Article pubs.acs.org/est
Accounting for the Biogeochemical Cycle of Nitrogen in InputOutput Life Cycle Assessment Shweta Singh and Bhavik R. Bakshi* William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States S Supporting Information *
ABSTRACT: Nitrogen is indispensable for sustaining human activities through its role in the production of food, animal feed, and synthetic chemicals. This has encouraged significant anthropogenic mobilization of reactive nitrogen and its emissions into the environment resulting in severe disruption of the nitrogen cycle. This paper incorporates the biogeochemical cycle of nitrogen into the 2002 input-output model of the U.S. economy. Due to the complexity of this cycle, this work proposes a unique classification of nitrogen flows to facilitate understanding of the interaction between economic activities and various flows in the nitrogen cycle. The classification scheme distinguishes between the mobilization of inert nitrogen into its reactive form, use of nitrogen in various products, and nitrogen losses to the environment. The resulting inventory and model of the US economy can help quantify the direct and indirect impacts or dependence of economic sectors on the nitrogen cycle. This paper emphasizes the need for methods to manage the N cycle that focus not just on N losses, which has been the norm until now, but also include other N flows for a more comprehensive view and balanced decisions. Insight into the N profile of various sectors of the 2002 U.S. economy is presented, and the inventory can also be used for LCA or Hybrid LCA of various products. The resulting model is incorporated in the approach of Ecologically-Based LCA and available online.
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INTRODUCTION Nitrogen is among the crucial elements that is essential for human well-being. Its biogeochemical cycle involves conversion of “inert” nitrogen to “reactive” nitrogen that can be further converted to other useful products such as food, fiber, plastics, etc. Perturbation of the nitrogen cycle by human activities has been reported in many studies,1−3 and a recent study 4 shows that this perturbation has exceeded the “safe operating zone” for humanity. In fact, according to this study, the N cycle is in much worse shape than the Carbon cycle, even though the latter cycle receives much more attention. There have been numerous efforts to account for anthropogenic perturbation of the N cycle and its impact on ecosystem degradation. Inventories of global emissions of NOx, NH3, and N2O have been constructed,5 emissions from agricultural activities are also monitored to find better agricultural practices that can reduce emissions,6−9 and farm balances have been developed to identify the net nitrogen loss.10,11 National nitrogen balances have also been developed12,11 to indicate a nation’s nitrogen trade and use. An indicator of the efficiency of nitrogen use in animal production was defined as the ratio of useful nitrogen produced to input nitrogen.13 Guidelines have been developed by EUROSTAT14 for developing regional and national nitrogen balances. The eutrophication potential of human food consumption pattern has also been studied.15 Similarly, a N footprint calculator traces the N loss to the environment based on the pattern of consumed goods.16,17 This calculator gives the N footprint in terms of N loss for the defined consumption pattern. Further, © 2013 American Chemical Society
an International Nitrogen Initiative (INI) is optimizing the use of N in food and energy, while reducing the negative impacts.18 INI carries out various research activities and assessment to help meet the objectives of efficient utilization of nitrogen. Another crucial aspect of nitrogen impact is the effect on ecosystem services. The US EPA has set up a program to study the effect of reactive N on various ecosystem services.19 This is a unique and much needed effort as it adopts an ecocentric view that has been missing in all the other efforts and could help in moving toward more judicious utilization of nitrogen for satisfying human needs. Most of the efforts for N accounting consider a relatively coarse scale and provide insight into only the overall N balance.11,12 Information about finer details such as the indirect impact of human activities on reactive N mobilization or on reactive N losses are usually not available, making it impossible to choose economic activities based on their life cycle impact on the N cycle. Some recent approaches do account for reactive N use based on specific human consumption at the process scale,16,15 but these lack the ability of capturing indirect effects of N use except for energy use.16 Generally, processes with high N loss are considered to be worse. Such a focus on direct emissions or use may miss the importance of indirect effects from other parts of the supply or demand chain, such as the Received: Revised: Accepted: Published: 9388
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Figure 1. Nitrogen flow in societal, economic, and ecological systems.
role of the demand for certain types of foods and textiles in causing emissions of reactive N. Similarly, the use of nitrogenous explosives is an example of an economic good that creates a huge demand for reactive nitrogen (Nr) mobilization but is usually ignored and is poorly understood.20 Hence, there is a need to devise data and methods to capture indirect effects of economic activities on reactive N losses and mobilization, which is the main focus of this work. Another way to reduce Nr loss is to identify potential areas of improvement that would enhance the overall efficiency of reactive N utilization. This needs development of an inventory which can help identify processes that use reactive N inefficiently. Defining a proper metric to understand the impacts of an economic activity on the N cycle is also essential to devise efficient management approaches. Such a metric for economic activities is not yet available. This article presents an integrated model of the U.S. economy and the nitrogen cycle. This is accomplished by connecting the 2002 U.S. economic input-output model with data about the flow of reactive nitrogen. This work continues
the development of the Eco-LCA model of the U.S. economy21,22 and the incorporation of the carbon cycle in it.23 Given the greater complexity of the nitrogen cycle as compared to the C cycle, this work classifies N flows into three broad categories - “reactive N mobilization,” “reactive N in products used”, and “reactive N losses”. Most previous studies13,9,16 have focused only on “reactive N losses,” but this focus is too narrow to quantify the broad human dependence on the biogeochemical cycle of nitrogen. The rest of this paper is organized as follows. The next section describes the N flow network for coupled naturalhuman systems, which will help identify the crucial components of N flow that need to monitored. This discussion also highlights the indirect effects of N consumption on various other N flows that are generally missed. Components of the developed Eco-LCA inventory and its details are in the latter half of this section. This is followed by a description of the nitrogen profile of the 2002 U.S. economy and a discussion about the types of insight that it can provide. 9389
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mentally extended IO models by connecting most of the flows depicted in this network diagram to economic sectors, thus capturing the indirect effect of economic activities on these flows. This effort adds to the Eco-LCA input-output model of the U.S. economy.22,26,23 N Inventory and Calculations. Table S.1 in the Supporting Information shows the different components of N inventory used in this work, along with the source of data, peripheral sectors through which the flow enters or leaves the economy, and magnitude of flows at the national scale. These values may be divided into three categories as described here. Each category forms the corresponding Vph vector for use in the model given by eq 1.
METHOD AND DATA Network of Nitrogen Flow. Figure 1 depicts the flow of N in the coupled natural human system considered in this work. It is divided into four compartments: N flows in the economy, N flows in agro-ecosystems, population N flow, and N flow in aquatic ecosystem. This figure provides a framework to study human impact on various N flows and indicates the key N flows that connect each system. The N flows of mature forest ecosystems are not explicitly shown in this figure but are part of the inventory through various components. The N flows can be classified into three categories: inert N flow, useful reactive N flow, and waste reactive N flow. The atmospheric pool of inert N provides the N for conversion to reactive N. The inert N flow is the flow of freshly converted reactive N from the atmospheric pool of inert N to the combined human-natural system. It is shown as “green N” flow in the system. This freshly converted reactive N is then converted to “useful reactive N” by industrial and biological methods in the respective compartments, as shown. This useful reactive N is further converted to various products in different compartments, and the exchange of these products across the subsystems provides the link between human and natural systems. This flow is depicted as “blue N” in Figure 1. Demand for these blue N components is what drives the flow of “green N” in the combined system. Further, during the process of conversion of this “blue N” to various products there is degradation of N to waste, which gets discarded to the atmosphere, soil, or water. This flow is depicted as “red N”. This is the flow that causes direct negative impact on natural systems and is not recovered for further use. Although there is some recirculation of these waste reactive N flows, quantification is complicated and unclear. The components from this diagram that are included in this work are given in Table S.1 in the SI. This diagram initiates a method to account for the direct and indirect effects induced by the anthropogenic system on N flows through coupled natural-anthropogenic systems. Understanding and quantifying the interactions between the parts shown in Figure 1 helps identify the sources of reactive N loss, path flow of Nr mobilized within industrial or ecological systems, and different products that create a demand for Nr. Hence, it helps in understanding the indirect role of the consumption of an economic good or service in altering N cycling through natural systems. For example, in the compartment “N flows in the economy” of Figure 1, the Nr consumed as plastics and synthetics drives Nr mobilization as ammonia and Nr losses to air. This affects the natural rate of circulation of N from inert form to reactive form in the atmosphere. Another flow of Nr consumed as animal feed also drives Nr mobilization as ammonia in the industrial system and creates further loss of Nr. This flow of consumption as animal feed connects N flow in the economy to N flow in the population. Hence, this flow further affects N flow as livestock N which moves to human population N stock when consumed by human beings as animal food such as meat. Thus, the demand for animal food by human beings indirectly drives the Nr flow in economic system through ammonia formation and N losses while producing N feed for animals. These interactions are difficult to track on a product by product basis, and many linkages might be missed due to the large and complex network. Methods based on input-output (IO) analysis can overcome this limitation24,25,22 and are the focus of this work. The resulting model adds to the rich literature on environ-
X ph = (I − GT )−1Vph
(1)
Here, (I − GT)−1 is called the Ghosh inverse, and Vph is the physical resource vector. The direct and indirect values shown in the figures are calculated as X̂ −1(I + GT)Vph and X̂ −1((GT)2 + (GT)3 + ...)Vph, respectively. Equation 1 partitions resources entering the economy through primary or peripheral sectors to sectors that buy goods from these sectors and so on throughout the economic network. The choice of Ghosh model over Leontief model to calculate the total embodied flow component was driven by the inuitive treatment of various Nr flows entering as resource to the economy. There has been some controversy with the use of the Ghosh model;27 however, the controversy pertains to the causal interpretation in supply of resources simulating the use of resources in other sectors but is not applicable in only the linkage interpretation.28,29 Causal interpretation is not the intent in this work, and the Ghosh model is simply used for the linkages that have helped in calculation of total resource and emissions vectors. In addition, as proven in ref 30, the Xph vector obtained by the Ghosh inverse is equivalent to that obtained by the Leontief inverse. This vector represents the embodied resource or footprint in each sector. It is also important to note that this work considers a national scale IO model for the US in calculation of all the embodied resources and ignores any flow related with imports and exports. This limits the resulting inventory for analysis of nitrogen flows for production within the US. Further details are in refs 23 and 31. Details about each component in the N inventory are in the rest of this section. Reactive N Mobilization. This category includes flows that convert “inert” nitrogen into its reactive form. This component can be considered as analogous to “mining” since it extracts resources from the natural pool and makes it available for human use. Previously, such useful reactive N was provided primarily by the natural N cycle. However, now anthropogenic activities have led to other modes of reactive N mobilization and have also enhanced the natural mode of reactive N mobilization. Hence, it is important to study the impact of different economic activities that drive changes in the natural N cycle by altering the mobilization of reactive N. The natural and artificial components of reactive N mobilization included in the model are the following: Nitrogen Input from Atmospheric Deposition, Nitrogen Input to soil from Leguminous Plants, Nitrogen fixed by micro-organisms in soil, and Industrial N fixation by Ammonia Production. These constitute the “green N” flows depicted in Figure 1. Calculation of flows for this N category by input-output algebra is analogous to partitioning the inert N entering peripheral sectors such as fertilizer manufacturing between sectors that buy from these sectors. 9390
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Figure 2. Reactive N mobilization categories for 2002 economic sectors. Legend for x-axis. 1: Agriculture, Livestock, Forestry & Fisheries, 2: Mining & Utilities, 3: Construction, 4: Food, Beverage & Tobacco, 5: Textiles, Apparel & Leather, 6: Wood, Paper & Printing, 7: Petroleum & Basic Chemicals, 8: Resin, Rubber, Artificial Fibers, Agri Chem & Pharma, 9: Paint, Adhesives, Cleaning & Other Chem, 10: Plastics, Rubber & Nonmetallic mineral prod, 11: Ferrous & nonferrous metal products, 12: Cutlery, Handtools, Struc. & Metal Containers, 13: Other metal hardware and ordnance manufacturing, 14: Machinery & Engines 15: Computers, Audio-Video & Comm. Equip, 16: Semiconductors, Elec Equip & Media Reproduction, 17: Lighting, Electrical Components & Batteries, 18: Vehicles & Other Transportation Equip, 19: Furniture, Medical, Equip & Supplies, 20: Other Misc. Manu., 21: Trade, Transportation & Commu. media, 22: Finance, Insu, Real-Estate, Rental & Leasing, 23: Professional & Technical Services, 24: Management, Admin & Waste Services, 25: Education & Health Care Services, 26: Arts, Entertainment, Hotels & Food Services, 27: Other services except Public Administration, 28: Government & Special Services.
and can be utilized more effectively to produce useful N products such as food. Losses of reactive N to air include N2O and NOx emissions from fuel burning, Land Use Land Cover (LULC) change, manure management, field burning of agricultural residues, forest fires, nitric acid production, adipic acid production, domestic wastewater treatment, and composting. Reactive N losses to water are due to runoff or leaching and include N loading of water as ammonia, inorganic N such as nitrates and nitrites, and organic nitrogen due to waste disposal. Reactive N loss to land includes the reactive nitrogen in sewage sludge, which is applied to agricultural land. However, reapplication of sewage sludge can be considered as recirculation of “reactive” N rather than N loss. In this work, it is treated as N waste to land.
Usually, reactive N is not consumed in the peripheral sectors but passes through them to other sectors such as fertilizer use. This is a supply side calculation, whose theoretical properties are discussed in ref 30. The data for “green N” flow to atmospheric pool by denitrification processes have not been included in this work due to its large uncertainty. Reactive N in Product Use. The mobilized reactive nitrogen is transformed to make economic products such as the ones included in this category and depicted as “blue N” in Figure 1. Most of the anthropogenically and ecologically mobilized N is converted into the following products, which are included in this work: Fertilizer, Organic manure, Plastics and synthetics, Explosives, Animal feed and others, Harvested Crops for human consumption, and Meat for human consumption. This inventory could be more comprehensive with more data and by tracking the use of agricultural N as feed and fodder, which is a difficult task, and outside the scope of this work. Input-output calculations with this category of reactive N flows are demandside calculations, as discussed in ref 30. Reactive N Losses. Loss of reactive N is due to discarding used resources as waste or as losses due to inefficiency during extraction, conversion, and use phases. Emission of N2O is the major component in this category. Although this gas is considered to be nonreactive due to its long retention time in the atmosphere, it slowly reacts to destroy stratospheric ozone and cause global warming. Therefore, in this work, N2O is considered to be a reactive N loss. Nr losses from land use change are also included in this category, because reactive N in soil comes mostly from the sources that mobilize reactive N
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NITROGEN PROFILE OF THE U.S. ECONOMY The N inventory at the economy scale described in the previous section has been used to study the N profile of different sectors of the 2002 U.S. economy. The results presented here are based on aggregate sectors using the aggregation scheme described in Table S.2. Figure 2 shows the life cycle contribution of each group of sectors to reactive N mobilization, which can be used to identify the activities that have high contribution to this green N flow due to direct or indirect interactions. As seen in this figure, “Agriculture, Livestock, Forestry and Fisheries (1)”, “Resin, Rubber, Artificial Fibers, Agriculture and Pharmaceuticals (8)”, “Food, Beverage and Tobacco (4)”, “Construction (3)”, and “Wood, Paper and Printing (6)” are 9391
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Figure 3. Product N category profile for 2002 economic sectors. Legend for x-axis is in Figure 2.
plantation of legumes has increased Nr mobilization many folds. This contribution appears in “Agriculture, Livestock, Forestry and Fisheries (1)” and “Food, Beverage and Tobacco (4)” sector groups. Further, “Arts, Entertainment, Recreation, Hotels, and Food services (26)” also contribute to Nr mobilization and most of this is due to indirect effects. This study of Nr mobilization for the US economy includes many paths of indirect reactive N flow that are missed by current studies, thus providing more complete insight into how economic activities depend on and affect reactive N mobilization. The inventory and models in this work capture these indirect effects, thus providing a link between economic productivity and the nitrogen cycle. After total reactive N mobilization, the next most important component of the N life cycle is reactive N in products used or blue N. The mobilized reactive N is transformed into many products by various industrial and ecological processes. The categories of reactive N mobilization such as biological N fixation (BNF), atmospheric deposition, and nitrogen fixation by plants go through complex cycling, making it difficult to analyze the direct end use of reactive N mobilized through these paths. However, end use of reactive N mobilized by industrial methods is relatively easy to track since the end use of ammonia is clearly defined and well controlled. End use of BNF and other agricultural flows can be quantified as N going to food products, forage, etc., which has not been included comprehensively in existing studies due to a lack of data and inadequate understanding of the flow of N in biological or agricultural systems. Flow in agricultural systems also includes that of artificial N applied as fertilizer, thus separating natural and artificial N end use may not be feasible in these systems. The current Eco-LCA inventory for reactive N end use mainly includes the end use of ammonia as different products like plastics and synthetics, explosives, nitrogenous fertilizers, and animal feed and other chemicals. Other two N end uses are livestock manure and human food consumption from harvested crops or as animal food products, which only partially cover the
the top 5 sector groups contributing to reactive N mobilization. “Agriculture, Livestock, Forestry and Fisheries (1)” causes the most reactive N mobilization mostly by direct flows, since this sector is involved in farming legumes, which causes very high reactive N mobilization. Further, the indirect reactive N mobilization for this sector is also high since it utilizes fertilizer that cause Nr mobilization as ammonia. “Food, Beverage and Tobacco (4)” has the second highest contribution to reactive N mobilization due to dependence of this sector on agricultural products. In this sector, the indirect Nr mobilization is higher than direct Nr mobilization which suggests a huge role of indirect demand in mobilizing more nitrogen. This indirect Nr mobilization is mainly due to industrial N fixation or N mobilization due to the use of artificial fertilizers. This demonstrates the importance of considering indirect effect on N flows. Such insight can help in understanding the anthropogenic impact on the N cycle and making decisions that will lead to better management of nitrogen as a resource. Figure 2 also shows the breakdown of the total N mobilization in terms of contributing categories to each aggregate sector. Industrial N fixation dominates in most of the sectors, clearly indicating the large role of anthropogenic processes in N mobilization. Atmospheric deposition is significant only in “Agriculture, Livestock, Forestry and Fisheries (1)”, “Food, Beverage and Tobacco (4)”, “Wood, Paper and Printing (6)”, and “Construction (3)” sector groups, which can be attributed to large direct or indirect land area required for the activities of these sectors. Although atmospheric deposition is a natural mechanism of reactive N mobilization and making it available to be reused, human activities have greatly affected the rate and magnitude of reactive N mobilization by this process. This is due to the enhanced presence of reactive N in the air due to activities like fuel combustion or land use change. Indirect effect on atmospheric deposition is high in “Wood, paper and printing (6)”which is a result of indirect effect on land use change due to requirement of this sector’s production activities. Similarly, 9392
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Figure 4. N emissions category profile for 2002 aggregated sectors. Legend for x-axis is in Figure 2.
end use of BNF along with the artificial N end use. Despite these limitations, the developed model is still useful for quantifying the role of different economic sectors in creating demand for particular reactive N in products. Figure 3 shows the reactive N in products used or the blue N profile for different economic sectors, including direct and indirect flows. The values of these flows in various sectors are listed in Table S.1 in the SI. “Agriculture, Livestock, Forestry and Fisheries (1)”, “Food, Beverage and Tobacco (4)”, “Paint, Adhesives, Cleaning & other Chemicals (9)”, “Arts, Entertainment, Hotels & Food Services (26)”, and “Resin, Rubber, Artificial fibers, Agri. Chemicals & Pharma (8)” are the top five sector groups that use products based on reactive N. Fertilizer manufacturing is the top product from reactive N, hence “Agriculture, Livestock, Forestry and Fisheries (1)” sector group dominates. “Food, Beverage and Tobacco (4)” sectors are next due to their strong dependence on fertilizer. However, in these sectors, the indirect reactive N use is almost as large as direct use. This reflects that activities in this sector do not only directly consume N products but depend on other sectors which consume a lot of N products such as the agricultural sector. The “Arts, Entertainment, Hotels & Food Services (26)” are next in consumption of N products. Most of the dependence in this sector is indirect manure and fertilizer due to inputs from food sectors. The next sector for reactive N end use in products is the “Paint, coating, Adhesives, Cleaning and Other chemicals (9)” which dominates due to the explosives manufacturing sector which results in high demand of reactive N for making its products. Other sectors that show significant consumption of reactive N end products have more contribution from indirect N end use, further confirming the importance of accounting for indirect flows. Figure 3 also shows the direct and indirect dependence of various aggregate sectors on different categories of N products. One important thing to note is that although “Resin, Rubber, Artificial Fibers, Agriculture and Pharma (8)” is the sector with the second highest contribution to reactive N mobilization yet its
contribution to reactive N in product use is low. This signifies that even though a sector mobilizes a lot of reactive N it might just pass the mobilized N as resource to other sectors and not use the N for making consumer products. In case of this aggregated sector, production of ammonia mobilizes N, but since this is an intermediate that goes to other sectors, this sector does not produce many products with embodied Nr. Most of the mobilized N goes to fertilizers which are used in agricultural sectors, and the rest goes to manufacturing of explosives, plastics, synthetics, and other chemicals. This is seen in the detailed plots for each sector in this aggregated group in Figure S.4. Thus, it is important to look at both the profiles of reactive N mobilization and reactive N end use to see the impact of economic activities on N flow. As discussed in the section about the network of nitrogen flow, some sectors create high demand for end use product N, whereas others serve as pass-through sectors for production of end use reactive N product. The next component of the Eco-LCA N inventory is Nr loss, which receives the most attention since this loss is the direct and most visible cause of negative impact of increased reactive N in the biosphere. Figure 4 shows the reactive N loss profile or red N profile for different aggregated sectors as the direct and indirect losses to air, water, and land. Perhaps not surprisingly, “Agriculture, Livestock, Forestry and Fisheries (1)” is the aggregated sector with maximum loss of reactive N. The losses to air and water dominate in this sector. Air losses include those from volatilization of manure as N2O, land use change losses, and burning of fuels. The water losses are highest due to runoff as ammonia, nitrite, nitrate, and organic N. This nutrient loss indicates highly inefficient use of the mobilized Nr and product with embodied Nr and an urgent need to improve Nr efficiency in this sector. This is an active area of research among agriculture scientists.32 “Mining and Utilities (2)” is the second largest aggregated sector for reactive N losses with air emissions, mostly due to fuel combustion dominating, as shown in Figure 4. The next highest emissions 9393
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are from the “Food, Beverage and Tobacco (4)” sector group. This group results in high N losses to water and land due to food scrap waste and other N emissions to water such as nutrient runoff from agriculture land on which this sector is directly dependent. It can be seen in Figure 4 that indirect emissions are very high in this group of sectors. Further, it can be seen that air emissions dominate in almost all sectors. These emissions are from fuel combustion and LULC change, thus emphasizing the need to reduce energy use or improving the combustion processes to reduce loss of such reactive N to the environment. Comparing reactive N mobilization, reactive N in products used, and reactive N losses in each sector can also yield useful insight. Figure S.19 in the SI plots these three quantities for each aggregated sector on log scale for N in Tg. The graph shows that few sectors have higher reactive N mobilization than reactive N product use and reactive N loss. Also, the diagram clearly shows the disparity between reactive N mobilization and reactive N product use. The sectors high in N end use are not necessarily the sectors that have high N mobilization. Such insight can be used efficiently to reduce N mobilization which is driven by reactive N product use. For such an analysis a more complete relation between different types of reactive N mobilization and reactive N product use needs to be mapped. Similarly, to reduce reactive N losses, the processes with lower efficiency of converting reactive N mobilized to reactive N end use products need to be improved. Further, the sector groups from 22 to 28, which are mostly service sectors also have higher reactive N mobilization than reactive N end use and reactive N losses. Service sectors do not have a direct role in mobilizing nitrogen in most of the processes; however, this indicates that indirect role of service sectors in reactive N mobilization is very important and deserves further research. The life cycle model developed in this work is only a beginning to analyze reactive N flow in the economy. The overall picture which involves reactive N mobilization, reactive N end use, and reactive N loss lacks the ability to track the path of mobilized N due to incomplete understanding of cycling of N through the combined economy and ecological network. Knowledge of use and reuse of already mobilized N will be really helpful for identifying potential opportunities of reducing reactive N mobilization and losses in the economy, thus reducing the impact of anthropogenic activities on the N cycle.
a single numeraire such as a nitrogen footprint. This is because all three components of the inventory have different implications over the nitrogen cycle and the environment. The reactive N mobilization component captures direct and indirect extraction of N into reactive form. This form of N is not always detrimental to the environment. So, a high reactive N mobilization should not always be considered to be a negative effect. Similarly, reactive N in products used gives the direct and indirect dependence of a particular economic activity on a specific N product such as plastics, fertilizer, etc. Thus, these two components only indicate the activities that have a large direct and indirect role in converting N from inert form to reactive form or to consume products based on reactive N. Flows due to mobilized N and N in products cannot be added without the risk of double counting since some of the mobilized N gets converted into products. The third component is much simpler to interpret as it is the direct and indirect N losses and is closest to existing work on N footprint but is able to consider direct and indirect emissions over the selected national scale. Thus, the Eco-LCA N inventory developed in this work presents a comprehensive accounting of N flow, from the point of extraction, to product use, to its loss to the environment. The current inventory is missing some components due to a lack of data or inadequate understanding of N flow in the economy. Missing pieces include flows such as the recycling of mobilized nitrogen from residuals to crops, utilization of lost N in soil by the next crop cycle, free organism mobilization in forest ecosystems, etc. These flow components are difficult to track and constitute an important part of nitrogen cycling that requires input from biogeochemists. The tracking of resource flow in the economy may also help overcome the limitation of understanding of the flow path of N. Such an activity is tedious to perform and requires effort to gather physical data of N based commodities which is lacking currently. Once such understanding develops, it may become possible to define a proper metric for a Nitrogen footprint of economic activities that can represent the real negative impact of reactive N utilized or lost in particular activities. The limitations of the current N inventory includes the uncertainty of available data, lack of completeness of inventory due to missing component data in the ecological-economic N flow network, and lack of welldefined nitrogen metrics. The high uncertainty in the values of several Nr flows prevents a rigorous uncertainty analysis for this work. Uncertainty values available from EPA reports33 for some of the nitrogen flow components that are considered here ranged from −80% to 211% that makes the sensitivity analysis irrelevant. Some of the data that are a result of census may have other uncertainty due to the process of collecting data34 but was not reported in the source. Since, the EEIO model is a coarse model, it should be combined with more specific data at fine scale to improve the reliability of results in decision making. Unfortunately, at the national scale Nr flows do show much variability.35 However, the input side uncertainty will be applicable to all sectors; so the average trend should not be different and can be used for developing an average understanding of reliance on these flows. Several of the other sources of uncertainty are discussed in refs 36−38 and will be applicable to this study due to similarity of the modeling techniques. A thorough treatment of uncertainty must be left for future research. The comprehensive inventory in this work emphasizes that accounting only for losses might miss the big picture of efficiently using the mobilized nutrient in the
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DISCUSSION The nitrogen-extended input-output model developed in this work is among the first efforts to comprehensively capture the direct and indirect impacts of economic activities on the nitrogen cycle. The components of this ecological-economic N model include reactive N mobilization, reactive N in product use, and reactive N losses. Other research has focused on monitoring nitrogen losses as emissions to air or waste to water due to various economic activities; however, there have been no efforts to quantify the role of economic activities in creating demand for making reactive N available due to anthropogenic activities. The developed model is incorporated in the ecologically based input-output LCA model22,26 and can capture the direct and indirect roles of different economic activities in creating demand for reactive N mobilization and reactive N in various manufactured products. Also, it can quantify the direct and indirect N losses due to various economic activities. In this work, the N data in the inventory have not been combined into 9394
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economy, which is important to reduce the stress on the disturbed natural nitrogen cycle. The results of this work can provide the basis of future research activities. For example, the integration of nitrogen flows between the environment and economic activities can be used for tasks such as policy planning, environmental taxation, or technology improvement efforts in several economic sectors. As a part of LCA, this work can support decisions about industrial products and processes based on enhancing the life cycle efficiency of reactive N flows. Network theory has been utilized to gain insight about resource flows in several industrial and ecological systems. Such efforts can be applied to the nitrogen network developed in this work and help initiate further research on cycling efficiency of Nr mobilized in economic and industrial life cycles.31
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ASSOCIATED CONTENT
S Supporting Information *
Details about the data and methods used in this work. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 1-614-292-4904. Fax: 1-614-292-3769. E-mail: bakshi.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank three anonymous reviewers for their feedback that has improved the paper significantly. Funding for this work was provided by NSF grant nos. ECS-0524924 and CBET-0829026. The authors are also thankful to EPA and USGS for developing the databases; Lori E. Apodaca and Norman Spahr for their responses and clarification about nitrogen databases; Daniel Sobota and Jana Compton for pointing us toward some relevant papers and answering queries; and Erin Gibbemeyer, a Ph.D. student at The Ohio State University for gathering 2002 economic data that formed the basis of this work.
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