C.6 Nitrogen and Phosphorus Cycles

Introduction

Nutrient cycling is a fundamental concept in ecology, describing the movement and exchange of organic and inorganic matter back into the production of living matter. The nitrogen and phosphorus cycles are critical for maintaining the productivity and sustainability of ecosystems. These cycles ensure that essential elements are available for organisms to build cells and tissues.

Nitrogen Cycle

Importance of Nitrogen

Nitrogen is a vital element for all living organisms as it is a key component of amino acids, proteins, and nucleic acids. Although nitrogen is abundant in the atmosphere (approximately 78% as $N_2$), most organisms cannot use this form directly.

Steps in the Nitrogen Cycle

1. Nitrogen Fixation $$ N_2 + 3H_2 \rightarrow 2NH_3 $$
   • Conversion of atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) or related compounds.
   • Carried out by nitrogen-fixing bacteria (e.g., Rhizobium) in the soil or in symbiotic relationships with leguminous plants.
   • Lightning and industrial processes also contribute to nitrogen fixation.
2. Nitrification $$ 2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H_2O + 2H^+ $$ $$ 2NO_2^- + O_2 \rightarrow 2NO_3^- $$
   • Conversion of ammonia ($NH_3$) to nitrate ions ($NO_3^-$).
   • This is a two-step process involving different bacteria:
     • Ammonia is first oxidized to nitrite ($NO_2^-$) by bacteria such as Nitrosomonas.
     • Nitrite is then oxidized to nitrate by bacteria such as Nitrobacter.
3. Assimilation
   • Plants absorb nitrate ($NO_3^-$) or ammonium ($NH_4^+$) from the soil and incorporate it into organic molecules like amino acids and proteins.
4. Ammonification (Decomposition) $$ Organic , N \rightarrow NH_3 + H_2O \rightarrow NH_4^+ $$
   • Decomposers such as bacteria and fungi break down organic nitrogen compounds from dead organisms and waste products, converting them back into ammonia or ammonium.
5. Denitrification $$ 2NO_3^- \rightarrow N_2 + 3O_2 $$
   • Conversion of nitrate ($NO_3^-$) back into nitrogen gas ($N_2$), thus returning it to the atmosphere.
   • Carried out by anaerobic bacteria such as Pseudomonas and Clostridium.

Human Impact on the Nitrogen Cycle

• Agriculture: Use of synthetic fertilizers increases nitrogen input, leading to nutrient runoff and eutrophication in water bodies.
• Fossil Fuel Combustion: Releases nitrogen oxides ($NO_x$), contributing to acid rain and atmospheric pollution.

Phosphorus Cycle

Importance of Phosphorus

Phosphorus is crucial for the formation of DNA, RNA, ATP, and phospholipids. Unlike nitrogen, phosphorus does not have a gaseous phase and is mainly cycled through rocks, water, soil, and living organisms.

Steps in the Phosphorus Cycle