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      Field-Based Estimates of Global Warming Potential in Bioenergy Systems of Hawaii: Crop Choice and Deficit Irrigation

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          Abstract

          Replacing fossil fuel with biofuel is environmentally viable from a climate change perspective only if the net greenhouse gas (GHG) footprint of the system is reduced. The effects of replacing annual arable crops with perennial bioenergy feedstocks on net GHG production and soil carbon (C) stock are critical to the system-level balance. Here, we compared GHG flux, crop yield, root biomass, and soil C stock under two potential tropical, perennial grass biofuel feedstocks: conventional sugarcane and ratoon-harvested, zero-tillage napiergrass. Evaluations were conducted at two irrigation levels, 100% of plantation application and at a 50% deficit. Peaks and troughs of GHG emission followed agronomic events such as ratoon harvest of napiergrass and fertilization. Yet, net GHG flux was dominated by carbon dioxide (CO 2), as methane was oxidized and nitrous oxide (N 2O) emission was very low even following fertilization. High N 2O fluxes that frequently negate other greenhouse gas benefits that come from replacing fossil fuels with agronomic forms of bioenergy were mitigated by efficient water and fertilizer management, including direct injection of fertilizer into buried irrigation lines. From soil intensively cultivated for a century in sugarcane, soil C stock and root biomass increased rapidly following cultivation in grasses selected for robust root systems and drought tolerance. The net soil C increase over the two-year crop cycle was three-fold greater than the annualized soil surface CO 2 flux. Deficit irrigation reduced yield, but increased soil C accumulation as proportionately more photosynthetic resources were allocated belowground. In the first two years of cultivation napiergrass did not increase net greenhouse warming potential (GWP) compared to sugarcane, and has the advantage of multiple ratoon harvests per year and less negative effects of deficit irrigation to yield.

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          Stability of organic carbon in deep soil layers controlled by fresh carbon supply.

          The world's soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6-0.8 m depth) stimulated the microbial mineralization of 2,567 +/- 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon.
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            Environmental and stoichiometric controls on microbial carbon-use efficiency in soils.

            Carbon (C) metabolism is at the core of ecosystem function. Decomposers play a critical role in this metabolism as they drive soil C cycle by mineralizing organic matter to CO(2). Their growth depends on the carbon-use efficiency (CUE), defined as the ratio of growth over C uptake. By definition, high CUE promotes growth and possibly C stabilization in soils, while low CUE favors respiration. Despite the importance of this variable, flexibility in CUE for terrestrial decomposers is still poorly characterized and is not represented in most biogeochemical models. Here, we synthesize the theoretical and empirical basis of changes in CUE across aquatic and terrestrial ecosystems, highlighting common patterns and hypothesizing changes in CUE under future climates. Both theoretical considerations and empirical evidence from aquatic organisms indicate that CUE decreases as temperature increases and nutrient availability decreases. More limited evidence shows a similar sensitivity of CUE to temperature and nutrient availability in terrestrial decomposers. Increasing CUE with improved nutrient availability might explain observed declines in respiration from fertilized stands, while decreased CUE with increasing temperature and plant C : N ratios might decrease soil C storage. Current biogeochemical models could be improved by accounting for these CUE responses along environmental and stoichiometric gradients. © 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.
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              Feedstocks for lignocellulosic biofuels.

              In 2008, the world produced approximately 87 gigaliters of liquid biofuels, which is roughly equal to the volume of liquid fuel consumed by Germany that year. Essentially, all of this biofuel was produced from crops developed for food production, raising concerns about the net energy and greenhouse gas effects and potential competition between use of land for production of fuels, food, animal feed, fiber, and ecosystem services. The pending implementation of improved technologies to more effectively convert the nonedible parts of plants (lignocellulose) to liquid fuels opens diverse options to use biofuel feedstocks that reach beyond current crops and the land currently used for food and feed. However, there has been relatively little discussion of what types of plants may be useful as bioenergy crops.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                1932-6203
                4 January 2017
                2017
                : 12
                : 1
                : e0168510
                Affiliations
                [1 ]Department of Natural Resources and Environmental Management, University of Hawaii Manoa, Honolulu, Hawaii, United States of America
                [2 ]Texas A&M AgriLife Blackland Research and Extension Center, Temple, Texas, United States of America
                [3 ]United States Department of Agriculture-Agricultural Research Service Grassland Soil and Water Research Laboratory, Temple, Texas, United States of America
                [4 ]Department of Biology, University of Hawaii Manoa, Honolulu, Hawaii, United States of America
                [5 ]Department of Tropical Plant and Soil Sciences, University of Hawaii Manoa, Honolulu, Hawaii, United States of America
                [6 ]Hawaiian Commercial & Sugar, Puunene, Hawaii, United States of America
                Pacific Northwest National Laboratory, UNITED STATES
                Author notes

                Competing Interests: We declare the commercial affiliation, Hawaiian Commercial and Sugar (HC&S), of co-author Mae Nakahata. This affiliation does not alter our adherence to PLOS ONE policies on sharing data and materials.

                • Conceptualization: MNP SEC MNM JRK RO.

                • Data curation: MNP SEC.

                • Formal analysis: MNP SEC MNM ADT.

                • Funding acquisition: SEC JRK MN.

                • Investigation: MNP SEC MNM RO AY.

                • Methodology: MNP SEC MNM JRK ADT.

                • Project administration: SEC.

                • Resources: SEC JRK MN.

                • Supervision: SEC MNM AY.

                • Validation: SEC MNM.

                • Visualization: MNP SEC.

                • Writing – original draft: MNP SEC MNM.

                • Writing – review & editing: MNP SEC MNM JRK ADT.

                Author information
                http://orcid.org/0000-0001-8735-1580
                Article
                PONE-D-16-33017
                10.1371/journal.pone.0168510
                5215395
                28052075
                de540e29-e32b-44ff-aa0a-48c23048bc30

                This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

                History
                : 17 August 2016
                : 1 December 2016
                Page count
                Figures: 5, Tables: 1, Pages: 19
                Funding
                Funded by: funder-id http://dx.doi.org/10.13039/100000015, U.S. Department of Energy;
                Award ID: DE-FG36-08GO88037
                Funded by: funder-id http://dx.doi.org/10.13039/100000006, Office of Naval Research;
                Award ID: N00014-12-1-0496
                Funded by: funder-id http://dx.doi.org/10.13039/100000006, Office of Naval Research;
                Award ID: N00014-16-1-2221
                Award Recipient :
                Funded by: funder-id http://dx.doi.org/10.13039/100007917, Agricultural Research Service;
                Award ID: 003232-00001
                Award Recipient :
                Funded by: funder-id http://dx.doi.org/10.13039/100005825, National Institute of Food and Agriculture;
                Award ID: HAW01130-H
                This work was supported by the Department of Energy http://science.energy.gov/programs/ [award number DE-FG36-08GO88037], the Office of Naval Research http://www.onr.navy.mil/en.aspx [grants N00014-12-1-0496 and N00014-16-1-2221], and the United States Department of Agriculture-Agricultural Research Service, http://www.ars.usda.gov/main/main.htm [award number 003232-00001]. This work was further supported by the USDA National Institute of Food and Agriculture, Hatch project https://nifa.usda.gov/program/capacity-grants (project HAW01130-H), managed by the College of Tropical Agriculture and Human Resources. The commercial affiliation, Hawaiian Commercial and Sugar (HC&S), provided support in the form of salary for author MN. Further resources were provided by HC&S in terms of 1) field site access within the commercial plantation to meet the identified objectives of the study and 2) cost-sharing to meet the 20% requirement of a federal funding source through use of space and infrastructure within the HC&S headquarters and processing facility for office, laboratory (sample processing), and storage needs for the project duration. As the representative for the direct stakeholder for the overarching project, MN provided input on the needs of HC&S operations moving forward from sugarcane cultivation to diversified farming (including alternative grasses for biofuel feedstock) and agreed to the final field location and fundamental experimental treatments, but did not have any additional role in the specific study design, data collection and analysis, decision to publish, or preparation of the manuscript. In her capacity as primary stakeholder, MN also assisted with funding acquisition by developing budgets to cover the costs of the HC&S field crew for plot installation and maintenance and field equipment repair or construction on site as needed. The specific role of this author is articulated in the ‘author contributions’ section. Otherwise, the funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology and Life Sciences
                Agriculture
                Crop Science
                Crops
                Sugarcane
                Biology and Life Sciences
                Organisms
                Plants
                Grasses
                Sugarcane
                Biology and Life Sciences
                Agriculture
                Agricultural Methods
                Agricultural Irrigation
                Biology and Life Sciences
                Agriculture
                Agricultural Soil Science
                Ecology and Environmental Sciences
                Soil Science
                Agricultural Soil Science
                Physical Sciences
                Chemistry
                Chemical Compounds
                Carbon Dioxide
                Biology and Life Sciences
                Agriculture
                Agrochemicals
                Fertilizers
                Biology and Life Sciences
                Organisms
                Plants
                Grasses
                Engineering and Technology
                Energy and Power
                Bioenergy
                Bioenergy Feedstock
                Earth Sciences
                Geography
                Physical Geography
                Custom metadata
                All data files are available in the ScholarSpace repository of the University of Hawaii ( http://hdl.handle.net/10125/42704).

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                Uncategorized

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