Atom Conversion Efficiency: A New Sustainability Metric Applied to

Perspective Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Atom Conversion Efficiency: A New Sustainability Metric Applied to Nitrogen and Phosphorus Use in Agriculture Joshua H. Urso† and Leanne M. Gilbertson*,† †

Department of Civil and Environmental Engineering, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States ABSTRACT: Agriculture fertilization suffers from inefficiencies that carry significant environmental and economic consequences. These consequences include high fertilizer production energy demand, on-field greenhouse gas emissions, and eutrophication. Additionally, inefficient fertilizer use is responsible for billions of dollars in annual economic losses in the form of resource loss as well as environmental burdens. Furthermore, the unsustainability of current fertilization practices and the reliance upon finite resources calls into question the ability of agriculture to meet projected increases in global demand. Herein, critical fertilizer system inefficiencies are highlighted and quantified with a new proposed metric, atom conversion efficiency (ACE), which captures inefficiencies of primary nutrient atoms (N and P) at each stage of the fertilizer life cycle, from synthesis to farm gate, for the model crop, corn. Conversion efficiencies for the most common forms of N and P used in conventional fertilizers range from 5% (diammonium phosphate) to 10% (ammonium nitrate). These low system efficiency values motivate advancements in agriculture sustainability through (i) improvements in the conversion of the raw forms of N and P into fertilizers, (ii) enhancement of nutrient use efficiency (NUE), and (iii) nutrient form selection, signaling an opportunity for advancement through innovative technologies. KEYWORDS: Green chemistry, Green engineering, Atom economy, Fertilizers, Nanotechnology, Nutrient Use Efficiency (NUE), Sustainable agriculture

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of the Haber−Bosch process in the early 1900s14 forever changed the agriculture industry, providing a reliable and affordable external source of nitrogen needed to replenish soils. Nitrogen-based fertilizers enabled continuous crop production and thus increased annual agriculture output. While the introduction and use of inorganic fertilizers provided many societal benefits, significant adverse environmental impacts have been realized and are exacerbated by the large-scale and inefficient nature of modern agriculture operations. For example, fertilizer production and use (i.e., the practice of excessive application) are major contributors to eutrophication and greenhouse gas emissions, resulting in numerous uninhabitable aquatic environments and rising global temperatures, the consequences of which have not yet been fully realized.15 Furthermore, the ability for the agriculture sector to meet future demand imposed by the anticipated global population growth and increasing global affluence has been called into question.13 Meeting the needs of a healthy global society necessitates a sustainable agriculture system, which at its core requires external sources of primary nutrients (N and P), the building blocks of life on earth. Agriculture’s

INTRODUCTION A steady decrease in world hunger and record high agricultural yields realized over the past century1,2 have been enabled, in large part, through the introduction of new technologies and improved production practices. These include the mechanization of farm operations, crop rotation, integrated pest management systems, fertilizers and increased application rates, and new pesticide formulations.2,3 Yet, the foremost challenge facing agriculture today is maintaining yield increases while simultaneously enhancing resource use efficiency, in particular, fresh water, energy, and external nutrients (microand macro-nutrients).4−7 While the inefficient use and interdependence of water and energy in agriculture are highlighted in several seminal studies6,8−10 and recent reviews,11,12 the criticality of nutrient resources has received relatively little attention. As such, the focus here is on the inefficiencies presented during the synthesis, application, and final product incorporation of primary nutrients, nitrogen (N) and phosphorus (P), in agriculture. Modern fertilization practices played a significant role in shaping modern society and accelerating global population growth from approximately 1.6 billion people in 1900 to 7.4 billion in 2017.13 Prior to the 20th century, replenishment of essential soil nutrients was limited by natural cycles, which in turn restricted the scale of agriculture production. The advent © XXXX American Chemical Society

Received: October 8, 2017 Revised: February 6, 2018 Published: February 13, 2018 A

DOI: 10.1021/acssuschemeng.7b03600 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic of the agriculture fertilizer system from synthesis to farm gate, highlighting the primary input and output material flows as well as critical emissions to air, water, and land.

for P for over 50 years.5,16−20,28,33,34 In 2011, approximately 5 million metric tons of urea were applied to U.S. croplands; therefore, 2.5 million metric tons are assimilated by the crop in the best-case scenario (50% NUE).35 The other 50% can follow several pathways through the environment and contributes to atmospheric N2O and aqueous N emissions. While described here for N, other macronutrients in conventional fertilizers, including P and K, suffer from similarly meager use efficiencies and combined contribute to significant impacts across the fertilizer life cycle. While there are nuanced variations in the definition of NUE, it is defined here as the mass ratio of nutrient taken up by the crop to the mass of nutrient applied.16,36,37 As such, low NUE values result from low nutrient assimilation; rather than being acquired by the crop, nutrients can remain bound to soil particles and inaccessible to crop roots (common for forms of P), leach through soil to surrounding surface and ground waters (e.g., nitrate, NO3−), and/or be released via conversion to gaseous forms (e.g., N2O).19 Additionally, applied nutrients may be utilized in natural processes of soil biota, in which case the nutrients play an important role in maintaining a healthy soil biome, yet still contribute to low NUE since they are not available for incorporation into crop biomass and/or the final agriculture product. Primary nutrient flows across the fertilizer life cycle, from cradle to farm gate, are compiled in Figure 1. The consequences of low NUE are significant and discussed further, highlighting the critical need and opportunity for improvement. Notably, applied nitrogen fertilizer can be emitted to the atmosphere as nitrous oxide, N2O, a potent greenhouse gas with 300 times the global warming potential of CO2 (based on 100 year timespan).38 A conservative estimate approximates that 1% of the fertilizer applied to farm fields (emissions measured directly from agriculture fields) is converted to N2O, while more comprehensive estimates, which consider background and off-field conversion, range from 4% to 5%.38 These seemingly small percentages translate into significant emissions when considering the full scale of agriculture fertilization and are equivalent to approximately 116 million kg of N2O for the

projected challenge in meeting demand is due to the following: (i) Nutrient use efficiencies (NUE) have remained constant and low.5,16−20 (ii) Soil quality is decreasing, which increases the demand for external sources of nutrients (i.e., fertilizers).21 (iii) Continuous use of fertilizers reduces soil organic matter, which leads to soil erosion.22,23 (iv) The system relies upon finite and increasingly scarce resources such as land, phosphorus, and potassium.24−28 Achieving agriculture sustainability in accordance with the United States Department of Agriculture’s definition requires a reduction of environmental impacts and increased efficiency of the fertilizer system, while (i) continuing to satisfy human fiber needs, (ii) maintaining an economically viable system, and (iii) enhancing the quality of life of farmers and society as a whole.29 Of the existing design frameworks that offer guidance on how to achieve sustainability at the intersection of society, the environment, and economic vitality, the Twelve Principles of Green Chemistry30 and Engineering31 provide quantifiable metrics to inform design of molecules, products, processes, and systems. In particular, atom economy critically highlights chemical process efficiency at the atomic level.32 The metric applied herein, atom conversion efficiency (ACE), combines atom economy with other established metrics that describe efficiencies at other stages of the fertilizer systemnutrient utilization efficiency (NUE) and the elemental composition of the desired agriculture productto measure the efficiency of N and P use at the atomic scale from synthesis to the farm gate. The compiled results highlight critical inefficiencies across the nutrient life cycle, expanding beyond the common benchmark of NUE, and further identify specific stages and opportunities to increase the ACE.

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ENVIRONMENTAL AND ECONOMIC BURDENS OF CURRENT FERTILIZATION PRACTICES MOTIVATE THE NEED FOR CHANGE Environmental burdens created by nutrient emissions to the atmosphere and aquatic systems are a consequence of low N and P NUEs in conventional fertilizers. NUEs have either decreased or remained constant at 20−50% for N and ≤25% B


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Figure 2. Nutrient application volumes of nitrogen (orange circles; size represents quantity, in kg, applied) and phosphorus (diamonds; size represents quantity, in kg, applied) in the United States by county. Blue circles represent coastal hypoxic waters identified by the National Oceanic and Atmospheric Administration (NOAA) and World Resource Institute.45 Nutrient application data was obtained from the United States Geological Survey (USGS) County-level estimates of nitrogen and phosphorus from commercial fertilizer for the conterminous United States: 1987−2006 database.125 The data analytics software, Tableau, was used to generate the final graphic.

California.42,44,45 The impact on these impaired waterways can be related to fertilizer application volumes, as presented in Figure 2, highlighting the pervasiveness of this issue across the U.S. (Note: The inclusion of impaired coastal waterways in Figure 2 is not to suggest that these are the only highly eutrophic waters but rather is a consequence of comprehensive data availability. Freshwater eutrophication, particularly in the Great Lakes, has been the subject of ongoing research.46−48) Further, there are significant economic consequences associated with eutrophication. In the U.S., economic loss due to fresh water eutrophication is estimated at $2.2 billion dollars annually, which is acknowledged as an underestimate of the true economic burden.49 Tourism and recreation, commercial fishing, property values, damages to human health, drinking water treatment, and mitigation efforts account for the greatest costs of the total economic burden estimate.50 Further, a comprehensive nitrogen assessment in Europe estimates that excess nitrogen costs the E.U. between $82 and $376 billion anually.51

conservative (1%) estimate and 582 million kg of N2O for the more comprehensive (5%) estimate.35,38 This makes the agriculture sector one of the largest contributors of N2O emissions in the U.S., with soil management contributing 75% of total N2O emissions.39 Moreover, associated emissions generated during the production of the fertilizers contribute to the agriculture sector’s global warming potential (GWP), making the agriculture sector the fourth largest contributor to total GWP, behind electricity generation, transportation, and “other” industrial processes.39 For example, approximately 45 MJ of energy are consumed and 2.6 kg CO2 equivalents are emitted per kilogram of anhydrous ammonia (the basis of all nitrogen fertilizers).33,38,40 Further, the energy consumption attributed to the total annual ammonia production, via the Haber-Bosch process, is estimated to be 1−2% of the annual global energy production, equivalent to 6−12 quadrillion BTU.38,41 In addition to atmospheric emissions, release of excess N and P not acquired by crops to surrounding aquatic environments is one of the most pressing environmental consequences of modern fertilization practices. Aquatic N and P emissions (e.g., to lakes, rivers, and oceans) initiate rapid growth of aquatic biota (e.g., algae and plants), which depletes natural waters of their dissolved oxygen supply. A water body becomes hypoxic when oxygen levels fall below 2 mg/L, making it impossible to support aquatic life (e.g., fish). In 2008, approximately 245,000 km2 of oceans worldwide were classified as a “dead zone” (waters too hypoxic to sustain aquatic life).42 Furthermore, 60% of coastal rivers and bays in the U.S. are classified as moderately eutrophic or worse,43 with the following most significantly impacted: the Mississippi River delta leading into the Gulf of Mexico, the Chesapeake Bay, the Great Lakes, Puget Sound, San Francisco Bay, and the southern coastal waters of

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DEPENDENCE ON FINITE RESOURCES AND MEETING FUTURE AGRICULTURE DEMAND Finite resources critical to the production of fertilizers include fossil fuels, arable land, and phosphorus. Currently, the production of fertilizers (nitrogen-based fertilizers, in particular) depend upon fossil fuels such as oil and natural gas to support the energy intensive Haber−Bosch process and mining operations associated with phosphate rock extraction. Phosphate rock, itself, is a finite resource, and deposits are located in geopolitically volatile regions. Finally, arable land, the foundation of large-scale crop production, is increasingly realized as a critically finite resource with global soil erosion rates of 10 million hectares annually.22,23 The following C


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ACS Sustainable Chemistry & Engineering sections highlight the urgent need to consider preservation of these finite resources as a necessary path toward reversing our current unsustainable trajectory. Finite Resources Used in Production of N-Based Fertilizers. N-based fertilizer use in the U.S. increased by an average of 198 t per year from 1960 to 2011.35 Production of N-based fertilizers is accomplished through the Haber−Bosch process, which transforms atmospheric nitrogen into liquid ammonia according to the following chemical reaction:40,75 3CH4(g) + 4N2(g) + 6H 2O → 3CO2 (g) + 8NH3(l)

(1)

While atmospheric nitrogen is a near infinite resource, several other components of this process are not. The significant activation energy barrier for this reaction to proceed necessitates a catalyst, commonly metal-based, which is sourced from rocks and naturally occurring ores.52 In addition, fresh water is increasingly recognized as a finite resource.53 Increased demands from other industrial sectors (e.g., energy generation) as well as agriculture processes further downstream (e.g., irrigation) will only add to an already stressed water system.54−56 Phosphorus: An Essential Finite Resource. Phosphorus is an essential element for life. In addition to being fundamental to biological energy transfer (via adenosine phosphates), it is an essential building block of RNA and DNA.57 Since there are no substitutes for P in these biological processes, there is added concern over “peak phosphorus” (defined as reaching maximum phosphate production) projected for 2033 and potential depletion of resources within the next 100 years.24,26,27 Peak resource occurrences, such as peak crude oil, have proven challenging to predict due to unforeseen geopolitical issues, discovery of new deposits, improved extraction technologies, and emergence of new competing resources.58−60 Yet, peak phosphorus is unique due to the followiong: (i) Mining of phosphate rock is currently the most economically viable source of large-scale P production for use in fertilizer.24 (ii) A large proportion of phosphate reserves are concentrated in geopolitically volatile regions.24,61−63 The recognized limit on P resources instills a sense of urgency to enhance the efficient use of this invaluable and finite element. Coupled with resource recovery technologies,26 there is the opportunity to not only prolong P availability but to also close the loop on this finite resource.64 Arable Land: The Foundational Finite Resource. Arable land is an essential resource for agriculture production that is becoming increasingly finite due to global population growth (adding diverse demands on finite land area), increased global affluence (causing shifts in demand for land-intensive animalbased products), desertification, soil erosion, and urbanization, all of which add stress to existing farmlands.65−69 Additionally, over half of the available global land area is already being used for agriculture (∼51 million km2).25 Multiple independent estimates of crop land availability70−72 predict being at or near maximum land use capacity for agriculture production73 (summarized in Figure 3). These historic and projected trends in active cropland motivate the need for “doing more with less”, that is, increasing yields using the currently available land to meet ongoing demands necessary to sustain a healthy global population.

Figure 3. Historical and projected active cropland in millions of hectares (ha) as determined by the Organisation for Economic Cooperation and Development (OECD),71 the International Assessment of Agricultural Knowledge (IAASTD),70 and the UN Food and Agriculture Organization (UN FOA)72 (left axis) plotted with the trend in global population presented by the United Nations (median projection, black trend line, right axis).13 Dashed lines indicate projections as determined by the respective institutions.

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ATOM CONVERSION EFFICIENCY (ACE): A METRIC TO QUANTIFY FERTILIZER SYSTEM EFFICIENCY The preceding sections highlight the inefficient use of finite resources by a sector that has direct influence on the well-being of our global society. Current fertilization practices have us on an unsustainable trajectory that includes continued environmental degradation, depletion of finite resources, and the potential inability to meet projected demand for agriculture products. Inefficiencies that cascade across the fertilizer life cycle are the impetus of the current unsustainable path, and as such, efficiency improvements are proposed as a way to enhance the sustainability of agriculture fertilization. The Twelve Principles of Green Chemistry (GC)30 and Green Engineering (GE)31 were established to facilitate the development of sustainable chemical and engineering design practices. A critical underpinning of both sets of Principles is the maximization of efficiency (of mass, energy, space, and time). Established GC metrics (e.g., atom economy and E-factor) focus on efficiency improvements to chemical processes, while the Principles of GE offer guidance for achievement of systemwide sustainability.30,31 Here, the principle of atom economy is combined with other existing efficiency metrics to track the conversion of N and P atoms from acquisition to final agriculture product incorporation. Corn is selected as a model crop due to its nutrient intensive nature and large-scale production (corn is the most produced crop in the U.S.74). Thus, the corn kernel is considered the final agriculture product.68 The application of green metrics to this model system aims to (i) highlight the meager efficiencies of the fertilizer system, (ii) capture those components of the fertilizer life cycle that suffer the greatest inefficiencies, and (iii) identify the stages that contribute most to improvements in efficiency. As such, the results are used to identify opportunities for increasing fertilizer efficiency and motivate the development of alternatives to address and improve system-wide sustainability. Atom Economy. Conventional chemical reaction efficiency, as percent yield, is defined as the ratio of the actual and theoretical yields. This approach to quantifying chemical synthesis efficiency encourages improvements to maximize the desired product yield and does not account for byproduct formation. Atom economy (AE) is a metric established to D


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ACS Sustainable Chemistry & Engineering ⎛ 60.07 g/mol ⎞ AE Urea = ⎜ ⎟ × 100 = 83% ⎝ 72.03 g/mol ⎠

evaluate the efficiency of chemical reactionsnamely, the conversion of reactants into the desired productwith the goal of informing redesign in a way that maximizes this atomic conversion.32 Atom economy is determined according to:30 ⎛ Mass of atoms in the desired product ⎞ ⎟ × 100 AE = ⎜ ⎝ Sum of mass of atoms in all reactants ⎠

The atom economy for the synthesis of urea is determined as the product of individual chemical reaction efficiencies, following a chain efficiency approach:77

(2)

and is applied herein to the production of primary N and P nutrient forms. Further, the concept of atom economy inspired the development of ACE, a metric that is applied here as a measure of fertilizer system efficiencytracking nutrients, N and P, from their raw form (akin to reactants) to their final form as essential nutrients in a food product (akin to the desired product). The determined ACE value for different nutrient forms offers a system-wide efficiency metric that moves beyond those traditionally reported for individual stages. The approach and associated calculations are presented in detail for urea and monoammonium phosphate (MAP), the predominant N- and P-based fertilizers used in the U.S.35 and presents the best-case scenario (i.e., uses the upper range of NUE, 50% and 25%, respectively). The same approach is applied to additional forms of N and P used in conventional fertilizer, and the results are compiled in Table 1.

Nutrient Form

AESynthesis (%)

NUEMax (%)

Urea Anhydrous Ammonia Ammonium Nitrate Diammonium Phosphateb Monoammonium Phosphateb Superphosphate

35 42 45 35 35 59

50 50 50 25 25 25

46 46 46 52 52 52

Eall = E1 × E2 × ... En

(8)

AE Urea Synthesis = AE NH3 × AE Urea

(9)

AE Urea Synthesis = 0.42 × 0.83 = 0.35 or 35%

Ca3(PO4 )2 (s) + 3H 2SO4 (aq) → 2H3PO4 (aq) + 3CaSO4 (s)

and MAP synthesis from phosphoric acid ACE (%) 8 10 10 5 5 8

(10)

Atom Economy Calculations: Monoammonium Phosphate (MAP). Conventional phosphorus fertilizers are sourced from phosphate rock, which is obtained through surface mining operations to produce calcium phosphate rock.78 The exact composition of the final product will depend on the geographic location and may contain other elements (e.g., halogens).79 Here, it is assumed that all phosphate rock is in the form Ca3(PO4)2 and that the primary use of monoammonium and diammonium phosphate (MAP and DAP, respectively) is for the provision of phosphorus to agriculture soils (i.e., only the phosphorus-based nutrient ions will be considered in the ACE calculations). Treatment with sulfuric acid converts phosphate rock into usable forms of P for fertilizer, generating either superphosphate or phosphoric acid, which is then used to produce MAP and DAP. Phosphoric acid synthesis proceeds according to the following79:

Table 1. Primary Forms of Nitrogen (N) and Phosphorus (P) Used in Conventional Fertilizer with Their Respective Atom Economies of Synthesis (AEsynthesis), Best Case Nutrient Use Efficiency (NUEMax), Corn Kernel N and P Content, and Determined Atom Conversion Efficiency (ACE) Kernel N or P Contenta (%)

(7)

(11)

80

H3PO4 (aq) + NH3(l) → NH4H 2PO4 (s)

(12)

Using the molecular weights of the constituents present in eqs 11 and 12, the associated atom economy calculations for MAP synthesis are as follows: AE H3PO4 =

Efficiency of nutrient incorporation into the final agriculture product is dependent on the crop. bFor nutrient forms that include both N and P, the calculated efficiencies represent atom economies for P only.

2 × 107.3 = 0.34 (3 × 98.08) + 328.24

(13)

a

AEMAP =

124.07 =1 107.3 + 17.04

AEMAP Synthesis = 0.34 × 1 = 0.34

Atom Economy Calculations: Urea. Urea, like all nitrogen-based fertilizers, begins with synthesis of liquid ammonia (via eq 1), which is then converted to urea as follows: 2NH3(l) + CO2 (g) → (NH 2)2 CO(s) + H 2O 76

The atom economy follows:

for reactions 1 and 3 is determined as

(4)

⎛ 112.08 g/mol ⎞ AE NH3 = ⎜ ⎟ × 100 = 42% ⎝ 268.35 g/mol ⎠

(5)

⎛ ⎞ MWUrea ⎟⎟ × 100 AE Urea = ⎜⎜ ⎝ MW2 mol NH3 + MW1 mol CO2 ⎠

(6)

(15)

Fertilizer System Efficiency: Determination of Atom Conversion Efficiency (ACE). The calculated atom economies for urea and MAP are used in combination with the established NUE of N and P and the known N and P composition of the final corn kernel to estimate the overall efficiency of N and P from acquisition (N2 from our atmosphere, P from phosphate rock) to farm gate (N/P in an agriculture product). The following best-case efficiency scenarios are used in the ACE determination, including the (i) highest reported NUEs for N and P (50% and 25%, respectively)5,17,19,20,28 and (ii) established kernel N and P contents of 46% and 52%81−83 (representing the percent of the total assimilated nutrient in the kernel), respectively. The system efficiencies of urea and MAP are outlined in Figure 4, and the respective dimensionless ACE values are determined as the product of the equally weighted efficiencies at each stage of the process. For the urea system,

(3)

⎛ ⎞ MW8 mol NH3 ⎟⎟ × 100 AE NH3 = ⎜⎜ ⎝ MW4 mol N2 + MW3 mol CH4 + MW6 mol H2O ⎠

(14)

E


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Figure 4. Schematic of the processes for nitrogen (N) and phosphorus (P) fertilizer systems, from raw material acquisition to incorporation into agriculture products as nutrients. The atom economy and atom conversion efficiency (ACE) results for each stage and the system, respectively, are determined following the approach described in the text for representative N and P fertilizers (urea and monoammonium phosphate, respectively).

ACE = AESynthesis × NUE × Kernel N Content

(16)

ACE = 0.35 × 0.50 × 0.46 = 0.08 or 8%

(17)

the selection of current readily available nutrient forms. For example, anhydrous ammonia offers a 7% gain in the AESynthesis over urea and a total system efficiency improvement of 2%. Despite not having the highest AESynthesis, urea remains the dominant N form of fertilizer used in the U.S.,35 most likely due to its lower market cost and relative stability (e.g., safer for storage and transfer) than anhydrous ammonia. While economics is the prevailing driver for choices surrounding nutrient form, the following examples indicate the opportunity to combine the choice of nutrient form with ongoing efforts to improve NUEs in a way that maximizes the overall system efficiency. Further, movement toward stricter regulation of agriculture nutrient emissions in an effort to restore aquatic environments87 may provide future economic incentives (e.g., water quality trading)88 that shift motivations in the choice of nutrient form. Of all stages in the fertilizer system, scientists and engineers have the greatest ability to manipulate NUE (e.g., developing technologies that deliver nutrients more efficiently to the crop) and thus improve NUE and the overall ACE value. For example, in the application of urea, an achievable 20% increase in NUE translates into a 3% increase in ACE. In addition to tangible benefits for the crop, there are significant environmental benefits to be realized, including an estimated savings of 3.5 billion kg of N per year (a value of roughly 2.2 billion U.S dollars based on 2011 annual U.S. application quantities and fertilizer prices). 35 Finally, if NUE improvements are considered in combination with increases in AESynthesis, system conversion efficiencies compound resulting in a 10% increase for urea. This result is greater than sum of individual stage improvements (i.e., in the examples provided, AESynthesis alone results in a 5% and NUE alone results in a 3% improvement in ACE). As expected, more aggressive increases in AESynthesis and NUE will further increase the system efficiency. Finally, the N and P content in the corn kernel (the final agriculture product included here) is 46% N and 52% P.81−83 The ratio of N and P in the corn kernels is governed by the crop demands and its established biochemical system. These values have remained constant since the earliest found reports in 192481 and indicate an established biochemistry of the crop phytobiome that results in the final composition. While the final product elemental content is biochemically constrained, it is also dependent on the specific crop and thus will vary based

and MAP system ACE = AESynthesis × NUE × Kernel P Content

(18)

ACE = 0.35 × 0.25 × 0.52 = 0.05 or 5%

(19)

Additional forms of N and P used in fertilizers are included in Table 1 accompanied by their respective efficiency values for each stage and the overall system efficiency (ACE). There are three important outcomes from this exercise: (i) The system conversion efficiency for all nutrient forms is very low, ranging from 5% to 10%. (ii) Identified low AEs depend on the nutrient form, meaning that AESynthesis is the primary distinguishing factor between the different nutrient forms. (iii) These low efficiencies underpin the opportunity for improvements, guided first by nutrient choice and second through NUE enhancements. The AESynthesis is governed by the nutrient form and by existing chemical processes that transform raw N and P into forms used in conventional fertilizer. While identifying alternative sources of N and P that improve synthesis stage efficiencies (or even better, eliminate the need for this conversion) would offer leapfrog system efficiency gains, there remains an absence of reliable abundant alternative sources of N and P. As such, the AE of synthesis is constrained to the established chemical reactions (e.g., liquid ammonia synthesis, phosphate rock extraction through mining). Still, significant research is being pursued to improve the Haber− Bosch process, particularly in the areas of enhanced catalysis (to lower the energy barrier of triple bonded N2 conversion to NH3) as well as the identification of alternative energy sources, which could potentially eliminate or replace methane with a lower molecular weight reactant compound.84−86 The latter has the potential to impact the AE of nitrogen-based fertilizers through alteration of both the numerator (e.g., reducing coproduct formation, such as carbon dioxide) and the denominator (e.g., reducing the total reactant molecular weight). A combined improvement in AE of 20%, accomplished through these synthesis advancements, would translate into a 5% system efficiency gain for urea. Yet, until these advancements are realized, improvements in AESynthesis are limited to F


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ACS Sustainable Chemistry & Engineering on final agriculture product of interest. Furthermore, because the N and P content of the desired agriculture product is constrained, it is impossible to achieve a 100% ACE value. Therefore, the goal should be to achieve the theoretical maximum (assuming 100% AEsynthesis and 100% NUE) ACE values of 46% and 52% for N and P, respectively. The focus on atom economy and conversion efficiency from acquisition to farm gate highlights inefficiencies of agriculture fertilization practices that can be overlooked by focusing exclusively on NUEs. Further, the outcomes of this exercise are intended to direct our attention to those individual stages in which we, scientists and engineers, have the opportunity to contribute to efficiency gains. Still, this approach is not without limitations. The calculated ACE values do not capture resource extraction efficiencies (e.g., arising from the range in phosphate ore grades), process energy efficiencies, nor the inherent hazardous nature of substances produced and/or consumed within the system. As such, in its current form, associated environmental impacts are not incorporated in the ACE value. Furthermore, ACE is independent from geographic location and organic soil nutrient management (e.g., replenishment through composting of nonproduct biomass, such as corn stover) because it is governed by the conversion efficiency of the nutrient atoms into a desired product selected rather than fertilizer application rates. These limitations should be considered when the outcomes of this exercise are applied to inform improved design of new approaches to enhancing efficiencies, as well as offer additional opportunities to further advance agriculture sustainability.

apply nutrients in a location that maximizes the probability of uptake by the root system.16,92 While FBMPs have proven to be an effective way to reduce excess unutilized fertilizer, increase productivity, improve water quality, and reduce the overall environmental burden of agricultural,93−95 the industry has not yet fully achieved agriculture sustainability through successful BMP implementation. This is due to the many challenges to implementation, including a required change in behavior, a lack of incentive for farmer participation, and/or limited oversight preventing widespread adoption.16,34,93,95,96 Additionally, BMPs oftentimes require capital and/or maintenance costs, which are not directly accrued by the farmer.97,98 For example, the estimated costs necessary to implement BMPs to meet the 2025 goal for reduction of nutrient loads in the Chesapeake Bay’s watershed outlined in the Watershed Implementation Plans (WIPs) is $3.6 billion, with an annual estimated $900 million thereafter required to maintain the proposed total maximum daily loads.97 As such, it is recognized that the demand for improvements in nutrient management exceeds the current capabilities of BMPs suggesting a need to couple with the development of new fertilizer product technologies.5,16 In addition to agriculture BMPs, there has been significant efforts to develop technological alternatives for delivering nutrients more efficiently to crops. An established and leading product alternative is CRFs,99 which are designed to improve NUE by altering the time scale that nutrients are delivered to plants such that nutrient availability is synchronized with natural plant growth patterns.100 There are two primary forms of CRFs, coated and uncoated. Coated CRFs use a solid thin layer, usually a polymer, that serves to stabilize the nutrient core enabling slow dissolution and release to the surrounding soil.101 Uncoated CRFs involve a combination of chemical compounds (e.g., nitrification inhibitors) and conventional fertilizers to hinder natural biological processes of enzymes and soil bacteria that transform nitrogen to its most mobile form, nitrate.99 Current CRFs have demonstrated advantages over conventional fertilization practices, including 20−30% reduction in fertilizer application rates to achieve the same yields, which has the additional benefit of reduced labor costs.101 Yet, there are several technological and economic challenges that restrict widespread adoption of CRFs, including limited control over nutrient release, potential to cause damage to crops, and high costs. Since CRFs rely on the presence of moisture and temperature to achieve their controlled nutrient release,102,103 the release of nutrients is governed by environmental conditions rather than crop needs. Further, the performance of CRFs is complicated by extreme weather conditions such as heavy rains and droughts. For example, when the rate of release increases significantly, rapid release of nutrients is concentrated and can cause damage to crop roots through soil acidification.102 To realize the true NUE benefit of CRFs, the total nutrient delivered to the soil and timing of the delivery should be carefully considered (e.g., more of a dynamic rather than scheduled application procedure) so as to optimize delivery efficiency and nutrient utilization to avoid concentration effects as well as ensure that unused nutrients do not remain in the soil after harvest.100,102 Finally, insoluble components of the CRF shell result in the accumulation on nonbiodegradable solids in agriculture soils.104 In addition to these technological challenges, economics are a primary barrier to widespread use of CRFs with costs ranging 2.5−8 times the cost of conventional fertilizers limiting their applicability to

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IMPROVING NUE: CURRENT APPROACHES AND CHALLENGES Most conventional fertilizers are produced and applied to agriculture fields as solid pellets. Nutrients are released as this pellet dissolves, primarily in the form of ions. These nutrient ions transport through and interact with the surrounding soil system, influencing the NUE. For example, electrostatic interactions cause nutrients to either be attracted to negatively charged soil particles or facilitates rapid transport through the soil column due to repulsive forces, both of which influence nutrient bioavailability to crop roots.78,89−91 Additionally, strong mineral bonding between phosphate ions and soil particles severely limit P accessibility.28,78,90 Volatilization (e.g., N2O or NH3) also influences NUE and results from biochemical processes (e.g., (de)nitrification) that occur in the soil, including those necessary to transform nutrients into a usable form.89 The range of known pathways that cause low NUE, and thus limit nutrient assimilation by the crop, has long motivated alternative approaches aimed at enhancing NUE. While established agriculture best management practices (BMPs) and technological alternatives (e.g., controlled release fertilizers, CRFs) have been developed, there remain challenges that pose barriers to widespread adoption. These two alternatives are discussed and serve as learning opportunities to inform improved design of ongoing innovative approaches. The International Fertilizer Association’s (IFA) established fertilizer BMPs (FBMPs), which are outlined in their Global 4R Nutrient Stewardship Framework, apply the right nutrient source at the right rate, right time, and right place.92 Examples of FBMPs include implementation of crop rotation to naturally replenish soil nutrients, tailor fertilization rates based on actual monitored soil nutrient levels, apply fertilizers at a specific stage of growing cycle when they are more likely to be taken up, and G


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ACS Sustainable Chemistry & Engineering large-scale crop production and thus realization of the maximum benefit through improved NUE.104

downstream emissions). Economics is an often-overlooked

EMERGING ALTERNATIVES TO IMPROVE FERTILIZER SYSTEM EFFICIENCY The discussion of the current alternatives and their challenges intends to highlight the opportunity for continued development of innovative solutions to enhance NUE and reduce environmental implications associated with agriculture fertilization. The informative outcomes from the application of ACE offers guidance for development of sustainable next generation fertilizers and nutrient delivery systems. The development of informative design guidelines and their application early in the development of new products is particularly germane given the recent and rapid emergence of nano-enabled fertilizers for enhancing NUE and agriculture production.105−110 These include proposed designs where the engineered nanomaterial (ENM) serves as the source of nutrient (typically for essential micronutrients, such as Fe, Mn, Zn, or Cu)111−115 or as carriers of primary nutrients (e.g., N as urea) for enhanced slow and controlled release capabilities.105,116,117 The enhanced surface area, small size, and ability to manipulate particle features in a controlled manner are all advantageous properties accessible at the nanoscale that are inspiring these innovative approaches. Further, there is the opportunity to take advantage of the nanosize scale through stabilization of nutrients, offering the ability to access the unique properties at the nanoscale without the physical ENM. Advantages to such an approach include the preclusion of potential adverse consequences yet to be evaluated for large-scale application of ENM platforms. Regardless of the design, trade-offs exist with any emerging alternative; considering the nano-enabled system, not merely the enhancement product performance, it is critical to confirm the potential to enhance fertilizer system efficiency. In addition to development of nutrient delivery platforms for improved NUE, emerging technological innovations further downstream aim to enhance system efficiency through nutrient resource recovery. This includes the recovery of valuable nutrients (e.g., N and finite P) from urine and wastewater.118−121 The products of these process are proposed as potential fertilizer alternatives (e.g., struvite122−124), which can be used to help close the loop on the nutrient cycles (e.g., by providing an alternative to raw P). Still, this approach does not address the inherent problem, which is inefficient current fertilization practices and thus should be considered in conjunction with solutions aimed at addressing upstream inefficiencies. Similar to the developing nano-enabled alternatives above, these nutrient recovery processes require resources (e.g., infrastructure, energy) and as such present trade-offs that must be considered holistically to ensure gains in fertilizer system efficiency are realized.

advantageous to both the user (i.e., the farmer) and the

aspect of initial design of alternatives and must prove

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producer (i.e., fertilizer companies). As such, it is critical to both consider and disseminate information on the economics of the system to prevent aversion to development and adoption. From the perspective of the producer, the reduction in total nutrient applied (i.e., the fertilizer product) must be balanced by the increased product cost in a way that remains favorable for their bottom line. From the perspective of the user, the increased product cost must be balanced by a reduction in total amount of product applied, increased yield, and/or potential cost savings realized through preclusion of other remediation strategies to limit nutrient emissions. While there are many factors that contribute to the design of next generation fertilizers, it is the nexus of these identified tradeoffs−technological performance, environment, and economics− that a sustainable design approach enables accomplishment of these goals.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (412) 624-1683. E-mail: [email protected]. ORCID

Leanne M. Gilbertson: 0000-0003-3396-4204 Notes

The authors declare no competing financial interest. Biographies

AN IDEAL NEXT GENERATION FERTILIZER Joshua H. Urso is a Ph.D. student in the Department of Civil and

A successful next generation fertilizer will preclude the identified limitations of current technologies (e.g., be composed of benign substances that can ideally all be utilized by the crop and/or surrounding phytobiome; does not require significant change in behaviors and can be integrated as seamlessly as possible into the existing infrastructure), while providing improved efficiency of nutrient use, yield enhancement, and reduced environmental burden of fertilizer practices across the life cycle (e.g., upstream embedded resource intensity and

Environmental Engineering at the University of Pittsburgh working with Dr. Gilbertson. Prior to attending the University of Pittsburgh, Josh earned his bachelor’s degree in biochemistry from Oberlin College. Josh’s doctoral research aims to enhance agriculture sustainability, focusing specifically on engineering solutions to improve nutrient delivery and use efficiencies. H


Perspective


(11) Gleick, P. H. Water and Conflict: Fresh Water Resources and International Security. Int. Secur. 1993, 18 (1), 79−112. (12) Lovarelli, D.; Bacenetti, J.; Fiala, M. Water Footprint of crop productions: A review. Sci. Total Environ. 2016, 548-549, 236−251. (13) World Population Prospects, 2015 Revision, Key Findings and Advance Tables; Working Paper No. ESA/P/WP/.241; United Nations, DESA, Population Division, 2015. (14) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of ammonia synthesis changed the world. Nat. Geosci. 2008, 1 (10), 636−639. (15) Melillo, J. M.; Richmond, T.; Yohe, G. W. Climate Change Impacts in The United States: The Third National Climate Assessment; U.S. Global Change Resarch Program, 2014; p 841. (16) Zhang, X.; Davidson, E. A.; Mauzerall, D. L.; Searchinger, T. D.; Dumas, P.; Shen, Y. Managing nitrogen for sustainable development. Nature 2015, 528 (7580), 51−59. (17) Cassman, K. G.; Dobermann, A.; Walters, D. T. Agroecosystems, Nitrogen-Use Efficiency, and Nitrogen Management. Ambio 2002, 31 (2), 132−140. (18) Smil, V. Nitrogen in crop production: An account of global flows. Glob. Biogeochem. Cycles 1999, 13 (2), 647−662. (19) Fageria, N. K.; Baligar, V. C. Enhancing Nitrogen Use Efficiency in Crop Plants. In Advances in Agronomy; Academic Press, 2005; Vol. 88, pp 97−185. (20) Baligar, V. C.; Fageria, N. K.; He, Z. L. Nutrient Use Efficiency in Plants. Commun. Soil Sci. Plant Anal. 2001, 32 (7−8), 921−950. (21) Pimentel, D. Soil Erosion: A Food and Environmental Threat. Environ. Dev. Sustain. 2006, 8 (1), 119−137. (22) Pimentel, D.; Burgess, M. Soil Erosion Threatens Food Production. Agriculture 2013, 3 (3), 443−463. (23) National Resources Conservation Service. Soil Erosion on Cropland; 2007 National Resources Inventory; U.S. Department of Agriculture, 2010. (24) Cordell, D.; White, S. Peak Phosphorus: Clarifying the Key Issues of a Vigorous Debate about Long-Term Phosphorus Security. Sustainability 2011, 3 (10), 2027−2049. (25) Roser, M.; Ritchie, H. Yields and Land Use in Agriculture; Our World in Data, 2017. (26) Cordell, D.; Rosemarin, A.; Schröder, J. J.; Smit, A. L. Towards global phosphorus security: A systems framework for phosphorus recovery and reuse options. Chemosphere 2011, 84 (6), 747−758. (27) Smil, V. Phosphorus in the Environment: Natural Flows and Human Interferences. Energy Env. 2000, 25, 53−88. (28) Roberts, T. L.; Johnston, A. E. Phosphorus use efficiency and management in agriculture. Resour. Conserv. Recycl. 2015, 105, 275− 281. (29) Sustainable Agriculture: Definitions and Terms; Alternative Farming Systems Information Center, NAL, U.S. Department of Agriculture. https://www.nal.usda.gov/afsic/sustainable-agriculturedefinitions-and-terms#toc2 (accessed Septmber 12, 2017). (30) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: Oxford, 1998. (31) Anastas, P. T.; Zimmerman, J. B. Through the 12 Principles: Green Engineering. Environ. Sci. Technol. 2003, 37, 94A−101A. (32) Trost, B. M. The Atom Economy-A Search for Synthetic Efficiency. Science 1991, 254 (5037), 1471−1477. (33) Mulder, A. The quest for sustainable nitrogen removal technologies. Water Sci. Technol. 2003, 48 (1), 67−75. (34) Weber, C.; McCann, L. Adoption of Nitrogen-Efficient Technologies by U.S. Corn Farmers. J. Environ. Qual. 2015, 44 (2), 391−401. (35) USDA ERS. Fertilizer Use and Price https://www.ers.usda.gov/ data-products/fertilizer-use-and-price/ (accessed September 14, 2017). (36) Reich, M.; Aghajanzadeh, T.; Kok, L. J. D. Physiological Basis of Plant Nutrient Use Efficiency − Concepts, Opportunities and Challenges for Its Improvement. In Nutrient Use Efficiency in Plants; Plant Ecophysiology; Springer, Cham, 2014; pp 1−27.

Leanne M. Gilbertson is an Assistant Professor in the Department of Civil and Environmental Engineering at the University of Pittsburgh. Her research group focuses broadly on sustainable design of emerging materials and technologies proposed for use in areas at the nexus of the environment and public health. Dr. Gilbertson earned her bachelor’s degree in chemistry from Hamilton College and her Ph.D. in chemical and environmental engineering from Yale University, supported by the NSF and EPA STAR Graduate Research Fellowships. She was a postdoctoral associate in the Center for Green Chemistry and Green Engineering at Yale prior to joining the faculty at the University of Pittsburgh.

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ACKNOWLEDGMENTS The authors acknowledge the generous funding support from the Department of Civil and Environmental Engineering in the Swanson School of Engineering at the University of Pittsburgh and Sarah Urso for creation of the TOC and abstract art.

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