Study Guide:Barnes; CH 19, part 2


Concept: Nutrient Cycling 2


1.  How does the accumulation of organic matter in the forest floor vary by climate and species composition? Why?


Forest floor (organic horizon >40% OM or 20% organic C) retains nutrients, water; improves infiltration and reduces erosion.


In very cold climates, decomposition is slow, and SOM accumulates.  Arctic and boreal forests can have thick organic layers.  In hot and humid climates, decomposition is rapid, and forest floors are less massive.


Deciduous leaves decompose more readily than conifer needles.


2.  What chemical compounds found in plant litter are broken down during microbial decomposition, and which break down faster than others?


Proteins are rapidly broken down; there’s a big demand by decomposers.

Carbohydrate: glucose is most available, more complex sugars, cellulose and hemicellulose go more slowly.

Lignin goes the slowest.


3.   What other factors besides the chemical constituents of plant litter affect the rate of decomposition?


Temperature and soil water potential (availability of water)

The common earthworm is a European exotic, which speeds up decomposition and incorporates OM into the mineral soil.  Microarthropods are important to breaking down material into smaller pieces.  Bacteria and fungi do most of the work.




Chemical symbols you should know:

NO3 nitrate (most oxidized)

NH4 ammonium

SO4 sulfate




PO4 phosphate





4.  What are the processes for  N immobilization, N mineralization, nitrification and denitrification . (Also, try drawing the N cycle within an ecosystem)


1. N immobilization

2. N mineralization

3. Nitrification

4. Denitrification

5. N fixation





5.  Which elements are more likely to be leached from an ecosystem? Why?


Nitrate is more readily leached than ammonium (which is held by cation exchange).

Ca, K elements that are weathered in large amounts will be important in streamwater.


6.  When is nutrient leaching greatest in temperate deciduous forests? 


Early spring:  trees are not taking up much water yet, so there is more to lose; snowmelt has high nitrate.