Mitigating Greenhouse Gas Emissions from Agroecosystems

Chapter 16

Downloaded by PEPPERDINE UNIV on August 24, 2017 | http://pubs.acs.org Publication Date (Web): October 11, 2011 | doi: 10.1021/bk-2011-1072.ch016

Mitigating Greenhouse Gas Emissions from Agroecosystems: Scientific Basis and Modeling Approach Changsheng Li* Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, U.S.A. *E-mail: [email protected]

The greenhouse gases (GHGs) commonly observed in agroecosystems, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), are byproducts of survival of the microorganisms living in soil, manure or animal enteric systems. When environmental redox potential (i.e., Eh) evolves between 650 and -350 mV, different groups of the microbes can be activated to gain energy by transferring electrons between corresponding reductive and oxidative agents. When oxygen, nitrate or carbon is utilized as electron acceptor under the varied Eh conditions, CO2, N2O or CH4 will be produced, respectively. It is the spatial and temporary coincidence of the three controlling factors (i.e., Eh, electron donor and electron acceptor) that results in production of the three gases. If any of the factors is limited or missing, the greenhouse gas production will be reduced or eliminated. This is the principle that guides us to mitigate greenhouse gas emissions from agroecosystems by altering management practices. To quantify impacts of management alternatives on the microbe-mediated redox reactions, process-based models have been developed to integrate the reactions with a group of environmental driving forces. A biogeochemical model, Denitrification-Decomposition or DNDC, was adopted in the study to explain how this kind of models can be constructed and how they could serve the GHG mitigation. Three case studies for mitigating agricultural CO2, N2O and CH4 emissions

© 2011 American Chemical Society Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


are described in the paper to demonstrate how to assess effectiveness of management alternatives with the modeling approach.

Fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), three major greenhouse gases (GHGs), are detected across almost all agro-ecosystems worldwide. During the period of earth evolution, the three gases played an important role in shaping the planetary climate and reforming the ecosystems. However, faced by the threats of contemporary climate change, people are longing to moderate the increasing trends of the gases in the atmosphere. Agro-ecosystems are apparently an ideal target for doing so due to their accessibility and manageability. How are the GHGs produced in agroecosystems? What farming management alternatives could mitigate the GHG emissions? How can we quantify effectiveness of the candidate mitigation options? This paper is going to answer the questions based on the latest developments in research.

Greenhouse Gases, Byproducts of Microbial Survival Carbon dioxide, CH4 or N2O can be produced in any environment where organic matter and microbes co-exist. In most agroecosystems, organic matter and microorganisms are the major players dominating soil fertility and nutrient cycles including carbon (C) and nitrogen (N) gas productions. In chemistry, the production of CO2, N2O or CH4 results from typical reductive-oxidative (or redox) reactions, which are characterized with electron exchange between reductants and oxidants. So the occurrence of the redox reactions is theomodically controlled by the environmental redox potential (Eh); and the reaction rates are determined by the concentrations of the coupled electron donors and acceptors. The microbes living in the systems play a key role in the process, who gain energy by transferring the electrons that results in GHG production (1)

CO2 Under aerobic conditions, the microbes living in soil or manure gain energy by breaking down the C bonds of the organic compounds existing in the same ecosystem. During the process, electrons are released from the C and must transfer to an electron acceptor. The electron acceptors or oxidants commonly existing in the soil or manure include oxygen (O2), nitrate (NO3-), 4-valent manganese (Mn4+), ferric ion (Fe3+) and sulfate (SO42-). Among the oxidants, O2 possesses the lowest Gibbs free energy and, hence, is the first candidate electron receiver. During the electron transfer occurring in the microbial cells, the ionized oxygen combines with the dissociated C to form CO2 while releasing the CO2 formation energy (ΔfG° = -94.26 kcal/mol). In biology, the process is called as microbial heterotrophic respiration. For any soil or manure system if it is well aerated, emissions of CO2 300 Guo et al.; Understanding Greenhouse Gas Emissions from Agricultural Management ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

from the system are always expectable. It is the major process that leads to the losses of organic C from agroecosystems into the atmosphere.


N2O Soil redox potential (Eh) varies driven by a number of natural or management factors. For example, during a rainfall or irrigation event, the top soil could be saturated by water, and hence diffusion of the atmospheric O2 into the soil profile will be blocked. Along with depletion of the residue O2 left in the soil pores, most of the soil decomposers relying on O2 as electron acceptor will be depressed. However, the low Eh conditions will stimulate another group of microbes, which are capable of utilizing nitrate as electron acceptor. Among the soil oxidants, nitrate possesses the second lowest Gibbs free energy, and hence is ready to be used as an electron acceptor when O2 is depleting in the soil or manure. After receiving an electron, nitrate will become nitrite (NO2-). Nitrite can be further reduced to nitric oxide (NO), nitrous oxide (N2O) and finally dinitrogen (N2). The sequential reactions are called as denitrification as they lead to the losses of soil N into the atmosphere. During the denitrification processes, N2O is produced as an intermediate, which can be further involved in the N2O reduction to be consumed. For example, if a soil or manure system is too wet, the N2O produced by the nitratedenitrifiers could be further reduced to N2 that will result in little N2O emitted. So the net emission of N2O is highly sensitive to the soil Eh dynamics. This character should be utilized to mitigate N2O emissions. Nitrification is another source of soil N2O but usually with relatively low emissions from most agroecosystems. CH4 If an organic matter-microbe co-existing system is under anaerobic conditions for a relatively long-term (e.g., several days or months), the major soil oxidants, such as O2, nitrate, manganese (Mn4+), iron (Fe3+) and sulfate, will be depleted by the decomposers, denitrifiers, manganese bacteria, iron bacteria and sulfur bacteria, respectively. In the case, the low Eh (

Mitigating Greenhouse Gas Emissions from Agroecosystems

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