The Nitrogen Cycle Learning Objective By the end

The Nitrogen Cycle

Learning Objective. By the end of this presentation you should understand the 4 main processes central to the nitrogen cycle. • • Nitrogen Fixing Decomposition Nitrification Denitrification Please make sure that you make adequate notes before our next lesson to help you apply these processes to the nitrogen cycle. We will then go on to answer some past exam questions.

The Importance of Nitrogen All life requires nitrogen compounds, – To make amino acids which form proteins – Forming part of nucleic acids. (nitrogenous bases, A. T. C. G. U)

• Air, which is 79% nitrogen gas (N 2), is the major reservoir of nitrogen. • But most organisms cannot use nitrogen in this form as nitrogen gas is INERT

Plants must secure their nitrogen in "fixed" form incorporated in compounds such as: – nitrate ions (NO 3 -) – ammonia (NH 3)/ammonium (NH+4) Animals secure their nitrogen (and all other) compounds from plants (or by eating animals that have fed on plants).

The 4 Main Processes involved in cycling nitrogen through the ecosystem • Nitrogen fixation • Decomposition • Nitrification • Denitrification

Nitrogen fixation • The nitrogen molecule (N 2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy. • Three processes are responsible for most of the nitrogen fixation in the biosphere:

Nitrogen fixation • biological fixation by certain microbes • atmospheric fixation by lightning • industrial fixation – Haber process

Nitrogen fixation This is performed by 2 types of microbe • Free-living in the soil bacteria such as Azotobacter (don‘t learn this one) • Symbiotic (mutualistic) bacteria Rhizobium (do learn this one) found in root nodules of legumes Root nodules on clover plant

Rhizobium contains an enzyme called nitrogenase that converts nitrogen and hydrogen from the atmosphere into ammonia. • The ammonia is used to make amino acids which the legume can use to make proteins in return for supplying the bacteria with carbohydrates

Important Nitrogen Fixing Plants Often referred to as Legumes Examples include peas, beans, clover, alfalfa Rhizobium bacteria form root nodules which then carry out Nitrogen Fixing. Useful for improving soil fertility (crop rotation) These plants can grow on soil with low mineral content e. g newly formed soils. Legumes often form early stages of colonisation during succession.

Decomposers Organisms that feed on dead organic matter, releasing molecules, minerals and energy that then become available to other living organisms in that ecosystem.

Decomposition • The proteins made by plants enter and pass through food webs just as carbohydrates do. • At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. • Decomposers break down the molecules in excretions and dead organisms into ammonium ions during putrification.

Nitrification • Ammonium ions can be taken up directly by plants - usually through their roots. • However, most of the ammonium ions produced by decay are converted into nitrates. • This is accomplished in two steps

Nitrification Step 1 Ammonium ions to nitrites NH₄⁺ NO₂⁻ Nitrosomonas bacteria Step 2 Oxidation of nitrites to nitrates NO₂⁻ Nitrobacteria NO 3 -

• These two groups or autotrophic bacteria are called nitrifying bacteria. • Through their activities nitrogen is made available to the roots of plants in the form of nitrates. • Nitrates can be absorbed by plants to make nucleotide bases (for nucleic acids) and amino acids (for proteins).

3. Denitrification Anaerobic, denitrifying bacteria NO 3 - → NO 2 - → N 2 O → N 2 The bacteria use the nitrates as a source of oxygen for respiration and produce Nitrogen gas Pseudomonas carry out denitrification. Reduces availability of nitrates/ nitrogen containing compounds for plants.

n ifi ca tio Death De n itr Nitrogen fixation excretion feeding Death + Decay Rhizobium Nitrification Nitrosomonas Nitrobacter N₂ +3 H₂ 2 NH₃

NH+4 Ammonium decomposition excretion N in animals N in excretory products Nitrogen fixation Feeding/assimilation Nitrification decomposition NO-2 N 2 Nitrite denitrification NO-3 Nitrate Absorption and assimilation N in plants

NH+4 decomposition excretion N in animals N in excretory products Nitrogen fixation Feeding/assimilation Nitrification decomposition NO-2 N 2 denitrification NO-3 Absorption and assimilation N in plants