Reactive NitrogenNitrogen Emissionspubs.awma.org/flip/EM-Sept-2015/rotz.pdfEmissions in Crop...

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by C. Alan Rotz and April B. Leytem

C. Alan Rotz is an agricultural engineer with the U.S. Department of Agriculture’s (USDA) Agricultural Research Service in University Park, PA, [email protected]; April B. Leytem is a research soil chemist with USDA Agricultural Research Service in Kimberly, ID, [email protected].

em • feature

NitrogenNitrogenEmissionsNitrogenReactive

from Agricultural OperationsA look at current approaches to mitigate nitrogen emissions from agricultural operations.

Reactive nitrogen (Nr) is essential to the growth of plants and animals and is typ-ically the most limiting nutrient in agri-cultural production. Currently, 40% to

60% of the global population depends upon crops produced with synthetic nitrogen fertilizer.1 Liu et al. (2010)2 estimated that in 2000, total N input

to global crop production was approximately 137 Tg N yr -1 with 50% coming from synthetic fertil-izer, 16% from biological N fi xation, 13% from manure, 11% from atmospheric deposition, 8% from recycled crop residue, and 2% from sedi-mentation. With a world population of 9 billion in 2050 and a global GDP that more than triples,

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crop production of food and feed may need to increase by more than 50% compared with 2010.3 This increase must occur mainly by intensifying existing cropland and as a result Nr sources will rise from 185 to 232 Tg N yr -1 over that period.

While N fertilizer has enabled the growing global population to maintain food production, the ineffi -cient and sometimes excessive use of N fertilizers has also contributed to degradation of air, water, and soil quality. Estimates of the conversion of

N used in food production to that actually con-sumed by humans ranges from 10% to 20%4,5 with greater than 50% of the N fertilizer applied to cropland lost to downstream and downwind habitats.1 The current rate of Nr loss to the envi-ronment due to agricultural production is more than 10 times the rate that occurred at the end of the 1800s.5 This loss is of increasing concern for its negative consequences on human, animal, and environmental health.

Emissions in Crop ProductionNitrogen transforms and is lost through many pathways as it cycles through agricultural oper-ations (see Figure 1). The goal is to capture the available Nr in the protein of the crops produced, but substantial losses occur. The primary forms of Nr loss are ammonia (NH3), nitrate (NO3), nitrous oxide (N2O) and N oxides (NO and NO2 or NOx). The major loss in crop production is often in the form of NO3 lost through leaching to groundwater and runoff in surface water. Ammonia emissions can also be substantial, particularly when urea fer-tilizer is applied to the fi eld surface. The amount volatilized is largely dependent upon the time on the soil surface and secondarily by soil tempera-ture and pH.6 Once the fertilizer is incorporated by tillage or precipitation, volatilization diminishes.

In the soil, N undergoes nitrifi cation and denitrifi -cation processes. With nitrifi cation, bacteria in the

ImportedFeed

ExportedFeed

ExportedManure

AnimalProduct

Fertilizer

Fixation

Deposition

NH3 N2O NOx

NH3 N2O

NO3

NO3

NOx

Crop Production

Animal Production

Figure 1. Nitrogen fl ows and Nr losses from agricultural operations. The width of arrows represents the relative amount of N among pathways. Adapted from Robertson et al. (2013).10

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soil convert NH3 to nitrites and then NO3. During this transformation, small amounts of N2O and NO are produced, which can be emitted to the atmosphere. Denitrification is a microbial process where nitrate is reduced to produce molecular N (N2). Incomplete denitrification occurs though producing N2O and NO, which may escape to the atmosphere. The losses through these path-ways are dependent upon many management and environmental conditions. A remaining source of reactive N loss is NOx emitted by combustion of fossil fuels. Operations for tillage, planting, harvest-ing, and handling of crops emit small amounts of NOx to the atmosphere. The amount of Nr lost from crop and pasture lands varies with N source, timing, application method, and climatic conditions (see Table 1).

Manure is often applied to cropland by broadcast spreading across the field surface. Essentially, all ammonium N remaining in the manure is rapidly volatilized as NH3 (see Table 1). After the remain-ing manure N is incorporated in the soil, sub-stantial NO3 loss can occur, particularly when the manure is applied at a time when the crop is not actively growing and taking up N.

Emissions in Animal ProductionAll forms of reactive N loss also occur in ani-mal production (see Figure 1). Most occur from manure during animal housing, storage, and field application.7 The major N loss is often in the form

of NH3. Manure temperature and pH control the amount of NH3 formed from manure N and thus the emission rate.6 Other factors such as the amount and type of bedding material used and the air velocity across the surface also influence the rate and amount emitted. When animals are housed on unpaved surfaces, leaching, nitrifica-tion, and denitrification processes can also emit NO3, N2O, and NO. Thus, the amount and type of Nr loss varies widely with the type of housing used (see Table 1).

Manure is stored in solid, slurry, or liquid forms. Solid manure containing large amounts of bed-ding material can be stored in stacks. Substantial amounts of NH3 can be lost depending upon the depth of the stack and the length of the storage period. Nitrification and denitrification processes can also occur in this environment (see Table 1). When manure is stored as slurry in a tank, NH3 loss can be relatively low depending upon the exposed surface area. Manure solids may form a crust on the surface that reduces NH3 emission but promotes nitrification and denitrification pro-cesses and N2O and NO emissions. Storing and processing liquid manure in an anaerobic lagoon increases total N loss, but denitrification processes are more complete with up to half of the N lost as nonreactive N2. Following field application of manure, remaining ammonium N is rapidly vol-atilized as NH3 until the manure is incorporated into the soil.

Table 1. Relative amounts of annual Nr loss from U.S. agricultural operations expressed as a portion of total N input to each phase.

Notes: aGeneral values drawn from lit-erature review (Rotz, 2004)7 and modeling experience with the Integrated Farm System Model (Rotz et al., 2014)11 and Ma-nure–DNDC (Li et al., 2012).12

bNH3 emissions are primarily influenced by exposure time, temperature and ambient air speed.

cNO3 leaching to ground water and runoff to surface water vary widely as influenced by the amount and distribution of precipitation and the timing of N application.

dN2O and NOx formation and emissions are influenced by moisture, temperature, and anaerobic conditions that control nitrification and denitrification processes in soil and manure.

SOURCE AMOUNT EMITTED (% of applied N)a

NH3b NO3

c N2Od NOxd

Inorganic fertilizer applied to cropland 0–10 5–35 1–6 0–2

Grazed pastureland 5–20 10–40 1–8 1–6

Open lot animal housing 30–60 1–5 1–4 1–3

Enclosed animal housing with daily manure removal 5–20 0 0 0

Enclosed animal housing with accumulated manure 25–50 0 1–2 1–2

Solid manure storage 10–40 0–2 0.2–1 0–1

Slurry tank manure storage 5–30 0 0–1 0–1

Liquid lagoon manure storage 20–40 0–5 0 0

Broadcast application of manure, no incorporation 15–30 2–25 1–4 1–4

Broadcast application of manure, rapid incorporation 5–15 2–25 2–6 1–5

Subsurface injection of manure 2–10 5–25 2–9 2–7

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Other minor emissions of reactive N can occur from feed and animals. When crops are ensiled for storage, NO2 is produced and emitted during the fi rst few weeks of ensiling. The relative amount of N emitted is small, but it is a toxic and dangerous gas for those working with silage. Minor amounts of reactive N may also be emitted from the open surface of silos. Losses of crude protein (total N) are known to occur and this loss is probably in the form of NH3. With poor ensiling conditions and spoilage, nitrifi cation and denitrifi cation processes might also occur. Ruminant animals may emit minor amounts of enteric N2O,8 and equipment operations for feeding and manure handling emit NOx through fuel combustion.

Combining crop and animal production in an inte-grated system can improve N use and reduce Nr losses through more effi cient recycling of manure nutrients in feed production. By applying manure to cropland at the right time and at the right amount for crop uptake, leaching and denitrifi cation

processes are minimized. When manure N is applied in excess of crop needs, nearly all of the excess is lost to the environment, mostly in a reac-tive form. When animals are grazed, closer integra-tion of nutrient cycling occurs where fecal and urine excretions are deposited directly on the pasture. These excretions apply very concentrated applica-tions of N on small areas, which can enhance leach-ing and denitrifi cation processes. Use of rotational grazing strategies and avoiding the congregation of animals under shade or around watering and feeding areas helps reduce these emission path-ways and improve nutrient use effi ciency.

Mitigation of EmissionsGains in N use effi ciency are required to reduce Nr emission to the environment. This can be achieved by either better utilizing the N input to increase production or reducing the N input while maintaining production. Galloway et al. (2008)9

estimated that increasing N use effi ciency of crops and using improved animal management could

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each reduce Nr creation by 15 Tg N yr -1. Man-agement strategies such as the 4R principles (right source, right rate, right time, and right place) are being promoted to reduce fertilizer N losses from cropping systems while maximizing crop uptake.10 These same principles apply to the use of manure nutrients in cropping systems. Use of complex crop rotations and cover crops to stabilize and utilize Nr in the soil can help reduce unintended losses. Inclusion of legumes crops in rotations that fix N can reduce the amount of synthetic fertil-izer used in crop production. Improved plant and animal genetics that have greater N use efficien-cies may be available in the future to enhance N utilization. Other practices such as buffer strips, wetlands, and bioreactors can help keep Nr out of sensitive ecosystems or transform Nr into non-reactive N2.

Mitigation of reactive N loss from animal produc-ing operations must begin with feeding. Gener-ally, 20% to 30% of the protein N consumed by animals is retained in the animal products pro-duced and the remainder is excreted.7 Feeding the right amount and quality of protein improves animal utilization and often improves production. This reduces the N excreted by the animal and often reduces the portion of that N excreted in a form that is readily lost as Nr. Feeding animals in groups with similar nutrient requirements is one method to more accurately meet pro-tein requirements. Feeding specific amino acids (protein building blocks) can also tailor diets to more accurately meet nutritional requirements. In ruminate animals, obtaining the right degradation of protein in the rumen and having an appro-priate bypass of protein to the intestinal tract is important. Feeding the right amounts and types of carbohydrates can also improve N utilization. Feed additives, such as enzymes, antibiotics, pro-biotics, organic acids, and growth hormones, may also reduce N excretion. Feeding well balanced diets to maximize performance and N utilization is often the most cost-effective way to reduce N excretion and thus mitigate Nr emissions from all manure handling processes.

After N is excreted by the animal, steps can be taken to mitigate emissions during manure handling.7

This can begin on the barn floor. By separating urine from feces, hydrolysis can be reduced and the N remains in a less volatile organic form. In dairy free-stall barns, NH3 emissions have been reduced by up to 50% by sloping floors to drain urine away from fecal material. When long-term manure stor-age is used, NH3 emissions can be reduced by up to 80% and N2O emissions can be eliminated using a storage cover. Bottom loading of a slurry tank allows a natural crust to form on the surface. This is a cost-effective way to reduce NH3 emis-sions from storage by up to 70%, but nitrification and denitrification processes are enhanced caus-ing N2O and NO emissions. Microbial and acid additives have been evaluated to reduce manure emissions, but safe and cost-effective mitigation materials have not been found. When manure is applied to cropland, N losses are greatly reduced through rapid incorporation into the soil. Use of direct, subsurface injection of manure reduces NH3 emissions following application by 70% to 95%, compared to broadcast application without incor-poration, but the higher concentration of N in the soil may enhance leaching, nitrification, and denitri-fication processes (see Table 1).

Management to reduce N losses requires a whole-farm approach. Changes can be made to reduce N losses in each step of manure management between animal excretion and crop uptake. How-ever, the benefit of reducing the loss in any one component is low if steps are not taken to reduce losses from subsequent components. For exam-ple, reducing NH3 emission in the housing facility has little benefit if that retained N is not main-tained through improved management during manure storage and field application. Reducing NH3 emissions also may not provide overall ben-efit if that additional manure N leads to greater losses through denitrification and leaching. Only by providing similar levels of mitigation to animal feeding, housing, manure storage, and field appli-cation can production systems be developed with reduced or optimal environmental impact.

ConclusionReactive N is a critical input to agricultural systems and a major contributor to the substantial improve-ment in food production made in the 20th century.

In dairy free-

stall barns, NH3

emissions have

been reduced by up

to 50% by sloping

floors to drain urine

away from fecal

material.©iStock.com/GraconDesign

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Nr losses cannot be eliminated, but strategies and technologies are available for reducing these losses and other alternatives can be developed. Mitigation of Nr emissions often increase the cost

of production so more economical methods are needed or strategies must be implemented to help cover the increased costs to maintain sustainable food production systems. em

References1. Davidson, E.A.; David, M.B.; Galloway, J.N.; Goodale, C.L.; Haeuber, R.; Harrison, J.A.; Horwath, R.W.; Jaynes, D.B.; Lowrance, R.R.; Nolan,

B.T.; Peel, J.L.; Pinder, R.W.; Porter, E.; Snyder, C.S.; Townsend, A.R.; Ward, M.H. Excess Nitrogen in the U.S. Environment: Trends, risks, and solutions; Issues in Ecology, report #15, 2012.

2. Liu, J.; You, L.; Amini, M.; Obersteiner, M.; Herrero, M.; Zehnder, A.J.B.; Yang, H. A high resolution assessment on global nitrogen fl ows in cropland; PNAS 2010, 107, 8035-8040.

3. Bordirsky, B.L.; Popp, A.; Lotze-Campen, H.; Dietrich, J.P.; Rolinski, S.; Weindl, I.; Schmitz, C.; Muller, C.; Bonsch, M.; Humpenoder, F.; Biewald, A.; Stevanovic, M. Reactive nitrogen requirements to feed the world in 2050 and potential to mitigate nitrogen Pollution; Nature comm. 2014; doi: 10.1038/ncomms4858.

4. Bodirsky, B.L.; Popp, A.; Weindl, I.; Dietrich, J.P.; Rolinski, S.; Scheiffele, L.; Schmitz, C.; Lotze-Campen, H. N2O emission from the global agricultural nitrogen cycle: Current state and future scenarios; Biogeosciences 2012, 9, 4169-4197.

5. UNEP and WHRC. Reactive Nitrogen in the Environment: Too Much or Too Little of a Good Thing; United Nations Environment Programme, Paris, 2007.

6. Rotz, C.A.; Montes, F.; Hafner, S.D.; Heber, A.J.; Grant, R.H. Ammonia emission model for whole farm evaluation of dairy production systems; J. Environ. Qual. 2014, 43, 1143-1158.

7. Rotz, C.A. Management to reduce nitrogen losses in animal production; J. Anim. Sci. 2004, 82 (E. Suppl.), E119-E137.8. Hamilton, S.W.; DePeters, E.J.; McGarvey, J.A.; Lathrop, J.; Mitloehner, F.M. Greenhouse gas, animal performance, and bacterial population

structure responses to dietary Monensin to dairy cows; J. Environ. Qual. 2010, 39, 1-9.9. Galloway, J.N.; Townsend, A.R.; Erisman, J.W.; Bekunda, M.; Cai, Z.; Freney, J.R.; Martinelli, L.A.; Seitzinger, S.P.; Sutton, M.A. Transformations

of the nitrogen cycle: Recent trends, questions, and potential solutions; Science 2008, 320, 889-892.10. Robertson, G.P.; Bruulsema, T.W.; Gehl, R.J.; Kanter, D.; Mauzerall, D.L.; Rotz, C.A.; Williams, C.O. Nitrogen–climate interactions in U.S.

agriculture; Biogeochemistry 2013, 114, 41-70.11. Rotz, C.A.; Corson, M.S.; Chianese, D.S.; Montes, F.; Hafner, S.D.; Coiner, C.U. Integrated Farm System Model: Reference Manual; USDA

Agricultural Research Service, University Park, PA, 2014; available at https://www.ars.usda.gov/SP2UserFiles/Place/19020500/ Reference%20Manual.pdf (Accessed 20 February 2014).

12. Li, C.; Salas, W.; Zhang, R.; Krauter, C.; Rotz, A.; Mitloehner, F. Manure–DNDC: A biogeochemical process model for quantifying greenhouse gas and ammonia emissions from livestock manure systems; Nutr. Cycl. Agroecosys. 2012, 93, 163-200.

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