research is needed. Carbon enters terrestrial ecosystems, including agriculture, through photosynthesis by green plants that assimilate CO2 and fix it into organic forms (figure 1). Some C eventually enters the soil, where its subsequent cycling and storage among SOC and soil inorganic carbon (SIC) pools determine its residence time and ultimately its return back…

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.