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Flashcards in Module 4 Deck (17)
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mechanisms protecting soil organic matter from decomposition

(i) physical inaccessibility
(ii) interactions with mineral surfaces or metal ions
(iii) presence of recalcitrant compounds (e.g., charcoal),
(iv) environmental conditions (e.g., temperature, water availability, soil pH, redox status).


describe the process of nitrification

The biological conversion of ammonium to nitrate is known as nitrification ammonium ions from organic matter, urea, and nitrogen fertilisers >>ammonium oxidised to nitrite then to nitrate >>produces hydrogen ions and decreases pH >> 2NH4+ + 4O2  2NO3- + 4H+ + 2H2O >> outer zone


describe N immobilisation

- Immobilisation is a microbial process in which the plant available ammonium and nitrate ions are converted to plant unavailable organic N.
- Carbon rich substances such as maize stubble and cereal straw promote this and reduce N available to plants


Describe the different types of N2 fixation systems

Chemical - The Haber-Bosch process, N2 +3H2 ⇔ 2NH3
Biological - happens due to micro soil organisms, Azotobacter, Beijerinckia, Spirillum and Enterobacter


Discuss the conditions under which inorganic nitrogen can be lost from the soil and the processes involved

Ammonium (NH4+) and nitrate (NO3–) are the predominate inorganic forms of nitrogen in soils.
These can be lost through leaching
nitrate moves with water so best in wet conditions


mycorrhizal symbioses

The beneficial effect of mycorrhizae on plant growth has mostly been attributed to an increase in the uptake of nutrients, especially phosphorus via
(1) root interception;
(2) mass flow; and
(3) diffusion.


rhizobial symbioses

part of biological nitrogen fixation, infects legume species to make efficient nitrogen fixation
- The bacteria grow in the nodules, fixing N2 gas into
NH4+ compounds which are available to plants
- The plant keeps the bacteria supplied with carbon compounds


effects of earthworms on soil fertility and the factors that affect their abundance.

earthworms improve texture of soil, promote aggregate stability, larger turnover in soils, burrowing increase porosity, increases moisture retention, increase chemical fertility though faces and intake of soil, incorporate organic matter, transforming nitrogen.
Abundant during high rainfall and cooler temps and in more alkaline soils.


microbial respiration and associated treatments



C Cycle

- driving force behind other cycles
- sources carbon from plant residue (lining, protein etc) and microorganism's


N Cycle

- transfers nitrogen from one form to another
- reactions such as mineralisation, immobilisation, nitrification, denitrification, ammonia fixation and leaching


sources of soil matter

Plant residue, micro and Macro organisms, Manure, Sewage Sludge


the implications of high C/N ratio in N Immobilisation

Release of greenhouse gases, leaching nitrate can become dangerous for drinking water.


describe the process of denitrification

NO3- => NO2- => N2O => N2 = the reduction of nitrate to nitrite to nitric oxide to nitrous oxide and nitrogen gas >> loss of valuable plant nutrient and release of green house gases >> outer zone.


distribution of microbial organisms in soils

top soil, rhizosphere (moist soil, defences against pathogenic), subsoil, saturated conditions = as you go down the layers there is less organic matter and oxygen for microorganisms.


How is lignin decomposed by fungi?



Exo-enzyme two methods of decomposition

Lock and key, the enzymes have an active site that fits a particular substrate
For lignin, they oxidise and create free radicals