CVEN9886 Chap.8 Carbon and Nitrogen Cycles
Carbon and Nitrogen Cycles
This chapter traces carbon among carbon dioxide, biomass, methane and mineral storage, then traces nitrogen through fixation, assimilation, ammonification, nitrification and denitrification. Each arrow is tied to microbial groups, oxygen condition and electron flow. Original balances show how to distinguish transformation from removal and how residual ammonium and nitrate diagnose different process limitations.
What this chapter covers
- 01
Carbon fixation into biomass and respiratory return to oxidised carbon dioxide
- 02
Anaerobic methanogenesis and methane consumption across redox boundaries
- 03
Calcium carbonate formation and longer-term mineral or sediment storage
- 04
Nitrogen fixation, assimilation into biomass and ammonification of organic nitrogen
- 05
Two-step aerobic nitrification from ammonium through nitrite to nitrate
- 06
Anaerobic denitrification using nitrate and an electron donor to form gaseous products
Worked example · free
Close a two-zone nitrogen balance
- +1Calculate 3 units of residual ammonium.
- +1Calculate 2 units of residual nitrate.
- +1Assign 7 units to nitrogen gas and close 3 plus 2 plus 7 equals 12.
- +1Interpret the two residuals as different process limitations.
Key terms
- Methanogenesis
- An anaerobic archaeal process that forms methane, including reduction of carbon dioxide with hydrogen.
- Nitrification
- Aerobic microbial oxidation of ammonia to nitrite and then nitrate.
- Denitrification
- Anaerobic respiration that reduces nitrate through nitrogen oxides toward nitrogen gas.
Carbon and Nitrogen Cycles FAQ
Why is nitrification described as two step?
Different oxidation stages convert ammonia to nitrite and nitrite to nitrate. The populations and conditions can respond differently, so nitrite may accumulate when the second step lags.
What does denitrification require?
It requires nitrate as an electron acceptor, an electron donor such as organic carbon and anoxic conditions. Missing any one of these can limit complete nitrate reduction.
How do I check a biogeochemical equation?
Count each element and charge on both sides before interpreting the process. Then state the oxidation direction, microbial group and environmental condition represented by the balanced relationship.
What is the difference between transformation and removal in a nitrogen balance?
Transformation changes nitrogen form within the defined system, such as ammonium becoming nitrate. Removal requires nitrogen to cross the selected boundary, for example as harvested biomass or nitrogen gas. Biomass incorporation may be temporary if solids later decompose inside the boundary.
List every dissolved, particulate and gaseous pool, convert them to a common nitrogen-mass basis and make the opening amount equal the closing pools and outputs. A disappearing nitrate measurement alone cannot establish denitrification without identifying its destination.
Exam move
Redraw both cycles from empty boxes until every arrow can be explained as a microbial conversion with a direction and condition. For carbon, begin with carbon dioxide, organic biomass, methane and mineral storage. Add fixation, respiration, methanogenesis, methanotrophy and carbonate formation one at a time.
Beside each arrow, write whether carbon becomes more oxidised or reduced and identify the oxygen or electron-acceptor condition. Distinguish pool size from flux: a large reservoir can turn over slowly, while a small reactive pool can support rapid microbial conversion. Balance the methanogenesis relationship by counting carbon, hydrogen and oxygen before linking it to anaerobic archaea and biogas production.
For nitrogen, write atmospheric nitrogen, ammonium, nitrite, nitrate, biomass and gaseous intermediates as separate pools. Add nitrogen fixation, assimilation, ammonification, the two steps of nitrification and denitrification. Mark nitrification as aerobic and denitrification as anaerobic respiration that still requires nitrate plus an electron donor.
Use arrows of different colours for oxidation, reduction and incorporation into biomass. Then practise boundary selection. Draw a box around the water phase, the whole treatment train or the receiving ecosystem and decide which movements count as internal transformations and which count as removal. Rework the twelve-unit example by listing opening nitrogen first and closing ammonium, nitrate and gas second.
Add biomass or nitrite only when the scenario includes them. Keep all terms on a common nitrogen-mass basis and check that the total closes. Turn residuals into diagnoses: ammonium suggests incomplete aerobic oxidation, nitrite can indicate imbalance between nitrifying steps, and nitrate suggests incomplete downstream reduction.
Connect each residual to a distinct control such as oxygen, retention time, anoxic zoning or carbon availability. Create condition-change drills by removing oxygen, usable carbon or nitrate and predicting the blocked reaction and accumulating pool. Do not call every low-oxygen change denitrification.
For monitoring answers, require paired compounds on both sides of the proposed conversion, the controlling physical condition and either a time series or inlet–outlet comparison. Ask whether dilution, storage or transport could explain a concentration change before assigning a reaction. Finish each cycle explanation with matter conservation first and energetic interpretation second.
That order prevents process names from replacing the balance and makes the engineering response traceable to the failed microbial step. Confirm the chosen boundary before interpreting any apparent loss from solution.