Acidification Flashcards
What is acidification?
decrease in buffering capacity of soils
What is soil acidification?
the buildup of unbound H+ ions in the soil, thus decreasing pH in the soil bc
pH = -log10(H+) –> more H+ smaller the pH
How does acidification happen?
- it is a natural process by leaching base cations (Ca, K etc) influenced by:
- rainwater (slightly acidic pH 5.5 + CO2)
- CO2 production plant roots and soil microorganisms
over long time! - accelerated by (anthropogenic) acid deposition - industry, traffic, agriculture, coal burning
What are the source of acid deposition?
- emissions (industry, cars, humans in general): NOx (nitrogen oxide acids) + SO2 (Sulphur dioxides) become DRY deposition when going into the atmosphere
–> photo-oxidation transformation - H(NO3) + H2(SO4)
becomes WET deposition when encountering water particles (clouds) - cattle emissions - NH3 (ammonia) becomes NH4 and becomes WET deposition when encountering clouds
RESULT in ecosystem
H+, NO3-
2H+, SO4^(2-)
H+, NH3
How does dry deposition affect vegetation?
Vegetation functions like an umbrella catching the deposition (that’s why the deposition of a system depends on the total leaf area)
What are the sources of soil acidification?
- direct H+ from deposition:
2H+ (SOx) + 1H+(NOx) - product from processes in soil:
- NH4+ uptake (plants, microorganisms)
–> Biota + NH4+ = (-NH3 amminoacids/amides) + H+
- Nitrification (microbial, aerobic)
–> NH4+ + 2O2 = H2O + NO3- + 2H+
- Pyrite oxidation (FeS) - (chemical, microbial) bc of desiccation (look at desiccation deck)
What is the relationship between deposition and pH?
The more the deposition the smaller the pH
Currently downward trend - however, accumulation over the years of passing thresholds
What are the effects of acidifying deposition on plants?
- Direct leaf damage: occult (fog or cloud water) + dry deposition
- Leaching cat-ions (Ca2+, K+, Mg2+, Mn2+): decline in plant growth rate
Natural process but going much faster due to anthropogenic pressures - Increased mobility of heavy metals (Al3+) - which lead to root damage
- Reduction in soil pH: causes germination problems and has an effect on soil organisms (decomp + nitrification)
- bc of 4 –> change in dominant N form - NH4:NO3 ratio
What is the aluminium toxicity ration (Al:Ca)?
if Al:Ca ratio is over 1 there is a risk of toxicity
What is the ammonium toxicity ratio?
NH4:NO3 needs to be under 5 to be safe (pH range: 4.6-5.9)
In short, what is the effect of acidifying compound depositions on the vegetation?
DECREASED plant growth + reproduction
INCREASED sensitivity to pests and diseases
What is the soil buffering capacity?
The soil buffering capacity is the capacity of the soil to neutralise acids
(e.g. if there is a small influx of protons the soil can buffer the pH and make it remain stable, that is lost once there’s a big enough influx of protons.)
Describe the Carbonate buffer range
The carbonate buffer range happens between a pH range of 8.6 to 6.2 (soil quite alkaline)
CaCO3 + H+ = Ca2+ + HCO3- (loosing the Calcium molecule)
HCO3- + H+ = CO2 + H20
This happens on young soils that haven’t experienced much weathering, loamy/clay soils and soils with Ca2+ rich ground/surface water
Describe the cation exchange
The exchange happens in a pH range between 6.2-5.0
The soil particles have a negative charge that attracts particles with a positive charge. Normally it would be cations but with the addition of protons, protons take the place of the cations.
The Cation Exchange Capacity (CEC) describes the amount of positive charge that can be exchanges.
The Base Saturation is the percentage of CEC occupied by cations such as Ca2+, Mg2+, K+, Na+.
What happens with a continued feeding of protons to the base saturation?
The base saturation drops and so does the pH eventually.
Describe the Silicate Buffer Range
The silicate buffer range happens in a pH range between 5.5-4.2 (weakly acidic soils)
MgSiO4 + 4H+ = 2Mg2+ + H4SiO4 (loosing the 2 magnesium molecules)
However, this is a very slow process
Describe the heavy metals buffering range
Heavy metals buffering range happens between pH of 4.2-3.8 for aluminium and less than 3.8 for iron
Al(OH)3 + 3H+ = Al3+ + 3H2O (release of heavy metals)
Fe(OH)3 + 3H+ = Fe3+ + 3H2O
this happens in old deposits, pleistocene sand and where there is no upward seepage of groundwater
What is the difference between high and low buffer capacity soils?
- High buffer capacity: a high amount of protons is needed in order to change the soil pH
- soils low in silicates but rich in carbonates (CaCO3, MgCO3)
- soils rich in clay and organic matter (bc of CEC)
- young soils + limestone
- mafic (rich in Mg) minerals pyroxine, amphibole, biotite, olivine
- influence Ca and HCO3 rich (bicarbonate) ground/surface water - Low buffer capacity: low amount of protons is enough to change the soil pH
- soils rich in silicates (Si)
- old geological deposits
- quartz (SiO2) predominant
(almost 80-90% of dutch sandy soils)
How to recognise acidification?
- Geology
- Soil chemistry
- Vegetation change
- Hydrology
How to recognise acidification? (geology)
- Old geological deposits
- Old (sandy soils) (Pleistocene > Holocene)
- lime-poor, Si rich deposits (granite)
- limited influence HCO3 rich surface/groundwater (desiccation)
- Pyrite or FeS rich sediments exposed to O2
How to recognise acidification? (soil chemistry)
- Base saturation:
>80% - pH around 6.5
<60% - degradation base-rich vegetation types
<40% - no longer cation exchange buffering range
<15% - pH may drop below 4.2 (Al toxicity) - pH below 4.2 –> Al buffering range thus risk of toxicity
- Al:Ca ratio > 1 (toxicity risk Al)
- NH4:NO3 > 5 (toxicity risk NH4)
How to recognise acidification? (Vegetation changes)
- most sensitive: plant communities weakly buffered soils (4.2>pH<5.5)
- shift in plant species composition:
reduction in base-loving species, increase acid tolerant species +
check indicator species, plant communities +
Ellenberg indicator values for acidity
