What is Reastive N and Why Should I Care?
Gerald May, Extension Air Quality Educator
Natalie Rector, Extension Manure Nutrient and Water Quality Educator
Nitrogen is an important component for all plant and animal growth, essential for the development of proteins and important in other living functions. In our environment, N exists in many forms. In its inert gaseous state, N2, it is very stable and makes up over 78% of the earth’s atmosphere but is unavailable for plant and animal growth.
Reactive N includes the biological and chemically active forms of N in the environment. The inorganic forms of N ammonia (NH3),ammonium (NH4), nitrite (NO2), nitrate (NO3), nitric oxide (NO) and nitrous oxide (N2O) included in the N cycle are all forms of reactive N (Figure 1). These forms of N are all interrelated and are constantly in flux in the environment. Learning to recognize and manage reactive N has future implications for agriculture.
Through the conversion of N2 to NH3 and NH4 (ammonia and ammonium), N becomes available for life uses. This conversion of N2 to its ammonia forms takes place through two natural processes: 1) N2 is converted to NH4 by bacteria living in the nodules of legume plants (clovers, alfalfa, beans, peas); and, 2) a burst of energy from a bolt of lightning converts N2 to NH4 (Vitousek et al., 1997). In the early 20th century the advent of the Harber (or Harber-Bosch) process for converting N2 to NH3 was commercially adopted and the manufacture of commercial N fertilizers began. Each process requires significant inputs, either bacterial or energy, to complete the conversion.
The burning of fossil fuels releases fixed N in the fuel and to a smaller degree provides high temperatures necessary for the conversion of N2 to NH3. The release of N from biological pools such as clearing forested land also contributes to reactive N in our environment (Vitousek et al., 1997).
The Nitrogen Cycle
Prior to adopting the use of commercial N fertilizers, farmers depended on lightning storms, crop residue, animal manure and N fixation by legumes to produce the N needed for plant growth (Figure 1). A portion of the N fixated by legumes is removed in harvested crops for use by animals and humans. The remainder is left in the field either as organic N in crop residue or in the soil as nitrate and nitrite. Denitrifying bacteria that live in anaerobic conditions in soil, riparian areas and wetlands, convert a portion of the NO2 and NO3 back to their inert state (N2) and to a lesser extent N2O (Vitousek et al., 1997; Killpack and Bucholtz, 1993).
Increased human activity, including burning of fossil fuels, clearing land and manufacturing commercial N fertilizers, have made significant contributions to the reactive N in the environment. Prior to the increased release of reactive N associated with human activity the amount of reactive N in the environment was held in balance by the denitrification process. Human activity has had both positive and negative impacts on the denitrification process but to what extent is not well understood (Vitousek et al., 1997). The net effect has been a significant increase in the reactive forms of N in our environment.
Increased use of N Fertilizer
Figure 2 shows the growth in commercial fertilizer use in the U.S. from 1960 to 2008. Phosphorus as P2O5 and potash as K2O use has stayed constant since the mid 1990’s or earlier. These two components of soil fertility are chemically bound to soil particles (phosphorus more so than potassium) and, if not used by one year’s crop, remain in the root zone and available for the following year’s crop.
In contrast, from 1960 to 2008 N use increased nearly 5-fold (Figure 2). Unlike P2O5 and K2O, if N is supplied beyond crop needs for a single growing season the N not utilized by the crop may be bound by organic matter, leach from the root zone, be lost through surface water runoff or it may volatilize into the air prior to the next year’s crop. Fertilizer use in the U.S. is indicative of other growing regions throughout the world. This annual input of commercial fertilizer worldwide is a significant contributor to the reactive N in our environment.
Why is the Increasing Amount of Reactive N a Concern?
The increasing release and conversion of N has positively impacted all of our daily lives. The nitrogen used in agriculture has made it possible for farmers to meet the world wide demands of a growing population’s daily needs for food, fiber and shelter. Fossil fuels create energy for virtually all of our daily processes that we consider routine but are often as basic to others as warming their homes. Limiting reactive N in the environment becomes more challenging when the importance of these daily activities is considered. The increasing levels of reactive N are leading to major environmental concerns.
Nitrate (NO3), has long been a concern in ground water. High levels of nitrates in ground water have been directly linked to increased cases of blue baby syndrome, a health concern in infants under 6 months old. It also causes health concerns for the elderly. The U.S. Environmental Protection Agency (EPA) has an established safe drinking water standard of 10 mg NO3-N per liter water (EPA, 2009). In 1998, EPA reported finding NO3 levels exceeding the 10 mg/L standard in 40% of the reporting hydrogeologic sites, but only 1% of the tested drinking water sources exceeded the limit (EPA, 1998). Excess reactive N ends up in streams, rivers and lakes through tile line flows, inorganic matter, top soil erosion. It contributes to eutrophication and algae blooms in lakes and coastal areas that receive waters from nutrient enriched rivers (EPA, 2011).
Nitric oxide (NO), and nitrogen dioxide (NO2), collectively referred to as NOx, contribute to smog and haze and have been linked with asthma in children and adults (EPA, 2011). Nitrous oxide (N2O), is a greenhouse gas that also contributes to acid rain (EPA, 2010). Ammonia deposition has been associated with the undesirable changes in forest growth (Pitcairn et al., 1998). The EPA is studying the positive and negative impacts of reactive N on a national and regional basis. The agency has contributed to the development of a National Ecosystems Services Atlas. This atlas, due out later this year, is intended to identify the ecosystem systems, and drivers, impacted by reactive N (EPA, 2011).
What is Agriculture’s Role?
Agriculture is considered a major contributor of excess reactive N and therefore the industry will be looked to for solutions. When one considers the increasing cost of N inputs, contributing to the solution may actually be a win-win situation for producers, agriculture and the environment. Practices that may reduce reactive N associated with agricultural production include:
- Applying the N source close to the time of crop uptake.
- Avoiding surface applied N fertilizers, especially on high pH soils and hot/dry conditions.
- Avoiding nitrogen fertilizer applications in the fall for next season’s crops.
- Taking all reasonable N credits from past cropping practices, reducing purchased N accordingly.
- Increasing use of cover crops to maintain nutrients in the root zone during the non-cropping season.
- Reducing ammonia losses from manure storage.
- Exploring new technologies that hold and retain N in the soil.
- Increasing use of conservation measures to reduce topsoil losses and retain nutrients in the root zone.
- Formulating non-ruminant diets based on amino acid requirements rather than total protein content coupled with increased use of synthetic amino acids.
Reactive N in the environment is a major emerging issue. Because of agriculture’s dependence on N for continued yield improvements it is an issue the industry will want to monitor as the impact of reactive N in our environment is explored.
In 2006 the EPA convened the Science Advisory Board Reactive Nitrogen Committee. The most recent draft report from that committee, including the committee recommendations were posted in January 2011.
That report can be found at: http://yosemite.epa.gov/sab/SABPRODUCT.NSF/81e39f4c09954fcb85256ead006be86e/69B45FA395DAC4AC8525780D006D0D33/$File/INC+Report_Quality+Review+Draft_1_20_11.pdf
EPA, 1998, Environmental Protection Agency National Water Quality Inventory: 1998 Report to Congress, Available: http://water.epa.gov/lawsregs/guidance/cwa/305b/98report_index.cfm.
EPA, 2009, Environmental Protection Agency National Primary Drinking Water Regulations, Available: http://water.epa.gov/drink/contaminants/index.cfm.
EPA, 2010, Greenhouse gas emissions, Available: http://epa.gov/climatechange/emissions/index.html#ggo
EPA, 20111, Nitrogen Research, Available:
EPA, 20112, Nitrogen oxides emissions,Available: http://cfpub.epa.gov/eroe/index.cfm?fuseaction=detail.viewInd&lv=list.listByAlpha&r=219685&subtop=34.
Killpack, S. C., and D. Buchholz, 1993, Nitrogen in the environment, Available: http://extension.missouri.edu/publications/DisplayPub.aspx?P=WQ255.
Pitcairn, C.E.R., I.D. Leith, L.J. Sheppard, M.A. Sutton, D. Fowler, R.C. Munro, S. Tang and D. Wilson, 1998, The relationship between nitrogen deposition, species, composition and nitrogen concentrations in woodland flora in the vicinity of livestock farms, Environmental Pollution, v102 s1 pp. 41-48.
Purdue University, Dept. of Hort. and Landscape Arch., Utilization of Nitrogen by Plants, Available: http://www.hort.purdue.edu/rhodcv/hort640c/.
USDA - Economic Research Service, Available http://www.ers.usda.gov/Data/FertilizerUse/.
Vitousek, P.M., J.D. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H. Schlesinger and D.G. Tilman, 1997, Human Alteration of the Global Nitrogen Cycle: Sources and Consequences, Ecological Applications, v7(3) pp. 737-750.
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