|Main authors:||Oene Oenema, Meindert Commelin, Piet Groenendijk, John Williams, Susanne Klages, Isobel Wright, Morten Graversgaard, Irina Calciu, António Ferreira, Tommy Dalgaard, Nicolas Surdyk, Marina Pintar, Christophoros Christophoridis, Peter Schipper, Donnacha Doody|
|FAIRWAYiS Editor:||Jane Brandt|
|Source document:||»Oenema, O. et al. 2018. Review of measures to decrease nitrate pollution of drinking water sources. FAIRWAY Project Deliverable 4.1, 125 pp
This article presents a brief overview of the global nitrogen cycle and of the nitrogen transformation processes. Nitrogen cycling and transformations are influenced by a range of processes and factors, which in the end influence both the production and transport of nitrate and thereby the pollution of groundwater and surface waters by nitrates. Nitrogen cycling is strongly associated with carbon cycling and with the cycling of water and other nutrients. Figure 2 presents an illustration of the nitrogen cycle of soil-plant systems. It shows how nitrate (NO3-) leaching is connected to a range of nitrogen pools and transformation processes, which ultimately affect the magnitude of nitrate leaching. In addition, N leaching losses may occur via dissolved organic N (DON), and also as ammonium (NH4+) in sandy and volcanic soils (Addiscott et al., 1991; Burt et al., 1993; Hatfield and Follett, 2008).
Understanding the sources, pools and transformation processes, as well as the factors that influence the sources, pools and transformation processes is needed for evaluating the effectiveness of measures to decrease nitrate leaching.
|1. Nitrogen cycling and transformation processes|
|2. Nitrogen use and losses in agriculture|
|3. Nitrogen use and losses in EU-agriculture|
Nitrogen (N) occurs in different forms and transforms from one form into the other almost endlessly (Figure 3). Molecular nitrogen (N2) is the dominant constituent of the atmosphere and the most abundant N form on earth (Galloway et al., 2003; 2004. Only a few microorganisms have the capability to utilize (fix) N2, converting it to organically bound N. The Haber-Bosch process converts N2 into ammonia/ammonium (NH3/NH4+) in a physical-chemical manner (Smil, 2001). The NH3/NH4+ can be taken up by plants (assimilation). Following the senescence of plants and organisms, the organic-N is transformed again into NH3/NH4+ (through mineralization). Autotrophic bacteria can utilize the energy contained in NH3/NH4+ through nitrification. Thereby, the oxidation status increases from -3 in NH3/NH to +5 in nitrate (NO3-). The NO3- can be taken up by plants (assimilation) or it is denitrified to nitrous oxide (N2O) and to di-nitrogen (N2) in anaerobic environments through heterotrophic bacteria or it can be leached to water bodies. Molecular N (N2) may be formed also through anaerobic ammonium oxidation (anammox; NH4+ + NO2- → N2 + 2H2O), by chemoautotrophic bacteria (Galloway et al., 2008).
A distinction is often made between reactive and non-reactive N. Reactive N (Nr) includes all forms of nitrogen that are biologically, photochemically, and radiatively active. Forms of nitrogen that are reactive include ammonia (NH3), ammonium (NH4+), amines (and other metabolizable organically bound N), nitrous oxide (N2O), nitrogen oxide (NO), nitrite (NO2-), and nitrate (NO3-).These forms are all involved in short-term cycling in the biosphere. Reactive forms of nitrogen support plant growth directly or indirectly and are capable of cascading through the environment and have impact through smog, acid rain, eutrophication, biodiversity loss, etc. Dominant forms of non-reactive N is N2, which makes up about 80% of the atmosphere, and the N locked-up in deep sediments and rock. These N forms do not contribute directly to environmental impacts (Galloway et al., 2008; Sutton et al., 2011; Fowler et al., 2013).
Figure 4 presents a quantitative picture of the global N cycle. The atmosphere, sediments and terrestrial rock have the largest pools of N, but this N is largely ‘non-reactive’. Large amounts of N cycle between atmosphere, terrestrial biosphere (agriculture and the urban and natural environments) and the marine biospheres (oceans, lakes). The cycling of N is related to the reactivity and mobility of the different N forms (Figure 3) and the presence of energy sources for transport. Sunlight fuels photosynthesis, the hydrological cycle (evapotranspiration) and wind and water currents (in combination with gravitational energy and internal particle energy). Natural gravity and the internal energy of particles govern the earth motion (seasonal and diurnal cycles), the physical interaction between elementary particles, including diffusion, and the physical transport of particles. The heat (energy) in the core of the earth governs tectonic uplift and volcanic activity (Smil, 2017). Humans have strongly influence the N cycle during the last few centuries, especially from the 1950s, with the help of fossil energy sources and technological developments (Smil, 2000).
The global N cycle is strongly influenced by anthropogenic activities. The changes in human diets towards more animal-derived protein have increased the total amount of N needed (to deliver the food of one person) to more than 100 kg per person per year in Europe (Smil 2013; Westhoek et al 2014). More than half of the food eaten by humans is produced now using N fertilizer from the Haber-Bosch process (Smil 2001; Erisman et al 2008). The industrial N2 fixation is now as large as or larger than the biological N2 fixation in the terrestrial system. In addition, large-scale deforestation and soil cultivation have increasingly mobilized N from the soil organic N pools, which have subsequently contributed to the increased N losses from the terrestrial system to the aquatic/marine system and to the atmosphere (Galloway et al 2008).
Nitrogen is needed in food and feed production in relatively large quantities for the production of amino acids (protein), nucleic acids and chlorophyll in plants. That is why farmers apply manures, composts and N fertilizers, to boost crop production. Synthetic N fertilizers became available and affordable in affluent countries from the 1950s and more recently in almost all countries (Smil, 2000). The availability of N in agriculture increased during the last 100 years also through the increased production of leguminous crops (beans, pulses, clover and alfalfa) that fix N2 biologically, through energy combustion that increases in NOx emissions and N deposition, and through the increased production of animal manures, and residues and wastes from industries and households (Herridge et al., 2008; Davidson, 2009; Sutton et al., 2013).
The N cycle in agriculture has been characterized as a leaky N cycle, because of the many opportunities of N molecules to escape (Figure 5). Nitrogen enters agriculture either via synthetic fertilizers, biological N2 fixation or atmospheric deposition. In addition, there is recycled N within the agricultural system, in the form of animal manure, compost, crop residues and mineralization of soil organic matter. Nitrogen leaves the system via harvested crop and animal products and via losses of various N forms to air and water.
The increased availability of N in agriculture has increased the losses of N to air and water bodies (Figure 5). Emissions of N to the wider environment occur via various N forms (NH4+, NH3, N2, N2O, NO, NO2-, NO3-), which can lead to problems related to human health and ecosystem degradation. The volatilization of ammonia (NH3), leaching of nitrate (NO3-), and the emissions of di-nitrogen (N2), nitrous oxide (N2O) and nitrogen oxide (NO) following nitrification-denitrification reactions are the main N loss pathways from agricultural systems and food systems. Possible human health and environmental effects of this reactive N include a decrease of human health, due to NH3 and NOx induced formation of particle matter (PM2.5) and smog, plant damage through NH3 and through NOx induced tropospheric ozone formation; a decrease of species diversity in natural areas due to deposition of NH3 and NOx; acidification of soils because of deposition of NH3 and NOx; pollution of groundwater and drinking water due to nitrate leaching; eutrophication of surface waters, leading to algal blooms and a decrease in species diversity; global warming because of emission of N2O; and stratospheric ozone destruction due to N2O (Sutton et al., 2011).
Fertilizer N use in Europe increased rapidly between 1950 and 1990, but stabilized thereafter at a level of about 10-11 Tg per year (Figure 6; Erisman et al., 2008; Sutton et al., 2011; Sutton et al., 2013). For comparison, the use of phosphorus (P) and potassium (K) fertilizer use are also shown in Figure 6; these are the most important nutrients next to N. Global N fertilizer use has increased from about 10 Tg in 1961 to almost 110 Tg in 2012, but there are large differences between continents. Fertilizer N use in Africa is staggering at a level of about 1-2 Tg per year during the last decade, while fertilizer N use in Asia has rapidly increased during last three decades by on average 2 Tg per year (not shown). The rapid decrease in European N use around 1990 is mainly related to the political restructuring of Eastern and Central Europe at this time. The slow decrease in fertilizer use in Europe between 1990-2010 is mainly related to EU agri-environmental policy. The rapid increase in N fertilizer use between 1950s and mid-1980s, concomitant with the rapid intensification of livestock production in EU in this period are at the base of the nitrate problems in groundwater and surface waters in EU. The total amounts of N in manure produced (~10 Tg/yr) were roughly similar to the annual use of fertilizer N (~11 Tg/yr) in the EU during the last 10 years or so. In addition, there were inputs via biological N2 fixation (about 1 Tg/yr ) and atmospheric deposition (2 to 3 Tg/yr) (De Vries et al., 2011).
About 50 to 60% of the total N input to crop land via animal manure, fertilizer, biological N2 fixation and atmospheric deposition is recovered in harvest crop in the EU. The remainder is lost from the crop land to the wider environment via ammonia volatilization, denitrification, leaching, overland flow and erosion. The losses to the environment in the EU are not well-known; the estimated total leaching losses, denitrification, and surface run-off differ by a factor of two between studies. Estimated N inputs to groundwater and surface waters range from 2.7 to 6.1 Tg in 2000 (De Vries et al., 2011).
Figure 7 shows the spatial distribution of N losses from terrestrial systems to the aquatic system (groundwater, rivers, lakes and seas) in the EU-27 for the year 2002. The pie diagram at the right side shows the split of the various N sources for the aquatic system. The contribution from agriculture is nearly 60%. Sewage systems contribute 22%. Minor inputs are from atmospheric deposition (mainly from agriculture and industry) and natural systems. The bar diagram at the right side shows which countries contribute most N into the aquatic system. Clearly, the loss of N (nitrate, NO3-) originates from many different sources, which are diffusely spread across EU-27, with the exception of the sparsely populated northern parts of Scandinavia and Scotland. Within this huge spatial variability various hot spots can be found, notably in Western Europe. The estimations shown in Figure 7 have not been checked and corrected by estimations at national scales by experts from Member States.
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