Lecture 9 CM Flashcards
Aerobic microbes are key to decomposition and nutrient cycling
– they function best in well-aerated soil
Aerobic microbes are key link between
soil inorganic and organic fractions, particularly the cycling of Nitrogen - the universal limiting factor for organic growth in soils
Nitrogenase, the enzyme that fixes atmospheric N2 into organic Ammonium (NH4+) is
only present in N-fixing bacteria such as Rhizobium
• Nitrifying bacteria such as Nitrosomonas convert Ammonium (NH4+) into Nitrite (NO3-)
Physical structure (mix of granule sizes [clays and sands]) and anion/ cation balance (equal +ve and –ve ions)
are key to aeration and water mobility as this provides for air spaces within the soil
Aerobic microbes control nutrient cycling in the soil
• Microbial oxidation of organic matter (OM) and complex organic compounds
is a principal source of energy for microbes, which release simple compounds and surplus nutrients as a byproduct
• CO2 largely routes back to atmosphere
(reversing uptake that originally took place through photosynthesis)
Other nutrients are retained and competed for by microbes
Net mineralisation
occurs when more nutrients are released than can be taken up and utilised by soil microbes – this increases nutrient availability for plants
Net immobilisation
occurs where addition of OM provides energy for microbes to utilise most of the nutrients produced by decomposition – leading to reduced availability for plants
Aerobic microbes drive nutrient cycling in the soil
Dead animal and plant matter is deposited within the soil
Microbial respiration via decomposition of organic matter
Bacteria degrade organic matter in the presence of O2
Where energy is high but nutrients are low [bacteria use all available nutrients] = Immobilisation
N, P, K, Mg, Na, Ca, Fe
Micronutrients are drawn into bacterial communities
Aerobic microbes support mineralization when soil carbon and nitrogen are in an equable ratio
Healthy soil (displaying overall mineralization) has an equitable balance between carbon and nitrogen components (called the carbon to nitrogen ration [C:N]), and the N cycle is stable - N is neither in surplus or deficit
• C:N of 20:1 to 30:1 is the typical ideal ratio in temperate arable soils
• For this reason, in farming the addition of OM/ fertiliser must be done very
carefully
• A low C:N ratio, eg. 10:1, causes release of excess N as NH4 by microbes
• A high ratio, eg. 50:1, results in foraging of N by microbes, depleting the N resources available to plants
Well-aerated soil with good OM content and healthy microbial diversity (which support N cycling) is key for sustainable farming
The mean residence time for soil nutrients varies with soil type and habitat/ biome
The combined impact of temperature, soil structure, organic matter content, water availability and aeration contribute to nutrient recycling rates
• This is reflected in mean residence times for key nutrients within the soil – in highly productive soils, nutrients enter and leave the available nutrient pool much more rapidly than in soils with low productivity
Soil productivity and vegetation structure are closely linked
The spatial mosaic of soil fertility and soil microbe activity across a landscape gives rise to a spectrum of productivity types
• In Ireland, although the entire island belongs to the temperate broadleaf forest biome, a wide range of soil fertility is observed
• In general, poorly drained soils (arising from either excessive rainfall, high water-holding capacity, or both) are less fertile than well-drained soils (arising from lower rainfall, reduced water-holding capacity, or both)
• This is because soil water saturation impacts on oxygen availability, pH and nutrient availability.
• Patterns of human settlement in Ireland have been greatly impacted by soil distribution, for example the Normans colonization (1100’s), the Elizabethan plantations (1500’s), and famine-era settlement (1800’s).
The distribution of soil types in Ireland
Digital soil Map of
Ireland (EPA & Teagasc, 2015)
Complex spatial distribution of soils that have evolved under the influence of bedrock geology, glacial and river sedimentation and rainfall levels
Human settlement and soil fertility are closely linked
Prior to the Famine in Ireland in the 1840’s, population expansion was facilitated by the ready cultivation of potato even in the poorest Mor-type soils.
‘Lazy beds’, which are abandoned potato plots, indicate cultivation of poor [mor soil] land by peasant farmers in the early 1800’s
Combination of potato physiology and improved drainage conditions created by lazy-beds allowed short-term cultivation with high yields
[Lazy beds were unsustainable because ….?]
Soil productivity and vegetation structure are closely linked – Global Patterns
At a global level, biome vegetation types differ in terms of:
(a) Standing plant biomass
(b) The ratio of nutrients apportioned to soil or biomass
• High energy ecosystems such as tropical rainforests
• very rapid nutrient cycling
• most biomass is located above ground
• Hyperefficiency of microbial biota within the upper soil horizon
• Lower energy ecosystems such as temperate or boreal forests
• slower nutrient cycling
• biomass often equally divided into below/ above ground portions
• lower microbial efficiencies within the upper soil horizon
