Nitrogen Cycle

Ecology, Biotechnological Applications and Environmental Impacts

Jesus Gonzalez-Lopez, Alejandro Gonzalez-Martinez, Jesus Gonzalez-Lopez, Alejandro Gonzalez-Martinez

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000352313
• Edition: 1
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

Anthropogenic activity has clearly altered the N cycle contributing (among other factors) to climate change. This book aims to provide new biotechnological approach representing innovative strategies to solve specific problems related to the imbalance originating in the N cycle. Aspects such as new conceptions in agriculture, wastewater treatment, and greenhouse gas emissions are discussed in this book with a multidisciplinary vision. A team of international authors with wide experience have contributed up-to-date reviews, highlighting scientific principles and their environmental importance and integrating different biotechnological processes in environmental technology.

CHAPTER
1

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The Nitrogen Cycle: An Overview

Massimiliano Fenice

In our planet, the total quantity of chemical elements is practically constant, but their location, combination and physical/oxidation state change continuously due to various cyclic reactions. The different elements, removed from natural reservoirs (generally located in the lithosphere), are transferred to other compartments (hydrosphere and atmosphere) where they become available for living organisms (biosphere), which actively participate in most of these reactions. Thus the life equilibrium on Earth is sturdily dependent by the recycling of essential chemical elements (principally carbon, nitrogen, oxygen, sulphur, ironand manganese) occurring through a series of transformations. Since all living organisms influence the flow of the elements on Earth, their global turnover reactions on the planet are linked in the so-called ‘bio-geochemical cycles’ which are somehow linked and make life possible. Due to their metabolic versatility microorganisms undertake the most important role in these transformations. Nevertheless, due to the high amount of carbon and nitrogen constituting the biosphere, their cycles are more strictly associated and have great environmental implications. Nitrogen is a key element controlling species composition, diversity and dynamics; it regulates the functioning of most terrestrial and aquatic ecosystems. The nitrogen cycle, essentially driven by the biosphere, largely depends on the microbial metabolism and some of its reactions are of exclusive competence of specialised microbial groups. On the other hand, on a global scale, the nitrogen cycle is heavily modified by a number of human activities: increased use of fossil fuels, increased demand for fertilizers and other nitrogen compounds. The nitrogen cycle was traditionally subdivided into three main processes: nitrogen fixation, nitrification and denitrificationbut the current understanding of the cycle implicate a more articulate flow of transformations mediated by different microbial groups. In brief, N₂ is reduced to NH₃ by N-fixing microorganisms; ammonia is assimilated in the NH₂ groups of proteins, both in oxic and anoxic environments, or can be transformed into nitrate by the nitrification process. Nitrification can require different groups of microorganisms performing separately the oxidation from ammonia to nitrite and from nitrite to nitrate or microorganismscanperform both reactions (COMAMMOX). Besides, nitrate can be assimilated by microorganisms and plants in the proteins or be subject to various reduction processes. Organic NH₂ groups are converted into ammonia by ammonification process. Nitrate can be reduced to ammonia by the ‘dissimilative reduction of nitrate to ammonia’ (DNRA) or reduced to nitrite which is subject to further reductions producing gaseous compounds (NO₂, N₂O, NH₃ and N₂) in the denitrification process. In the ANAMMOX reaction, nitrite and ammonium are converted directly into N2 and water.

1.1 INTRODUCTION

Earth represents an extremely complex physicochemical system that, under the thermodynamic point of view, is ‘open’ concerning energy and ‘closed’ while considering the global balance of matter. Although the total quantities of chemical elements of the planet are practically constant (negligible variations could be due to nuclear reactions and/or by inputs from meteorites), their location, combination, and physical/oxidation state change continuously due to a series of cyclic reactions. Hence, all chemical elements are removed from natural reservoirs (generally located in the lithosphere) and transferred to other compartments (hydrosphere and atmosphere) where they are available for all living organisms (biosphere), which actively participate to most of these reactions and some of them also contribute to regenerate the reservoirs.

Thus, the equilibrium state of life on Earth is strongly dependent on the recycling of essential chemical elements (principally carbon, nitrogen, oxygen, sulphur, iron, and manganese) occurring through a series of physical transformations, chemical and biochemical reactions and combinations of these processes. Since all living organisms influence the flow of chemical elements on Earth, the global turnover reactions of the elements on Earth are linked to the so-called ‘bio-geochemical cycles’.

The various human activities (i.e., deforestation and wood/coil combustion) always affected the cycles (those of carbon and nitrogen in particular). However, the most important unbalance of the cycles (mainly carbon’s, due to the CO₂ release) caused by anthropic activities started during the Industrial Revolution. Particularly relevant was the massive use of electricity, chemistry, and fossil fuels, which started from the second half of the nineteenth century, accompanied by subsequent further extensive deforestation due to intensive farming and livestock production. For these highly negative footprints on the environment, caused by humans in the last two hundred years and producing environmental degradation at a very high rate, a new ‘geological era’ has been defined by some scientists: ‘The Anthropocene’ (Crutzen 2006, Bellarby et al. 2008, Mondelaers et al. 2009, de Vries and de Boer 2010, Fowler et al. 2013, Lewis and Maslin 2015, Crutzen 2016).

In any case, all nutrient cycles are somehow linked and the various transformations of the elements correlated to the metabolism of living organisms, make life possible. However, due to the high amount of carbon and nitrogen present in all organisms, the cycles of these elements are more strictly associated and this association has a great environmental significance. For example, the fixed nitrogen is the principal limiting factor to primary production (carbon fixation) and it has been demonstrated that small variations in the ratio of nitrogen fixation to denitrification can significantly change atmospheric carbon dioxide concentrations (Falkowski 1997). Nitrogen is a key element that controls species composition, diversity, and dynamics; it regulates the functioning of most terrestrial, freshwater, and marine ecosystems (Vitousek et al. 1997).

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Department of Ecology and Biology (DEB), University of Tuscia, Viterbo Italy.
E-mail: [email protected]

However, due to their metabolic versatility, microorganisms (prokaryotes in particular) undertake the most important and active role in these transformations which move the elements through the biosphere between the various compartments. The role of microorganisms within the bio-geochemical cycles is even more important since a number of metabolic processes, very important at a global level, can be almost exclusively carried out by Bacteria and/or Archaea (i.e., nitrogen fixation, methanogenesis, methanotrophy, and sulphate reduction).

1.2 THE NITROGEN AND ITS CYCLE

Nitrogen is the fifth most common element in the solar system and our planet; it exists both in organic and inorganic forms and also in a number of oxidation states (from +5 to –3) (Table 1.1).

It is the most abundant element in the atmosphere where it is present principally in its diatomic form (N₂, ca. 78% of air composition, pairs to approximately 4 × 10¹⁵ tons) with traces of NH₃ and various oxides, such as N₂O, NO, and NO₂, resulting from the combustion and some biological processes (i.e. denitrification). The atmosphere plays a central role in the redistribution of nitrogen compounds (N-compounds) in terrestrial and marine ecosystems. In some cases, it passively behaves as a carrier; while in other circumstances, it participates in a pattern of complex chemical reactions, which transform the various N-compounds before they return to Earth (soil and waters) with different deposition velocities and various ecological and environmental effects (Galbally and Roy 1983, Nielsen et al. 1996, Hertel et al. 2011, Fowler et al. 2013).

TABLE 1.1 Different oxidation states (N° of oxidation) of nitrogen and corresponding compounds

Oxidation State	Formula	Common Name
–3 reduced	NH3, NH^4+, N^3–	Ammonia, ammonium, nitride
–2	NH2-NH2	Hydrazine
–1	NH2-OH	Hydroxylamine
0	N2	Dinitrogen
+1	N2O	Nitrous oxide
+2	NO	Nitric oxide
+3	N2O3, HNO2, NO2^–	Nitrous anhydride, nitrous acid, nitrite
+4	NO2, N2O4	Nitrogen dioxide, dinitrogen tetroxide
+5 oxidized	N2O5, HNO3, NO3^–	Nitric anhydride, nitric acid, nitrate

It has been also estimated that a lot of nitrogen (> 2 × 10¹⁷ tons) is present in the Earth most inner layers, but only a limited amount is transferred to the surface or to the atmosphere. This transfer is mainly due to volcanic activities that contribute to the global cycle. By contrast, in the Earth’s crust, nitrogen concentration is rather low and this element contributes scarcely to the compositions of minerals. The evaporitic nitrate de...