Lecture 16-19 Flashcards
1
Q
What are the 3 types of salt enrichment?
A
Rainwater
Groundwater
Artificial (irrigation)
2
Q
How does groundwater salt enrichment work?
A
Water moves upward through capillary rise and leaves behind salts as water evaporates
3
Q
Why is salt enrichment prominent along river and lake oasis in dry and arid regions?
A
Water moves up with capillary fringe
4
Q
How does salt enrichment through irrigation happen?
A
Occurs on surface but also subsurface of soil
Water percolates quickly after wetting but also evaporates in soil layers –> creates subsurface salt enrichment
5
Q
What are Sabakh soils (morning soils)?
A
Attract water from fog and dew in the morning –> enriches the soil surface with salt
Creates darker colour as salt enriched soils become saturated by water from atmosphere (during the day as it dries, soil gets more white)
6
Q
Are salt crusts good a retaining soil moisture?
A
Yes, because crust will dry rapidly however inside will remain wet for longer
7
Q
How do we measure salt enrichment?
A
Measuring electric conductivity (ECe)
- measured by difference in electrostatic potential
- larger than 15 mS/cm –> salt enriched soil
Measuring sodium adsorption ratio (SAR)
Measuring exchangeable sodium percentage (ESP)
Basically, measure sodium and compare it with other cations present in the soil
8
Q
How does high salt content impact plants?
A
Induces physical drought due to osmotic potential (drought stress)
Toxic concentrations of certain ions (Boron or chloride toxicity)
Induces unfavourable nutrient ratios (K/Ca)
Influences soil physical properties (change of soil structure, compaction, etc.)
9
Q
What are some very salt-sensitive crops?
A
Fruits, nuts, citrus, avocado
ESP=2-10
10
Q
What are tolerant and very tolerant crops?
A
Wheat, cotton, alfalfa, tomato (ESP=40-16)
Some grasses (ESP>60)
11
Q
What is a typical symptom are stress caused by salinity?
A
Plant loses green colour
12
Q
What is a human-induced salt enrichment issue in Quebec?
A
Salting the roads during the winter
13
Q
What is the solubility sequence of salts in increasing order?
A
CaCO3 (calcium carbonate)
NaHCO3 (sodium bicarbonate)
Na2CO3 (sodium carbonate)
CaCl2 (calcium chloride)
KCl (potassium chlroride)
MgCl2 (magnesium chloride)
NaCl2 (sodium chloride)
however this depends partly on temperature
14
Q
Why is it mrore problematic when a salt is more soluble?
A
Bad for plants due to osmotic potential
15
Q
What type of soil develops from the accumulation of calcium carbonate?
A
Calcisol
16
Q
What type of soil develops from the accumulation of calcium sulfate?
A
gypsisol
17
Q
What type of soil develops from the accumulation of sodium chloride?
A
solonetz
18
Q
What type of soil develops from the accumulation of other salts?
A
solonchak
19
Q
What type of soil develops from the accumulation of silicon dioxide?
A
durisol
20
Q
Solonchak soil
A
Experiences accumulation of salt on topsoil
High water table –> capillary fringe brings groundwater up to the surface –> salt from groundwater evaporates at the surface
High soluble salt content and soil water conductivity
Sources of salt include parent material, residual as water evaporates, coastal salt spray, groundwater
Low OM content due to limited plant growth
Usually well-structured soils because high salinity promotes flocculation
pH generally 7 to 8
Limited agricultural value because of high salinity
Fairly distributed in the world, but mostly found in inner land of central asia and North African prairies
21
Q
What are the soil forming processes of solonchaks?
A
Usually in inland areas where evapotranspiration is considerably greater than precip.
Differentiation between external and internal solonchack
- salt acc. on surface –> external
- salt acc. at depth –> internal
22
Q
How are solonchaks managed?
A
Salt has to be leached from soil for good yields
- can be done by irrigation
- However, the problem is that the irrigation water itself contains too much salt which aggravates the problem
If a lot of water present, it can be sufficient to leach out salts from soils
Planting crops with deep rooting system so it reaches the water table and lowers it
- able to reduce effect of groundwater salt enrichment
23
Q
What is solonetz soil?
A
Poorly structured impervious surface layer
Natric subsurface horizon (high Na concentration)
- shows a ESP of 15 or greater in the upper 40 cm of the horizon
Columnar subsoil structure with clay translocation
- translocation occuring due to presence of Na
In the dry season:
- formation of cracks
- accumulation of salts due to capillary rise of water
In the wet season:
- peptization of clay and OM due to high Na concentrations in upper soil horizons
- illuviation of clay and OM in the Btn horizon
Found in Australia and middle east
24
Q
What is soil dispersion caused by?
A
High Na and low salt levels in sodic soils can cause clay dispersion, degradation of aggregate structure and loss of macroporosity
If there is sufficient amount of water in usually dry season, topsoil becomes saturated –> high Na content leads to dispersion of clay and OM, destroys soil aggregate and forms soil crust on surface impeding plant growth
25
How do hydrated ions affect bonding forces ?
No hydrated ions: colloid are bound by van der walls forces
Hydrates ions can go between colloid and weaken van de waals forces
hydrated Na ions prevent binding of colloid because of loosely bound Na ions which pull colloid away (no more van der waals forces)
Results in dispersion of microstructure of soil and soil aggregates
26
What are calcisols?
Accumulation of calcium carbonate typically in subsoil
Not very soluble there fore still allow for some crop growth
Accumulation occurs beacuse:
- CaCO3 dissolves in topsoil --> leaches --> precipitation of secondary carbonate at deeper soil layers
- Leaching into groundwater --> capillary rise --> precipitation of carbonates in soils
Mostly found in arid and semi-arid regions such as Northern Africa and Middle East
27
What are calcisols used for?
Used for grazing
If precipitation exceeds 400 mm/yr, rainfed agriculture with drought tolerant crops such as wheat or sunflower is possible
When irrigated, calcisols allow the production of a wide range of fruits, grain and fodder crops
Careful irrigation is necessary to prevent salinization
- The topsoil of calcisols is easily eroded. Where this topsoil is lost, the calcic horizons exposed to the surface form calcrete crusts (rock solid)
28
How to remove salts from soils?
Addition of irrigation water removes soluble salts but leaves cation exchange complex dominated by Na
Na leads to dispersion and poor soil structure
Na combines with carbonic acid in soil solution producing Na2CO3 and raising pH (strong base weak acid)
Whilst removing soluble salts, add CaSO4 (gypsym)
- Ca preserves flocculated structure
- excess Na removed as neutral Na2SO4 (strong base strong acid)
29
Aridisol (desert soil)
Almost complete absence of water --> creates desert pavement of gravel
Old, oxidized, Fe-rich horizon
- have developed in geologic past when soil was exposed to more rainfall
Silcrete (Si-cement horizon)
- Silica cementation
Desert pavement of gravel
30
what are desert soil processes?
Sparse vegetation and rapid decomposition after rainfall leads to low OM content of soils
- no plant cover = a lot of erosion (fluvial or aeolian)
Dev. of stone pavement on surface, either through aeolian or fluvial erosion of fine particles
Dominance of previous soil formation processes, especially in pluvial periods during Pleistocene
- Fe-rich material --> red colour
- CaCO3 acc. --> caliche
- silica cementation leads to silcrete
31
How come we can find islands of vegetation in deserts?
Created by onset of vegetative cover
Over time, vegetative cover can become larger and create prockets of more fertile and better at retaining soil water
Because acc. of OM, and vegetation cover, lower evaporation and enhanced soil water and nutrient availability
32
What are organic soils (histosols)?
Organic material accumulates in wet places where plant growth exceeds the rate of residue decomp.
In such areas, residues accumulate over the centuries from wetland plants
Possess very high capacities to hold water and cations
- higher than clay-rich mineral soils on a weight basis, but similar to mineral soils rich in 2:1 silicate clays when considered on a volume basis (water or cations held per liter of soil)
33
Where are peatlands found?
Abundant in the North (places with wet and cool conditions that allow acc. of OM)
Some tropical peatlands exist nonetheless
34
What causes OM to accumulate in organic soils?
Litter input eventually reaches anaerobic zone in soil profile (below water table). Decomp. rates under anaerobic conditions are 20 times slower than under aerobic conditions
Some wetland plants have slow inherent decomp. rates (produce biomass that have slow deomp. rates --> internal resistance of material)
- shrubs, sedges, tree leaves, mosses
35
What are the 4 types of peat materials?
Type of peat depends on the vegetation that contributes to the OM
1) Moss peat: the remains of mosses such as sphagnum
2) Herbaceous peat: residues of herbaceous plants such as sedges, reeds, and cattails
3) Woody peat: from the remains of woody plants, inclduing trees and shrubs
4) Sedimentary peat: remains of aquatic plants (algae) and of fecal material of aquatic animals
36
Explain the development of a peatland
Onset of peatland dev. from a pond or lake
- hollow filled with water, there are some plants on the edges as well as some aquatic plants
Accumulation of OM on the pond floor
- the dead biomass will fall down and will reach the bottom of the lake in anaerobic conditions
Infilling of pond with OM and dev. of peatland veg.
- accumulation of OM slowly starts to fill the hollow
Fomes bog with drainage impedance and paulidification
- vegetation will start to grow on top of OM
- the parent material in the top part of the soil is completely disconnected from the mineral layer on the bottom
- OM is building the parent material
37
What is terrestrialization?
Growth of OM inwards
38
What is paludification?
Growth of OM outwards
39
What directions does peat grow?
Bottom to top
Outwards
Inwards
40
Bogs vs. fens
Bog is raised above mineral layer, therefore the inflow of water only goes into fen layer
Dev. of fen is still in contact with mineral soil, therefore inflow of water in valley leads to accumulation of fen peat
41
What are the 3 types of organic soils in Canada
Fens: mainly in arctic/subarctic regions, usually 50-200 cm think
- usually minerotrophic
Bogs: primarily in boreal zone, up to 600 cm thick
- often ombrotrophic
Swamps: mainly in temperate zone, 100-200 cm thick
- can be omrbotrophic or minerotrophic
42
Minerotrophic definition
Nutrition is mineral rich since mineral from mineral soils are transported to fens --> more fertile for plant growth
43
Ombrotrophic definition
Peatland surfaces that are supplied only with rain and snow (atmospheric deposition)
44
What are the characteristics of swamps?
Vegetation: trees and shrubs
Hydrology: topography controlled, waterlogged in the spring, dry in the summer
Acidity: variable (5-7)
Rate and degree of decomp.: fast and high (mainly humisols and mesisols)
- dry summer --> higher decomp. rates
Utility: when drained, high crop production (vegetables)
45
What are the characteristics of fen?
Vegetation: sedges and shrubs
Hydrology: topography controls drainage, usually flowing water throughout the year, can be rich in bases
Acidity: moderate pH (5-6.5)
Rate and degree of decomp.: medium, moderate (fibrisols and mesisols)
Utility: none
46
What are the characteristics of bogs?
Vegetation: mosses and trees (and shrubs)
Hydrology: low hydraulic conductivity of humified horizons, depends on rainfall (no inflow from river or groundwater)
Acidity: very acid (<5)
Rate and degree of decomp.: slow and low (mainly fibrisols)
Utility: production of peat moss and peat for energy
47
Is the oxic zone below or above the water table?
Above, it goes oxic zone --> oscillating water table (varies between summer and winter) --> anoxic zone
48
How does the minerotrophic layer get nutrients vs. ombrotrophic layer?
Minerotrophic: in contact with mineral layer and rich in mineral nutrients
Ombrotrophic: from wet and dry deposition (soil dust, anthropogenetic input, ashfall, marine aerosols)
- above water table, therefore runoff never flows in there
49
What happens to methane emissions as water table rises closer to the surface?
CH4 emissions increase
50
Is the top layer of peat alive of dead biomass?
Combo of alive and dead root biomass --> transition from living biomass into decomposed biomass
Acrotelm: transition from living biomass to dead biomass (transient carbon storage)
Catotelm: permanent anoxic conditions (long-term storage)
Note: cotton grass roots are deep enough that they go beyond the catotelm layer (can access nutrients beyond)
51
Explain the carbon budget of peatlands
Vegetation takes up CO2 through photosynthesis (input)
Plant/microorganisms respire CO2 back into the atmosphere (output)
Decomposition produces CO2, CH4 and dissolved organic carbon (output)
52
Why do peatlands have a negative C budget?
Exchange of CO2 (uptake through photosynthesis, release through respiration/decomposition)
Emission of methane (CH4) to the atmosphere under anaerobic conditions
Export of dissolved organic carbon from decomposition of OM
Imbalance of C uptake vs. loss leads to long-term C accumulation --> significant reduction in atmospheric CO2
53
Explain the carbon cycle in peatlands
Plants take from atmospheric CO2:
- aeorobic degradation in the acrotelm --> CO2 emissions back into the atmosphere
- anaerobic degradation
Peat in catotelm
- releases CH4 from the catotelm that oxidizes in the acrotelm and releases CH4 to the atmosphere
- releases CO2 that oxidizes in the acrotelm and releases CO2 to the atmosphere
54
What is the Eddy covariance tower used for?
Used to measure CO2 fluxes
Continous measurements of water, energy and CO2 and CH4 fluxes
55
Explain the net ecosystem exchange (NEE) of CO2 throughout the year
NEE is positive in the autumn/winter months because there is no photosynthesis (outflow of CO2) --> increases
In april, we still have an increase despite photosynthesis beginning since decomp. rates are still higher (decomp. strong due to warming temperatures) --> april is the peak positive value of NEE
May and onwards, plant cover increases --> fluxes of CO2 begins to go the other direction (NEE is negative)
Around July, there is a tipping point --> system taking up more CO2 and stored in peatlands (more inflow than outflow)
Afterwards, NEE negative until stabilizes
- measurements are very dispersed due to accumulated uncertainties
56
Explain the changes in concentrations of calcium with varying precipitation rates
High rainfall: Ca will be dissolved and washed out
- low concentrations
Threshold of about 2000 mm/yr: water balance is not enough to leach it out
- sharp increase
Decreases when rainfall is too low since plants are limited
- concentrations decrease
57
Explain the changes in concentrations of phosphorus with varying precipitation rates
High rainfall: leached and mobilized
- low concentrations
Threshold of about 2000 mm/yr: decreased rainfall means that soil dev. is not as dominated by allophanes therefore larger amount of P
Decreases when rainfall is too low
58
What happens to soil pH when rainfall increases?
It decreases
59
List a few important GHGs
Water vapour (H2O
Carbon dioxide (CO2)
Methane (CH4)
Carbon monoxide (CO)
Nitrous oxide (N2O)
Nitric oxide (NO)
Ozone (O3)
60
What GHG has the highest residence time?
N2O>CO2>CH4
61
What is the GHG with the highest concentration in the atmosphere currently?
CO2>CH4>N2O
62
What GHG has the highest radiative absorption potential?
N2O>CH4>CO2
Nitrous oxide is therefore dangerous due to its long residence time and radiative absoprtion potential
63
Why are CO2 emissions lower in the Northern hemisphere?
More land mass, therefore more potential for photosynthesis
64
What was the dominant source of annual CO2 emissions until 1950? What is it now?
Before, it was land-use change
Now, it is bruning of fossil fuels
65
What are important carbon sinks?
Land sinks (soil): taken up by vegetation and stabilized in soil
Ocean sinks
66
What are some contributors to carbon emissions?
coal, oil, gas, cement, land-use change, etc.
67
What is the effect of land use change on soil carbon?
Temperate forest: loss of 34% of soil carbon
Temperate grassland: loss of 29% of soil carbon
Tropical forest: loss of 21% of soil carbon
Tropical grasslands: 46% of soil carbon
68
How much carbon is loss from agricultural conversion a year?
Approx. 0.1-5 Pg C/yr
Therefore, return of agricultural land to forest will increase soil C
69
Why are land-use change emissions declining?
Transfer of croplands to forests/grassland ecosystems
Management practices have been improved (low tillage, de-intensification of production)
70
What type of soil stores the most carbon?
Histosols
71
Why are SOC stocks highest in higher latitudes even though above ground carbon in vegetation is small?
Because decomposition processes in cold regions is slow which is why SOC accumulates
Warming temps may lead to oxidation of huge amount of carbon stored in higher latitudes
72
What happens to C sequestration when there are changes in SOM?
Most natural ecosystems appear to be in equilibrium with SOM content: input=output
Rates of C sequestration of initially high as ecosystem develops, then fall to low values as ecosystem reaches equilibrium
Example: tropics are very old and very large C sequestration with a lot go vegetation, but also a lot of leaching of DOC and a lot of decomposition
The exception to this pattern: wetlands
- decay is OM is slowed by anaerobic and cold conditions
- most northern peatlands have accumulated OM and appear to be continuing
- therefore, peatlands are major terrestrial sink of CO2 since deglaciation 10,000 yeras ago
73
How can be increase C sequestration in soils?
Increase rate of input of OM into soil
- change land use to higher NPP
- leave plant material on soil surface
- convert cropland to pasture and pasture to forest
- change tillage practices
Reduce rate of decomp. in soil or increase ability of soil to adsorb and retain SOM
C sequestration greatest where NPP enters soil from roots and where soil either cold, anaerobic or clay-rich, which slows down rate of decomp.
73
How can be increase C sequestration in soils?
Increase rate of input of OM into soil
- change land use to higher NPP
- leave plant material on soil surface
- convert cropland to pasture and pasture to forest
- change tillage practices
Reduce rate of decomp. in soil or increase ability of soil to adsorb and retain SOM
C sequestration greatest where NPP enters soil from roots and where soil either cold, anaerobic or clay-rich, which slows down rate of decomp.
74
What factors are important to assess how global warming will impact vegetation?
Depends on effect on NPP of vegetation
- change in temp.
- water availability
- elevated atmospheric CO2
- changes in soil nutrient availability (especially N and P)
Redistribution of vegetation in response to climate change (migration from south towards North?)
75
What are the inputs of OM in soils?
SOM = NPP - decomp. - leaching
Litter (plant debris and metabolites, animals carcasses and excrements)
Leachates (throughfall and stemflow containing nutrient ions and soluble organic compounds)
Soil organisms
Roots (rhizosphere detritus and secretions)
76
What happens to the water table during the summer?
Water table will lower since there is less rain, less transpiration from vegetation, more evaporation
Soil profile becomes more aerobic --> increase in carbon emissions
77
What happens to the water table during the winter?
It will rise and a larger amount of the soil profile will become anaerobic
78
What are methane fluxes controlled by?
Soils can be a source of methane under reducing conditions: function of redox potential (Eh)
Methane flux to atmosphere from wetland soils controlled by microbial activity (methanogenesis)
Depends on:
- temp.
- water table position (aerobic vs. anaerobic portion of the soil profile)
- plant cover (some plants act as conduit of methane to atmosphere)
79
What are some mechanisms leading to methane emissions from wetlands?
Methanogenesis converts organic carbon from roots and plant tissues into methane in the soil that can be oxidized, diffused, transported out by vascular transport, or bubbled out
80
What is ebullition of methane?
Organic compounds are reduced into methane which can bubble out into the atmosphere
81
How can methane be consumed?
Can be consumed by methotrophic bacteria in well-drained soils
Rates are small compared to methane emissions from wetlands, but upland soils cover large areas
HOWEVER, consumption rates are reduced by increased N availability in soils (by atmospheric N deposition from fertilizers or combustion of fossil fuels)
- because microbes no longer have a competitive advantage over other organisms when N (limiting nutrient) becomes available
82
What are some sinks of methane?
Tropospheric OH
Stratosphere
Soils (well-drained)
83
What are some sources of methane?
Wetlands, lakes and freshwater, termites, wildfires, biomass burning, landfills, fossil fuels, geological, ruminents, paddy fields
There are more sources than sinks --> increase in methane in the atmosphere
84
What have humans done to increase methane over the years?
Creation of paddy soils and flooded areas (due to hydro-electric dams)
Decreased C sink in upland soils because of increased N deposition
85
What have humans done to decrease methane over the years?
Drainge of wetlands soils --> changes from a source to a potential sink
However, this increases CO2 emissions
86
What is the effect of climate change on methane?
Methane consumption has very limited dependence on temp. so unlikely to change
Methane production is very temp. dependent so will increase methane flux
- warming will lead to permafrost melting, waterlogging areas and icnreasing methane emission from subarcitc and polar wetlands
However, primary control is water table position: may be lower --> reducing emission rates
87
Where is permafrost found?
Mostly located in subarctic and arctic
Also found in rocky mountains
88
How are abandoned oil/gas wells and pipes contributed to methane fluxes?
Abandoned pipe becomes a conduit for deep soil methane to be released into the envr.
89
What is the most potent GHG?
Nitrous oxide
90
How are N gases released during nitrification and denitrification in soils?
Under anaerobic conditions, N becomes electron acceptor and is reduces to N2 or creates N2O
Therefore, the application of N-rich fertilizers can lead to oxidation and release of N2O and NO
- disturbance could be reduced by more efficient application of N fertilizer
Riparian zones (zones where snowmelt and agricultural drainage ends up) --> hotspots for N2O emissions due to high denitrification
(REVIEW N-CYCLE)
91
What are N2O emissions from soils controlled by?
Low O2 contents associated with high water contents
- much more present during denitrification
High temps
Rapid N cycling
Disturbance, through annual crops, forest clearance
Addition of N fertilizers
92
What soils are the biggest sources of N2O?
Fertilized soils>tropical forest soils=oceans,estuaries,wetlands
93
Why is nitrous oxide difficult to monitor?
Fluxes from natural systems are very episodic (compared to CH4 or CO2)
No real evidence that fluxes will change greatly in response to climate change
94
What is the role of soils in controlling atmospheric concentrations of CO2, CH4 and N2O?
CO2: through decomp. of OM and stabilization of SOC
CH4: through methanogenesis under anaerobic conditions and CH4 consumption under aerobic conditions
N2O: through leakage of N cycle
95
How have human activities contributed to increased concentrations of CO2, CH4 and N2O?
CO2:
through change in land use converting forests and grasslands to cultivation
- reducing OM content
- drained wetland soils, respired as CO2
CH4:
- through increased flux from rice paddies and other flooded soils
- reduced CH4 consumption in N-affected soils
- reduced emission from drained wetland soils
N2O:
- through increased leakage of N cycle
- from N fertilizer addition and increased disturbance
96
What happens to CO2 and CH4 concentrations when a wetland is drained?
Increase CO2 and decrease in CH4
Overall, we still contribute more to GHG effect by draining a wetland
97
How is climate change going to affect CO2, CH4 and N2O emissions from soils?
CO2:
- through balance of plant production and decomposition of OM
- A lot of uncertainty --> may lead to positive feedback (accelerate warming)
CH4:
- through change in water table and wetland area
- decrease in temperature and tropical regions but increase in permafrost terrain
N2O:
- unknown effect, likely small
98
