Lecture 10 : Summary

Elements necessary for life
C, H, O
N and P: most commonly limiting to growth
Cause problems in downstream aquatic ecosystems if they are lost from terrestrial ecosystems
Ca, K, Mg, S : Macronutrients
Fe, Mn, and others : Micronutrients

Mineral weathering in soil provides a long-term source of most nutrients in many systems

Nitrogen fixation
“N fixation”: biological conversion of atmospheric N2 gas into biologically reactive forms of N: primarily NH3
This is an energy-consuming reaction. It requires ATP
Cyanobacteria = blue-green algae
N fixers in both aquatic and terrestrial ecosystems
Live in water column, or form films on surfaces, or in soils or sediments, or in decaying logs
In terrestrial systems
When “free-living” bacteria, in which case it is called non-symbiotic N fixation
Can be bacteria living in symbiosis with plant roots, requiring anaerobic conditions, called symbiotic N fixation
Root nodules of plants seal out the oxygen. The N-fixing bacteria live in these nodules in a mutualistic symbiosis with the plant.
Some N-fixing plants :
Red alder (tree of the Western US)
Black locust (tree of the Eastern US)
Some herbaceous plants

Available nutrients and soil sorption and exchange
Tropical soils in low, geologically stable areas, can be so old that the clays themselves have all broken down. P is generally more limiting than N in tropical soils formed by lateritic process.

Transport of dust from Africa to Amazon basin: important P source to the Amazon
“A unique combination of global wind pattern and topography forms a vigorous dust source that emits an average of more than 0.7 Tg of dust per emission day and is active mostly during the winter–spring, which is different from most other Saharan dust sources.
“We estimate that between November and March, the Bodélé depression sends more than half of the dust that is deposited annually in the Amazon forest. “
In the tropics, slash-and-burn makes nutrients available for a short period, but soils generally poor in fertility thereafter

Plant and microbial uptake and assimilation
N in deposition moves quickly into soil organic matter and soil heterotrophic microorganisms
Types of fungi (mycorrhizae) live in a mutualistic symbiosis with plant roots
Arbuscular: penetrate cell walls in roots and form branched structures close to cell membranes
Ectomycorrhizae: cover the exterior of small roots and penetrate root cortex spaces; most often on woody plants
Mycorrhizae help in P assimilation only when P is the limiting nutrient.
Saprotrophic organisms feed on dead organic matter.

Immobilization and mineralization in plant litter
Fungi are the main decomposers in terrestrial ecosystems: net release of nutrients to soil

Reactive N and global change
N fixation in crops (legumes)
N emissions from fossil fuel burning, followed by elevated N deposition
N in agricultural fertilizer

Amount of human-fixed N globally, since ~1980s, surpasses natural N fixation globally

Elevated N deposition, N retention, and N saturation
What determines whether a particular nutrient input will be retained in a particular ecosystem or lost (exported)?
Depends on mobility of the nutrient, e.g. NO3 vs NH4 in soils.

  • Depends on soil or sediment properties and how much of the nutrient the soil or sediment can hold, and whether there is a kinetic limit to how it can be retained.
  • Depends on hydrologic flushing.
  • Depends on microbial and plant uptake demand, or strength of the uptake mechanisms.
  • Depends on whether the element has a gas phase that is produced through microbial activity. N does, but P, S, Ca, K, Mg do not.

Historical perspective: Forest N retention and N saturation
Forests were historically very good at retaining N.
Historically, N inputs were low.
Elevated N deposition really rose in the mid to late 20th century.
When the Clean Air Act was enacted in the early 70s, S emissions and deposition were appreciated, but not N emissions and deposition.
Result: S emissions declined after Clean Air Act. But N deposition has continued to be an issue.
N retention, cycling, and loss
Historically forests retained N inputs -> Soils were good N sinks -> Sink capacity was large but growth rate was low -> Vegetation increased N uptake, cycling -> When rate of sink capacity growth equaled the rate of N cycling increase, NH4 build up ensued -> More nitrification to NO3. -> NO3 is mobile Lost from forest ecosystem

Why worry about forest N saturation … isn’t it likely that fertilized agricultural land will always export more N than even N saturated forests?
We are trying to achieve nutrient reductions to receiving waters
Riparian forest buffers will reach N saturation
Expect an interaction with disturbance
Also important to remember: nitrification in soil is an acidifying process
NH4+ à H+ + NO3-
Releases H+ into the soil
NO3- can be taken up by a plant, leaving the H+ in soil
NO3- is mobile in soil; can be carried to streamwater
If H+ accompanies it, this acidifies the streamwater
If H+ stays behind in soil, and a different cation (Ca2+, K+, Mg2+) accompanies the NO3- in the streamwater, this acidifies the soil, and strips the soil of a macronutrient

N saturation and small watershed losses: Fernow, WV as a case study
Fernow, WV: net nitrification nearly equal to net N mineralization was a symptom of N saturation

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