EPHM7110 Chap.3 Nutrient Cycles and Where Human Activity Enters
Nutrient Cycles and Where Human Activity Enters
Three questions decode any cycle diagram
A nutrient is an atom, ion or molecule an organism needs in order to live, grow and reproduce. Nutrient cycles, also called biogeochemical cycles, move those nutrients between the non living environment of air, water, soil and rock and the organisms that use them, and the course puts the point memorably: the cycles tie the life of the past to the life of now and of later.
Every cycle diagram in the teaching material uses the same four parts, so three questions decode all of them. What are the reservoirs, what are the processes, and which pathway have people changed.
The third question is where the marks are.
The four cycles are not variations on a theme
Nitrogen makes up seventy eight per cent of the troposphere and is nonetheless often the nutrient in short supply, because almost none of that reservoir is in a form a plant can use. Carbon dioxide sits at a tiny fraction of the same atmosphere and is rarely limiting.
Phosphorus has almost no atmospheric reservoir at all, cycles slowly through rock and ocean sediment, and is commonly the limiting factor for both land and water.
A pollution problem is rarely a shortage of somewhere for a substance to go; it is a rate mismatch, with material arriving in a reservoir faster than the process that would move it onward can work.
Water carries the other three
Evaporation and transpiration are driven by solar energy, water returns as precipitation and moves on as surface runoff or as infiltration into an aquifer, and the major reservoirs are lakes and reservoirs, aquifers, the ocean and glaciers.
Three human changes are named. Withdrawal lowers the water table and brings groundwater depletion and seawater intrusion. Clearing vegetation and eroding soils raises runoff, lowers infiltration, and increases flooding, erosion and the delivery of nutrients and pollutants. Filling wetlands removes both the flood storage and the infiltration surface.
Because every nutrient reaches an aquatic system by surface runoff, restoring infiltration acts on several problems at once while a downstream treatment acts on one.
Eutrophication is a sequence, and the fish kill is its fourth link
Nitrate from agricultural and domestic activity and phosphate from fertiliser, detergent and mining waste enter the same first link.
Algal growth follows, decomposition rises, oxygen is depleted and fish die. That is why aquatic pollution so often presents as an oxygen problem rather than as a nutrient problem, and why a measure aimed at the last link is aimed at a symptom.
What this chapter covers
- 01
Reservoirs, processes and human affected pathways as a reading method
- 02
What separates the four cycles: atmospheric reservoir, speed and limiting behaviour
- 03
The water cycle and its three human entry points
- 04
Runoff up and infiltration down, and the five direction words that travel together
- 05
The carbon cycle, its two fluxes and its two named human effects
- 06
Nitrogen fixation, nitrification, assimilation, ammonification and denitrification
- 07
The acid rain chain from burning fuel to damaged trees and buildings
- 08
Eutrophication as a five link sequence with two entry points
- 09
Phosphorus as the slow, commonly limiting nutrient
Attribute four symptoms in a developed catchment, then pick one measure
- 3Attribute the blooms and the fish kills.
- 3Attribute the falling water table and the salinity.
- 3Name the measure that reaches most symptoms and justify it by position.
Key terms
- Nutrient
- An atom, ion or molecule an organism needs in order to live, grow and reproduce.
- Biogeochemical Cycle
- The circulation of a nutrient between the non living environment and living organisms.
- Reservoir
- A place where an element sits between transfers, such as the atmosphere, soil, rock or ocean sediment.
- Infiltration
- The movement of water from the surface into the soil and onward into an aquifer.
- Nitrogen Fixation
- The conversion of atmospheric nitrogen into usable compounds by lightning or by bacteria and cyanobacteria.
- Denitrification
- The return of soil nitrogen to atmospheric nitrogen gas by anaerobic bacteria, described as the only natural return path.
- Eutrophication
- Nutrient enrichment of water that stimulates algal growth, raises decomposition, depletes oxygen and kills fish.
- Limiting Factor
- The nutrient whose available quantity controls the pace at which producers can grow.
- Aquifer
- An underground body of rock or sediment holding groundwater, recharged by infiltration.
Nutrient Cycles and Where Human Activity Enters FAQ
Why is nitrogen so abundant and so often in short supply at the same time?
Almost the whole atmospheric store is in a form no plant can take up. It has to be converted first, either by lightning or by particular bacteria, and only after that conversion can it be absorbed and built into proteins and nucleic acids.
Abundance in the air therefore says nothing about availability in the soil, and that gap is why fertiliser manufacture changed agriculture and why the same fertiliser now drives so much water pollution.
A lake was cleaned up years ago and still blooms every summer. Why?
Look at the sediment rather than the inflow. The element that most often controls algal growth moves slowly, and its natural return leg runs through ocean and lake beds over geological time, so material delivered decades ago is still there and is still released back into the water column.
Cutting a pathway that is already closed changes nothing, and the measurement that would settle it is taken from the bed rather than from the pipe.
Does draining a floodplain faster reduce flooding downstream?
It moves the peak rather than removing it. Hard surfaces and channels cut the share of rainfall that soaks into the ground and raise the share that runs off, so the same volume arrives sooner and more concentrated. The confusion is between a local drainage outcome and a catchment scale one, and it is the same confusion behind filling wet ground for flood control while removing the storage that was moderating the peak.
Which cycle should a wetland presentation usually build on?
Whichever one the site's symptoms belong to, but water is almost always part of the answer because it is the carrier. Nitrate, phosphate and eroded soil all reach open water along the same route, so a measure acting on that route acts on several problems at once.
Naming the carrier explicitly also makes the recommendation testable, since infiltration and runoff can both be measured while general statements about degradation cannot.
Exam move
Take one catchment you can find figures for and draw all four cycles onto the same map of it, marking the human affected pathway on each with a dashed line. Then rank the four by how quickly a change at that pathway would show up downstream, and check your ranking against the speed column.
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