2011 Annual Report
1a.Objectives (from AD-416)
1. Improve databases and provide other information on the soil C stocks, C-sequestration, and greenhouse gas (GHG) emission and fluxes of current and/or new soil management and cropping systems by use of measurement and/or model based approaches.
2. Develop sustainable biofuel production systems and practices that result in improved soil C sequestration, efficient use and recycling of applied nutrients (especially N), and minimal GHG emissions.
3. Characterize and measure the influences and processes of environmental and plant-related controls on C and N cycling and storage in soils.
4. Develop processes, primarily microbiological, that.
1)minimize soil-borne disease thereby maintaining soil ecosystems and crop productivity,.
2)enhance biofuel production, and.
3)remediate damaged, and protect undamaged, soil and water.
5. Develop management practices and decision support tools that improve nutrient use efficiency and reduce losses in agricultural systems.
1b.Approach (from AD-416)
This research addresses priority science questions in the strategic plan for the
Climate Change, Soils and Emissions Research (NP212) and provides a scientific
foundation for decision-making and policy development. The five objectives in this
project integrate studies focusing on effects of management practices on soil
processes and sustainable crop growth and yield. Objective 1 addresses the need for
improved databases and provides other information on the soil C stocks, C sequestration, and GHG emission and fluxes of current and new soil management and
cropping systems. These data bases will be used to improve models and decision
support systems. Objective 2 studies address sustainable biofuel and irrigated
conservation tillage production systems and practices to improve soil C
sequestration, efficient use and recycling of applied nutrients (especially N), and
minimize GHG emissions. Objective 3 studies focus on characterizing and measuring
the influences and processes of environmental and plant-related controls on C and N
cycling, pool dynamics, and storage in soils. Objective 4 examines soil productivit and remediation tools that reduce soil-borne diseases and protect soils and soil waters from damage from soil contaminants. The goal is to utilize microbial isolates and communities for bioenergy production, biocontrol of soil-borne pathogens, and to remediate damaged soils. Objective 5 studies focus on improving nutrient use efficiency, especially N, and developing more advanced decision support tools (e.g., indices and models) for improved nutrient management. The overall SPNR research focus is on biological processes and management practices that influence SOC, GHG emissions, microbial and rhizosphere biology, pesticide/nutrient/contaminant removal from soil, bioenergy production, and nutrient use efficiency. All of these research projects share the common goal of improving or maintaining farm profitability while practicing sustainable and environmentally friendly agriculture.
Objective 1: Improved databases are required to assess land uses to reduce greenhouse gas (GHG) emissions. A web based database with GHG flux data is a product of GRACEnet. The software and a protocol is developed using actual GRACEnet data. Released the U.S. Agriculture and Forestry Greenhouse Gas Inventory.
Objective 2: Impacts of biofuel using DayCent were done at scales us to river basin size. Field and laboratory work were completed as scheduled, and manuscripts prepared and published on effects of N fertilizer on grain and residue yields, and impact on soil C and N under plow tillage and no-till. Manuscripts published on N source and nitrous oxide emissions under irrigated, no-till corn and on measuring ammonia loss in small plots.
Objective 3: Soil carbon sequestration was found to be underestimated for maize and switchgrass grown for bioenergy. Underestimation was by 60-100 percent when soils depths of 30-100 cm depths included rather than only to 30 cm as done previously. Manuscript submitted. The dynamics of soil C and soil N, although related, are not identical, thus soil C or N cycling can be targeted by management to improve ecosystem functioning and maintain soil organic matter dynamics to help minimize impacts of greenhouse gases on global change. Manuscript accepted.
Objective 4. Shallow vadose zone biobarriers remove nitrate from percolating water. A pilot scale biobarrier is designed and installed to intercept runoff from a cattle pen. Aerobic and anaerobic biobarrier to remediate sulfacholorpyridazine contaminated groundwaters completed. Investigation of selenium movement through soils was complete. Microbial fuel cell study published. Samples collected for microbial diversity from a potato field trial in southern CO. Nearly 2000 bacterial isolates have been collected and isolates are being screened for bioactivity against Phytophthora spp. and for lignin degradation activity. A novel lignin degrading isolate has been identified and is appropriate culture collection.
Objective 5: Ho 5.1a: All planned field and laboratory work completed as scheduled. Another year of field work was started in FY2011 for the needed observations to complete the studies field phase. A manuscript was accepted for publication on cooperative N management work with CSU at Rocky Ford, CO. Nitrogen losses from agricultural systems impact environmental and new tools are needed to help assess these losses. New USDA-ARS tools include the Nitrogen Trading Tool, Nitrogen Index 4.3, and NLEAP-GIS 4.2, now all calibrated and validated, and released via the ARS webpage (http://www.ars.usda.gov/npa/spnr/nitrogentools). These tools have been extensively downloaded for use both nationally and internationally to improve practices to decrease nitrogen losses to the environment.
U.S. Agriculture and Forestry Greenhouse Gas Inventory published by USDA. Interest in climate change and efforts to reduce the environmental impacts of agricultural production systems highlight the need for more accurate methods to quantify greenhouse gas emissions from the US agricultural sector. The recently published U.S. Agriculture and Forestry Greenhouse Gas Inventory 1990-2008 features state of the art methods to calculate emissions and their associated uncertainty ranges. The report partitions emissions spatially and by source category so policy makers can identify where mitigation efforts should be targeted. Results from this inventory are also included in the annual Inventory of US Greenhouse Gas Emissions and Sinks published by EPA and reported to The United Nations Framework Convention on Climate Change (UNFCCC). The website hosting the report has received several hundred visits per month.
Environmental impacts of biofuel feedstock production systems. The Energy Independence and Security Act of 2007 mandates the production of 36 billion gallons of biofuel by 2022 and that the greenhouse gas emissions associated with producing this fuel are at least 20% lower than petroleum based fuel. ARS Researchers in Fort Collins, CO, used field data from biofuel feedstock plots in the central and eastern US to verify the ability of a commonly used computer model called DAYCENT to represent the ethanol yields and greenhouse gas emissions for different crops. They then used the model to quantify yields and greenhouse gas emissions for lands currently used for corn ethanol production and to project yields and emissions if this land was converted to cellulosic ethanol production using perennial crops. Results suggest that if land currently used for corn ethanol production were converted to perennial crops, ethanol production could increase from 7 to 12 billion gallons while greenhouse gas emissions from soil would decrease by approximately 400%. These finding are critical for efficient energy production from plant feedstocks for the United States.
Book, “Advances in Nitrogen Management for Water Quality”. Reactive nitrogen losses increase nitrate (NO3-N) concentrations in groundwater resources, potentially impacting drinking water quality and safety, and contribute to surface water impacts such as hypoxic zones. A book was published about recent advances in nitrogen management that can help reduce reactive nitrogen losses; this book was a cooperative effort with researchers who presented at the 7th joint symposium of the Soil and Water Conservation Society and the Soil Science Society of America. The book, Advances in Nitrogen Management for Water Quality (2010) can be purchased at http://www.swcs.org/en/publications/advances_in_nitrogen_management_for_water_quality/ and select chapters are accessible for free online. This book discusses viable management solutions to reactive nitrogen losses as well as management approaches to increase field-level nitrogen use efficiencies, and can be used by peers working in nitrogen management in the U.S. and internationally.
Web-based Book, “GRACEnet Sampling Protocols”. GRACEnet is an acronym derived by contraction of the project’s title, “Greenhouse Gas Reduction through Agricultural Carbon Enhancement network.” GRACEnet represents a coordinated national effort that was established in 2005 by the Agricultural Research Service of USDA. As part of this effort guidelines were established to allow research collaborations and common sampling and sample collection protocols across many locations and within different agro-ecosystems as appropriate to assess the role of agricultural management systems at local, regional, and national scale. The use of common management scenarios, consistent sampling protocols, and detailed record keeping will facilitate cross-location and cross-regional comparisons and ensure quality control even though the soils, crops and condition will be location specific. The chapters in this book were prepared by leading ARS scientist. It is located on the ARS GRACEnet website < http://www.ars.usda.gov/research/GRACEnet > and is available to everyone who visits the website. Copies on CD have also been shared with about 30 countries who are participants in the Global Research Alliance (GRA).
Historical land use changes on greenhouse gas exchange in the U.S. Great Plains. To reduce the impacts of current agricultural practices on greenhouse gas emissions it is useful to understand how historical cropping systems impacted emissions. Detailed land use data from 21 representative counties in the Great Plains were used to provide inputs for computer model simulations from 1883 to 2003. Initial cultivation of native grass and continued farming produced a significant loss of soil carbon over the initial decades, and declining soil fertility led to reduced crop yields. Once soil carbon levels stabilized at a new, reduced level, greenhouse gas emissions associated with soil carbon losses were also mitigated. Later, irrigation, fertilizer application, and reduced plowing intensity restored soil fertility and increased crop yields, but led to increased nitrous oxide (a potent greenhouse gas) emissions that reversed the decline in net greenhouse gas release. This shows that greenhouse gas emissions from both loss of soil carbon and application of fertilizer and other farm inputs need to be considered to accurately assess net greenhouse gas fluxes.
GRACEnet web accessible database. GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network) systems are needed to organize and facilitate the accessibility of large amounts of data collected to assess the impacts different land management practices on crop and forage yields and greenhouse gas emissions. A data entry template was approved by the GRACEnet data management team to standardize the reporting of land management, weather, soils, and greenhouse gas data. The template was populated with data from 5 ARS units to serve as a database test case. The data from these 5 units is currently available on the GRACEnet SharePoint site and software is being written to perform queries and download data.
Knowledge of the pools and fluxes of C and N is required to interpret ecosystem functioning and improve biogeochemical models. Modeling of changes during incubation demonstrated that two-pool first-order kinetics effectively described losses of microbial biomass C and N and concurrent N mineralization. Microbial biomass N content and soil N mineralization rates were strongly affected by soil type and soil management. Non-acid hydrolyzable C was increased in both amount and mean residence time by cultivation and incubation. Hydrolysis removed a large amount of N as compared to pyrolysis. Data from pyrolysis of soils in a stream of argon at 550°C reflected microbial decomposition processes. This technique should be investigated to identify the recalcitrant forms of C and N in soils. The dynamics of soil C and soil N, although related, are not identical, thus soil C or N cycling can be targeted by management to improve ecosystem functioning and maintain soil organic matter dynamics to help minimize impacts of greenhouse gases on global change.
Biosolids and tillage effects on physically isolated fractions: Implications for conservation management of three Virginia coastal plain soil series. In the Virginia Coastal Plain, growers have practiced best-management practices of rotational no-tillage (RT) and continuous no-tillage (NT) with and without biosolid application to improve soil quality for over 20 years, but few studies document the effects on C and N sequestration in producer-managed fields. We sampled 48 grower’s fields in 2004 and 2006 that representing three important soil series that were under RT and NT with half of the fields having received biosolids application in 2001. Soils under NT with biosolids application had significantly greater SOC stocks than the RT sites with no biosolids application in two of the three soil series (Alta Vista and Kempsville/Emporia), and the soil C stocks were 3-3.4 Mg C ha-1 greater than those previously reported, suggesting a greater C sequestration potential in these soils. Most of the SOC was mineral-associated and showed no management effect, indicating mineral C saturation. Long-term best management practices that decrease soil disturbance and increase C input are necessary to increase C stocks but further C storage will be primarily in aggregate and labile pools that could potentially be lost through subsequent changes in management practice.
Qualitative analysis of volatile organic compounds in biochar. Volatile organic compounds (VOCs) associated with biochar may mechanistically influence plant and microbial responses to biochar amendments, since VOCs can directly inhibit/stimulate microbial and plant processes. As a potential agronomic soil amendment, these responses are crucial to know when selecting biochar to improve soil quality or C sequestration and there is limited information on chemical characteristics of sorbed VOCs associated with these biochars. Our objectives were to evaluate effects of feedstocks, pyrolysis technology, and pyrolysis temperatures on the qualitative properties of sorbed VOCs on over 70 biochars. These biochars possessed over 140 individual chemical compounds. There were no clear feedstock dependencies to the sorbed VOC composition, suggesting a stronger linkage with biochar production conditions coupled to post-production handling and processing. Thus potential users need to recognize the high inconsistency and chemical dissimilarity of sorbed VOCs that may help explain the response variability to biochar addition to soils and the need for VOC characterization before land application.
Vegetation effects on soil organic matter chemistry of aggregate fractions in a Hawaiian forest. We examined chemical changes from leaf tissue to soil organic matter (SOM) to determine the persistence of plant chemistry into soil aggregate fractions. We characterized a slow- (Dicranopteris linearis) and fast-decomposing species (Cheirodendron trigynum) and surface (O) and subsurface (A- horizon) SOM beneath each species using pyrolysis gas-chromatography-mass spectrometry, with and without derivatization. Despite differences in leaf chemistry, SOM chemistry was similar between soil aggregate fractions, but different between horizons with the O-horizon containing primarily lignin and polysaccharide biomarkers while the A-horizon contained polysaccharide, aromatic, and N-derived compounds, indicating considerable microbial processing of plant litter. The soils beneath Cheirodendron inherited greater lipid signal whereas the soils beneath Dicranopteris contained greater aromatic biomarker content, possibly derived from plant lignins. This study indicates that although plant-derived OM is processed vigorously, species-specific biomarkers and compound class differences persisted into the soils and that plant chemical properties may influence soil development even after considerable reworking of plant litter by microorganisms.
Evaluation of angiosperm and fern contributions to soil organic matter using two methods of pyrolysis-gas chromatography-mass spectrometry. Plant species are known to imprint specific biochemical signatures to soils including aliphatic leaf waxes, lignins, and tannins that could influence soil organic matter (SOM) biochemistry and stability. Pyrolysis gas-chromatography mass-spectrometry (py-GC/MS) analyzes biochemical fragments which can be related to lignin, polysaccharides, lipids, nitrogen-bearing, aromatic, and phenol source compounds while methylation using tetramethylammonium hydroxide (TMAH) combined with py-GC/MS improved the detection of lignin and lipid derived acids including cutin, suberin and tannin-derived compounds. We characterized non-polypod and polypod fern and angiosperm live tissues, roots using both methods. Py-GC/MS provided a broad biochemical overview of compound groups including lignin, polysaccharide, lipid, N-bearing, aromatic and phenol groups while TMAH-py-GC/MS detailed lignin units and fatty acids at the expense of the other categories. Both methods detected differences in lignin units between species, although p-coumaric and ferulic acids, predominantly found in ferns, were only observed with TMAH-py-GC/MS, but both py-GC/MS and TMAH-py-GC/MS methods are most useful when combined for their complimentary results.
Nitrogen fertilizer source influences nitrous oxide emissions from strip-till corn. Nitrous oxide (N2O) is a potent greenhouse gas emitted when inorganic N fertilizers are applied to agricultural crops. ARS scientists in Fort Collins, CO, compared N2O emissions from alternative N fertilizer sources to commonly used granular urea and liquid urea-ammonium nitrate (UAN) fertilizers in an irrigated, strip-till corn system. Some alternative N sources reduced growing season N2O emissions up to 70% compared to urea and 49% compared to UAN. Cropping system managers and policy makers need to consider selection of N fertilizer source as a mitigation practice for reducing N2O emissions and global warming potential in the semi-arid western U.S.
No-till sequesters more soil organic carbon than conventional tillage. Converting from irrigated, conventional moldboard-plow tillage (CT) production system to a no-till (NT) system has potential to reduce soil erosion and fossil fuel consumption while enhancing soil organic carbon (SOC) and total soil nitrogen levels. ARS scientists in Fort Collins, CO, found that a NT production system sequestered SOC with time (10 years) in the surface soil, while the CT system did not under irrigated continuous corn. Conversion of irrigated CT to a NT production system is a management practice that needs to be considered by decision and policy makers as a way to mitigate global warming in the semi-arid western U.S. by storing atmospheric carbon in the soil.
Bioremediation of sulfacholorpyridazine. Sulfacholorpyridazine is a major agricultural antibiotic and a significant surface water contaminant in areas where cattle are raised. We evaluated a vegetable oil based biobarrier and an oxidative biobarrier and found that the oxidative barrier was more effective at removing sulfacholorpyridazine providing a remediation recommendation for an important agricultural water contaminant.
Little is known about the number and variety of endophytes that may reside within a single plant. Endophyte are fungi and bacteria that reside within nearly all plant species and tissues and, by definition, do no obvious harm to the living plant. The root endophytic bacterial community of 20 potato cultivars was analyzed using high-throughput DNA sequencing and it was determined that 470 + 73 different bacterial taxa were residing within the roots of a single plant. In addition, the endophytic community differed significantly between each cultivar and was significantly correlated with plant biomass and tuber yields. Future studies are being planned to identify how plants regulate this invisible bacterial community in order to provide plant breeders with the ability to regulate this community and its contribution to potato production and yield.
improve Nitrogen management released online. Nitrogen losses from agricultural systems impact soil, water, and air quality. There is a need for new tools that can help us assess reactive nitrogen losses from agricultural systems. New USDA-ARS tools such as the Nitrogen Trading Tool, Nitrogen Index 4.3, and NLEAP-GIS 4.2 were calibrated and validated, and were released in December 2010 via a new ARS webpage (http://www.ars.usda.gov/npa/spnr/nitrogentools). These tools have been downloaded hundreds of times and are being used by international agencies, universities, and national and international peers to assess the effects of management practices on nitrogen losses in order to reduce these losses to the environment.
Soil carbon sequestration is under-estimated for maize and switchgrass grown for bioenergy. There is large void in the information base on net benefits of soil organic carbon (SOC) sequestration by bioenergy crops including maize and perennial grasses such as switchgrass. Assumptions and associated averaged estimates of SOC sequestration rates used in most modeling studies on net benefits of maize and switchgrass bioenergy crops are greatly underestimated. The SOC sequestration by switchgrass was two- to four-fold greater than that used for estimating it by nearly all the major computer models utilized to determine the net benefits or liabilities of growing bioenergy crops. In a nine-yr study in Nebraska, we measured SOC sequestration by maize was that was as great and as deep as that observed for the switchgrass. For both switchgrass and maize, over 50% of the increase in SOC was below the 30 cm depth, the soil depth to which most previous soil C sequestration data and modeling is based. Presently, computer models used to evaluate the potential environmental effects of bioenergy crops are based on published data with tillage-zone soil sampling depths of 30 or 40 cm. The greenhouse gas (GHG) emissions for cellulosic biofuels for the USA Environmental Protection Agency (EPA), and the California Air Resources Board’s Low Carbon Fuel Standards and similar standards for Europe are based on these models. If significant changes in SOC occur at deeper depths in bioenergy crop, then the results predicted to date based on these models are erroneous. If adjustments are not made, the modeling results will not give an accurate life cycle accounting of bioenergy sustainability.
Dabney, S.M., Meisinger, J.J., Schomberg, H.H., Liebig, M.A., Kaspar, T.C., Delgado, J.A., Mitchell, J., Reeves, D.W. 2010. Using cover crops and cropping systems for nitrogen management. p. 230-281. In Delgado, J.A. and R.F. Follett (eds) Advances in Nitrogen Management for Water Quality, Ankeny, IA.
Delgado, J.A. 2010. Crop residue is key for sustaining maximum food production and for conservation of our biosphere. Journal of Soil and Water Conservation. 65:111A-116A.
Delgado, J.A., Follett, R.F. (eds) 2010. Advances in nitrogen management for water quality. Soil and Water Conservation Society. pp. 1-424.
Delgado, J.A., Gagliardi, P.M., Shaffer, M.J., Cover, H., Hesketh, E. 2010. Chapter 14. New tools to assess nitrogen management for conservation of our biosphere. p. 373-409. In Delgado, J.A. and R.F. Follett (eds). Advances in Nitrogen Management for Water Quality. SWCS, Ankeny, IA.
Delgado, J.A., Secchi, S., Groffman, P., Nearing, M.A., Goddard, T., Reicocky, D., Lal, R., Salon, P., Kitchen, N.R., Rice, C., Towery, D. 2011. Conservation practices to mitigate and adapt to the effects of climate change. Journal of Soil and Water Conservation Society. 66(a):118A-129A.
Dijkstra, F.A., Morgan, J.A., Von Fischer, J., Follett, R.F. 2011. Elevated CO2 and Warming Effects On CH4 Uptake in a Semiarid Grassland Below Optimum Soil Moisture. Journal of Geophysical Research 116, G01007, doi:10.1029/2010JG001288.
Follett, J., Follett, R.F., Herz, W. 2010. Environmental human impacts of reactive nitrogen. Chapter 1. In Jorge A. Delgado and Ronald F. Follett (eds).Advances in Nitrogen Management for Water Quality. Soil and Water Conservation Society, Anken, IA. 424p.
Follett, R.F. 2011. GRACEnet sampling protocols. 69 pages. www.ars.usda.gov/research/GRACEnet
Gross, C., Delgado, J.A., Shaffer, M., Gasseling, D., Bunch, T., Fry, R. 2010. Chapter 15. A tiered approach to nitrogen management: A USDA perspective. p. 410-424. In Delgado, J.A. and R.F. Follett (eds) Advances in Nitrogen Management for Water Quality. SWCS, Ankeny, IA.
Halvorson, A.D., Del Grosso, S.J., Alluvione, F. 2010. Nitrogen source effects on nitrous oxide emissions from irrigated no-till corn. Journal of Environmental Quality 39:1554-1562.
Herrera, J., Delgado, J.A. 2010. Integrated nitrogen management. Soil and Water Conservation Society. p. 94-127. In Delgado, J.A. and R.F. Follett (eds). Advances in Nitrogen Management for Water Quality. SWCS, Ankeny, IA.
Hunter, W.J., Manter, D.K. 2011. Increased electrical output when a bacterial ABTS oxidizer is used in a microbial fuel cell. Current Microbiology. 62(2):633-638.
Hunter, W.J., Manter, D.K. 2011. Pseudomonas seleniipraecipitatus sp. nov.: A selenite reducing - proteobacteria isolated from soil. Current Microbiology. 62(2):565-569.
Hunter, W.J., Shaner, D.L. 2011. Studies on removing sulfachloropyridazine from groundwater with microbial bioreactors. Current Microbiology. 62(2):1560-1564.
Jantalia, C.P., Halvorson, A.D. 2011. Nitrogen fertilizer effects on irrigated conventional tillage corn yields and soil carbon and nitrogen pools. Agronomy Journal. 103:871-878.
Johnson, J.M., Wilhelm, W.W., Karlen, D.L., Archer, D.W., Wienhold, B.J., Lightle, D.T., Laird, D.A., Baker, J.M., Ochsner, T.E., Novak, J.M., Halvorson, A.D., Arriaga, F.J., Barbour, N.W. 2010. Nutrient removal as a function of corn stover cutting height and cob harvest. BioEnergy Research. 3:342-352.
Lal, R., Delgado, J.A., Salon, P.R., Dell, C.J., Groffman, P., Miller, N., Rotz, C.A. 2011. Soil management to mitigate climate change. Journal of Soil and Water Conservation. 66(1):276-285.
Lavado, R., De Paz, J., Delgado, J.A., Rimski-Korsakov, H. 2010. Evaluation of best nitrogen management practices across regions of Argentina and Spain. p. 313-342. Delgado, J.A. and R.F. Follett (eds). Advances in Nitrogen Management for Water Quality. SWCS, Ankeny, IA.
Manter, D.K., Hunter, W.J., Vivanco, J.M. 2011. Enterobacter soli sp. nov.: a lignin-degrading y-Proteobacteria isolated from soil. Current Microbiology. 62(3):1044-1049.
Nielsen, D.C., Halvorson, A.D., Vigil, M.F. 2010. Critical Precipitation Period for Dryland Maize Production. Field Crops Research 118:259-263. doi:10.1016/j.fcr.2010.06.004
Shaffer, M.J., Delgado, J.A., Cross, C., Follett, R.F. 2010. Simulation Processes for the processes for the nitrogen loss and environmental assessment package (NLEAP). Soil and Water Conservation Society. p. 361-372. In Delgado, J.A. and R.F. Follett (eds) Advances in Nitrogen Management for Water Quality, SWCS, Ankeny, IA.
Stewart, C.E., Neff, J., Amatangelo, K., Vitousek, P. 2011. Vegetation effects on soil organic matter chemistry of aggregate fractions in a Hawaiian forest. Ecosystems. 14: 382-397.
Sugiyama, A., Vivanco, J., Jayanty, S., Manter, D.K. 2010. Pyrosequencing assessment of soil microbial communities in organic and conventional potato farms. Plant Disease doi:10.1094/PDIS-02-10-0090.
Wilhelm, W.W., Johnson, J.M., Lightle, D., Karlen, D.L., Novak, J.M., Barbour, N.W., Laird, D.A., Baker, J.M., Ochsner, T.E., Halvorson, A.D., Archer, D.W., Arriaga, F.J. 2011. Vertical distribution of corn stover dry mass grown at several U.S. locations. BioEnergy Research. 4(1):11-21.