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Nitrogen – Pollution and Its Impacts

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Nitrogen – Pollution and Its Impacts NITROGEN POLLUTION AND ITS IMPACTS OUTLINE: 1. Background – forms and cycling 2. Pools and Sources 3. Cycling, transport dynamics and loadings from watersheds 4. Landuse and N exports 5. Management Options to reduce N 6. Effects and impacts of excess N on ecosystems Notes based on – • Environmental Protection Agency. 2002. Nitrogen: Multiple and Regional Impacts. EPA-430-R-01-006. • Follett, R. 2008. Chapter 2: Transformation and ransport processes of nitrogen in Agricultural Systems. In Nitrogen in the Environment: Sources, Problems, and Management. • Passeport et al. 2013; Environmental Management 1 Forms of N: Inorganic: Reduced + 1. NH4 (ammonium – g, aq, s) 2. N2 (nitrogen – g) 3. N2O (nitrous oxide – g) 4. NO (nitric oxide – g) 5. NH3 (g, aq) Oxidized - 1. NO2 (nitrite – aq) 2. NO2 (nitrogen dioxide - g) - 3. NO3 (nitrate – aq) Organic N – urea, amines, proteins and nucleic acids Tied to carbon molecules 2 Reactive vs. Non-reactive species Non-reactive – Nitrogen gas N2 – atmosphere - largest N store on the planet (78%) Reactive (Nr) – • Ammonia • Ammonium • Nitrogen oxides • Nitric acid • Nitrous oxide • Organic compounds N2 ↔Nr Concern – Anthropogenic activities increasing Nr accumulation in environmental reservoirs – and increasing impacts on these reservoirs. 3 Nitrogen Pools Table 1. Estimates of the active pools in the global nitrogen cycle. million tonnes N Air N2 3 900 000 000 N2O 1 400 Land Plants 15 000 Animals 200 of which people 10 Soil organic matter 150 000 of which microbial biomass 6 000 Sea Plants 300 Animals 200 In solution or suspension 1 200 000 - of which NO3 -N 570 000 + of which NH4 -N 7 000 Dissolved N2 22 000 000 air land sea 4 plants animals soil organic matter 5 Sources Natural Sources: 1. lightning N2 + O2 → 2NO 2. biological nitrogen fixation N2 → NH3 → amino acids → proteins Symbiotic bacteria in rhizobium/roots of plants Types of plants that host these bacteria? Other types of bacteria? 6 Plants – alfalfa, clovers, peas, beans, etc. Prior to anthropogenic influences - there was a balance between BNF and denitrification which kept Nr low Anthropogenic sources 1. Burning of fossil fuels 2. Widespread cultivation of N fixing crops 3. Use of fertilizers in agriculture – synthetic and animal manures 4. Urban, suburban, rural sources – wastewater, lawn fertilizers, septic tanks, CSOs. Human activity has had a profound effect on the N cycle. 1. Energy production - under high temperature combustion processes, unreactive nitrogen is converted to reactive nitrogen by two processes. a. Thermal NOx o 1000 K N2 + O2 → 2NO 7 b. Fuel NOx - the oxidation of organic N in fuels Natural gas - very low 0% Coal - up to 3% 2. Fertilizer-Most anthropogenic fertilizers are either NH3 or urea produced from NH3. This material is produced by the Haber-Bosch process. 4N2 + 12H2 = 8NH3 8 Types of N fertilizers – • Ammonium nitrate • Ammonium sulfate • Calcium nitrate • Sodium nitrate • Urea [CO(NH2)2] • Urea-ammonium nitrate 3. Production of legumes and other crops allows for the conversion of N2 to reactive N by increasing biological nitrogen fixation. Legumes include: Soybeans 9 Ground nuts (peanuts) Pulses (lentils) Forage (alfalfa, clover) 10 Global Population and Reactive Nitrogen Trends 200 6 150 Tg Tg N yr 4 Natural N Fixation N yr - - 1 100 1 2 50 Human Population (billions) Human Population (billions) 0 0 1850 1870 1890 1910 1930 1950 1970 1990 2010 Popul ati on Haber Bosch C-BNF Fos sil Fue l Total Nr From Galloway et al. 2002. In review. 1 Tg = 1012 grams Increase in anthropogenic N from 1860 to 2000 (Tg N /year) Process 1860 2000 Cultivation of 15 33 legumes – BNF Combustion 1 25 Fertilizer 0 100 Total Increase = 16 to 158 Tg N / year 11 Major Sources 1.Power plant emissions 2.Vehicle emissions 3. Septic tank leakage 4. Manure/livestock runoff 5. Agricultural emissions 6.Fertilizer runoff (e.g. lawns and fields) 7. Wastewater effluent 8. Food and feed imports 12 What happens to inputs of Nr? • Denitrification (loss as N2 gas) • Biomass storage (crops, forests, animals) • Soil storage • Groundwater storage • Exported to streams/estuaries • Exported in food and feed 13 Cycling, Transport Dynamics and N loadings from watersheds • Nitrate – high mobility • Ammonium – low mobility, fixation with clay particles in subsoils • Organic N – moderate mobility • Particulate versus dissolved species (?) 14 Key N Cycle Processes: Mineralization & Immobilization: • Mineralization: Conversion of organic N into inorganic N + - forms (NH4 , NO3 ) – via processes like decomposition -- organic N forms decompose and form ammonium first. • 1.5 to 3.5 % of the organic N may mineralize annually • Immobilization: Conversion of inorganic N forms to organic N forms (via processes like plant & biological uptake) o NH4+ preferably immobilized compared to NO3- The carbon and nitrogen pool ratios in soils and implications for mineralization and immobilization 15 Organic & Inorganic C, N, P, pools in soil P LITTER N C ORGANIC decomposition POOLS P N HUMUS C mineralization immobilization NH4 NO3 P C – carbon INORGANIC POOLS N – nitrogen NH4- ammonium P - phosphorus NO3 - nitrate 16 Fixation/Sorption by Clay Minerals • Positively charged ammonium ions are adsorbed on the negatively charged clay & humus sites – remember the CEC! • Because of the adsorption of ammonium to clay particles ammonium is the less mobile form of N --- nitrate (negatively charged) is more mobile that ammonium. • Thus soils with higher clay content will retain or tie-up more ammonium (compared to sandy soils) 17 • Clay type (e.g., vermiculite) will also influence this fixation Ammonia Volatilization • Conversion of ammonium from the soil to ammonia gas – NH3 • Typical sites for occurrence – locations where animal manures are spread on land. • Moist and high pH conditions increase the rate of volatilization. ******Nitrification • Conversion of Ammonium Nitrate • Performed by bacteria classed as autotrophs – because they obtain their energy from the oxidation of ammonium ions. • Two types of bacteria involved – Nitrosomonas and Nitrobacter – two stage process of conversion • Ammonium Nitrite Nitrate • Nitrification reaction results in the release of hydrogen ions – which means nitrification increases soil acidity! 18 • Factors that regulate Nitrification: o Availability of ammonium o Aeration – nitrifying bacteria are aerobic and need O2 o Moisture – nitrification is retarded at high soil moisture conditions (poor aeration) o Carbon – don’t need carbon as an energy source o Temperature – optimum 20C to 30C. o Exchangeable base forming cations: Nitrification proceeds at a higher rate in soils that have an abundance of exchangeable base forming cations like Ca2+ and Mg2+ o Clay types: (e.g., smectite & allophane) Clays that stabilize organic matter and hold ammonium ions tightly can result in reduced rates of nitrification o Pesticides & nitrification inhibitors Denitrification • Process of conversion of nitrate NO3- to nitrogen gas N2 • Typically occurs in saturated soils – like wetlands, riparian soils, flooded rice fields,……. • Bacteria involved – are heterotrophs – extract their energy from carbon (as opposed to autotrophs) • Conditions for Denitrification: o Nitrate availability o Energy source – Carbon 19 o Anaerobic conditions – soil air less than 10% O2 or less that 0.2 mg/L o Temperature between 2 and 50C – optimum is between 25 and 35C. o Acidity – very low acidity (pH < 5.0) can inhibit denitrification • Denitrification can occur in large saturated expanses of soil or even in small pockets of saturation called micro-sites • Denitrification rates could vary from 5 to 15 kgN/ha/yr for agricultural fields to as high as 30 to 60 kgN/ha/yr for wetlands, saturated soils receiving high N loads or riparian soils. • During a year - denitrification losses will be high when there is sufficient moisture in the soil and the soil is warm. 20 Hydrologic delivery processes: 1. Precipitation 2. Surface runoff 3. Sediment and soil associated 4. Vertical drainage, leaching 5. Groundwater flow Surface & sediment drainage Groundwater flow N flux = (N concentration * water flux) 21 Primary pathways of transport for various N forms Either in dissolved or particulate forms Dissolved < 0.45 or 0.7 micron Particulate > 0.45 or 0.7 micron + - . Surface runoff – DON, diss-NH4 , NO3 + - . Vertical drainage - DON, diss-NH4 , NO3 + - . Near subsurface flow - DON, diss-NH4 , NO3 + - . Groundwater flow - DON, diss-NH4 , NO3 + . Sediment - PON, particulate-NH4 DON – dissolved organic N PON – particulate organic N Concentration of nitrogen forms in runoff will depend on sources and the intersection of those sources by hydrologic flow paths 22 Storm event patterns of N across various seasons – how they relate to sources and hydrologic flow paths NO3 NH4 DON Typical solute trajectories during a summer rainfall event Trajectories are dictated by – • Where the N source is located in soil profile (near surface versus deep) • Which hydrologic flow path intersects this source 23 NO3 NH4 DON Possible solute trajectories during a snowmelt event in a forested watershed PN export with storms – • PN exports increases with discharge and typically peaks on the rising limb of the hydrograph. • Large storms can contribute large PN exports 24 Inamdar et. al., 2015. – • Irene in 59 hours exported 1/3rd the annual N runoff export from a 12 ha watershed. • 87% of that N export from TS Irene was in particulate form! 25 N exports with landuse 26 27 Forested watersheds: • Primary inputs – atmospheric deposition
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