Question

Questions (answer all 10)
1. Describe the main steps of the nitrogen cycle and the organisms or processes responsible for each step.
2. Explain why nitrogen gas (N₂) in the atmosphere is not directly usable by most organisms and how it becomes biologically available.
3. Compare and contrast nitrogen fixation and nitrification. Include where each occurs and why they are important.
4. Describe denitrification and its ecological significance, including where it commonly happens.
5. Outline the main reservoirs and fluxes of phosphorus and how phosphorus moves from rocks to living organisms.
6. Explain why phosphorus is often a limiting nutrient in freshwater systems.
7. Define eutrophication and describe the roles of nitrogen and phosphorus in causing algal blooms.
8. Identify three human activities that increase nitrogen or phosphorus loading in waterways and explain the consequences.
9. Propose two realistic mitigation strategies to reduce eutrophication in a local watershed. Explain how each strategy reduces nutrient inputs.
10. Using a real or hypothetical example, describe how altering the nitrogen or phosphorus cycle can affect an ecosystem's food web.

Answer 1

1) Main steps of the nitrogen cycle and who/what does each step
- Nitrogen fixation: N2 → NH3/NH4+. Carried out biologically by free‑living and symbiotic bacteria (e.g., Rhizobium in legume root nodules, cyanobacteria, Azotobacter), abiotically by lightning and industrially by the Haber–Bosch process.
- Assimilation: Plants and microbes take up NH4+ or NO3− and incorporate nitrogen into organic molecules (amino acids, nucleotides); animals acquire N by eating plants or other animals.
- Ammonification (mineralization): Organic N from dead organisms and waste → NH4+. Performed by decomposer microbes (bacteria and fungi).
- Nitrification: NH4+ → NO2− → NO3−. Carried out by aerobic chemoautotrophic bacteria/archaea (e.g., Nitrosomonas oxidize NH4+ to NO2−; Nitrobacter or Nitrospira oxidize NO2− to NO3−).
- Denitrification: NO3− → N2 (and N2O intermediate). Done by facultative anaerobic bacteria (e.g., Pseudomonas, Paracoccus, some clostridia) in low‑oxygen conditions.
- Anammox (anaerobic ammonium oxidation): NH4+ + NO2− → N2. Performed by specialized Planctomycetes in anoxic sediments and oxygen‑deficient waters.

2) Why atmospheric N2 is not directly usable and how it becomes available
- N2 is chemically inert because of a very strong triple bond; most organisms lack the enzymes (nitrogenases) needed to break it.
- It becomes biologically available when converted to reduced forms (NH3/NH4+) by nitrogen fixation (biological fixation by microbes, lightning, or industrial Haber–Bosch). Fixed N can be taken up by plants or further transformed (nitrification) into NO3−.

3) Compare and contrast nitrogen fixation and nitrification
- What they do: Fixation converts N2 → NH3/NH4+; nitrification converts NH4+ → NO2− → NO3−.
- Who/where: Fixation: nitrogenase‑containing bacteria/archaea (symbionts in root nodules, free‑living in soils, aquatic cyanobacteria), and abiotic lightning or industrial processes; occurs in soils, aquatic systems, root nodules, atmosphere (lightning) or factories. Nitrification: aerobic chemoautotrophs (Nitrosomonas, Nitrobacter, Nitrospira) in well‑oxygenated soils and water column/sediment interfaces.
- Importance: Fixation supplies new biologically available N from the atmosphere, supporting primary productivity. Nitrification converts ammonium (often produced by decomposition or fixation) into nitrate, a highly mobile and plant‑available form but also more prone to leaching and to supplying substrates for denitrification.

4) Denitrification and its ecological significance
- Process: Sequential microbial reduction of NO3− (and NO2−) to gaseous forms (NO, N2O, N2) under low‑oxygen conditions by facultative anaerobes.
- Where: Waterlogged soils, wetland sediments, lake/river sediments, anoxic zones in estuaries and marine waters, wastewater treatment systems.
- Ecological significance: Removes bioavailable nitrogen from ecosystems by returning N to the atmosphere, helping limit excess N and mitigate eutrophication; however, it can produce N2O, a potent greenhouse gas, and reduce fertilizer efficiency in agriculture.

5) Main reservoirs and fluxes of phosphorus and how P moves from rocks to organisms
- Major reservoirs: Geological (phosphate minerals/rocks, e.g., apatite), soil inorganic P (adsorbed to minerals), soil organic P, freshwater and marine dissolved inorganic phosphate (PO4^3−), biota, and sediments/sedimentary rock.
- Fluxes and pathway: Weathering of phosphate‑bearing rocks releases dissolved phosphate into soil and water → plants and algae take up phosphate (assimilation) → moves through food web via consumption → returned to soil/water by excretion and decomposition → some P adsorbs to soil minerals or is buried in sediments (long‑term sink). Over geological time uplift and erosion recycle sedimentary P back to rocks.

6) Why phosphorus is often limiting in freshwater systems
- Phosphorus has no significant gaseous phase and is released slowly by rock weathering, so inputs are limited. In freshwater P strongly adsorbs to soil and sediment particles (Fe/Al oxides), making it less bioavailable. Because available dissolved phosphate is typically low, primary production in many lakes is P‑limited—small increases in P can produce large algal responses.

7) Define eutrophication and roles of N and P in algal blooms
- Eutrophication: Enrichment of aquatic ecosystems by nutrients (mainly N and P) that stimulates excessive growth of algae and plants, often leading to oxygen depletion, loss of biodiversity, and water quality problems.
- Roles of N and P: Both are essential nutrients; in freshwater systems phosphorus is frequently the limiting nutrient, so added P commonly triggers algal blooms. In many coastal and marine systems nitrogen is more often limiting, so N inputs drive blooms there. Excess N and P together accelerate growth of phytoplankton and cyanobacteria; when blooms die and decompose, microbial respiration consumes oxygen, causing hypoxia or anoxia and sometimes producing toxins.

8) Three human activities that increase N or P loading and consequences
- Agriculture: Excess fertilizer application and manure runoff add high loads of dissolved N and P to waterways. Consequences: algal blooms, hypoxia, nutrient‑driven shifts in species composition, fish kills, degraded fisheries.
- Wastewater discharge and septic systems: Untreated or partially treated sewage releases N and P. Consequences: local eutrophication, harmful algal blooms, drinking water contamination (e.g., cyanotoxins).
- Urban stormwater and land disturbance (construction, deforestation): Increased erosion and runoff transport P bound to sediments and N from fertilizers, pet waste, and atmospheric deposition. Consequences: increased turbidity, sedimentation, nutrient loading, degraded aquatic habitat.

9) Two realistic mitigation strategies for a local watershed and how they work
- Riparian buffer strips and constructed wetlands: Planting/maintaining vegetated buffers along streams and restoring wetlands intercepts surface runoff, traps sediments (which carry P), promotes plant uptake of N and P, and creates anaerobic zones where denitrification removes nitrate. Result: less nutrient and sediment delivery to streams.
- Improved agricultural nutrient management: Precision fertilization (soil testing, right rate/timing/method), cover crops, reduced tillage, contour farming, and manure management reduce surplus fertilizer and soil erosion. Result: lower nutrient runoff and leaching into waterways.
(Alternative: upgrade municipal wastewater treatment to include biological nutrient removal/tertiary treatment to directly lower N and P discharges.)

10) Example of how altering N or P cycles affects a food web
- Real example (Lake Erie / western Lake Erie basin and Toledo, Ohio): Decades of increased phosphorus runoff from agricultural land and urban sources led to recurrent toxic cyanobacterial blooms (Microcystis). These blooms reduce water clarity and block light, killing benthic plants and altering habitat for invertebrates and fish. When blooms senesce, decomposition causes hypoxia, killing or displacing fish and benthic organisms and collapsing local fisheries. Toxins forced drinking‑water advisories (e.g., Toledo 2014). The altered nutrient regime favored opportunistic cyanobacteria over native phytoplankton and submerged macrophytes, restructuring the food web from diverse, benthic‑based energy flow to surface‑dominated, bloom‑driven dynamics.
- Hypothetical terrestrial example: Chronic N deposition from air pollution can shift a nutrient‑poor grassland to N‑enriched conditions, favoring fast‑growing invasive grasses that outcompete native forbs, reducing insect diversity and altering food resources for birds and small mammals.