Pedogenesis

Question 1

Soil Genesis: Pedogenesis

Factors of Soil Formation (Hans Jenny 1941)

Soil f (Cl, O, R, P, T,…)

Answer 1

Cl 	Climate
O 	Organisms
R 	Relief
P 	Parent material
T 	Time
…. 	Stochastic factors e.g. fire, flood, humans

Question 2

High rainfall increases soil acidity through element leaching

Answer 2

Biomes and soil types show strongly linked distributions associated with climate.

Question 3

There are 12 soil orders

Answer 3

Entisols: Little, if any horizon development
Aridisols: Soils located in arid climates
Alfisols: Deciduous forest soils
Ultisols: Extensively weathered soils
Gelisols: Soils containing permafrost
Andisols: Soil formed in volcanic material
Inceptisols: Beginning of horizon development
Mollisols: Soft, grassland soils
Spodosols: Acidic, coniferous forest soils
Oxisols: Extremely weathered, tropical soils
Histosols: Soils formed in organic material
Vertisols: Shrinking and swelling clay soils

Question 4

Plants and associated microbiota can alter soil - making it more or less favourable for their own growth.

Answer 4

Plant-driven effects on soil can drive ecosystem changes with time- driving succession towards ‘climax communities’. As plants evolved over geological time their interactions with soil have transformed the Earth’s surface, and changed biogeochemical cycles of elements into the oceans and atmosphere.

Question 5

Our hypothesis: As land plants evolved they increased in size and structural complexity, required more nutrients and water, and invested more photosynthate into supporting their mycorrhizal fungal partners-intensifying soil formation and driving biogeochemical cycles.

Answer 5

First ‘trees’
~385 Ma

First roots & vascular system ~ 407 Ma

First roots & vascular system ~ 407 Ma

Question 6

Effect of plants on soil formation

The scale of the effects

Nanoscale molecular interactions 
Mycorrhizal fungus-root-soil interface
Grain scale
Individual plants/ soil profile
Ecosystem
Soil life-cycle 
Globally, contemporarily* 
Globally, geologically*

Answer 6

*Both global biome / soil type relationships, evolutionary and geological time effects and possible deployment of rock for enhanced weathering.

Question 7

The life-cycle of soils

Answer 7

Weathering of parent material and accumulation of mineral grains

Vegetation accelerates weathering and traps particles

Soil horizons

Mature profile of fine
material enriched in
organic matter, clay
and sesquioxides

Soil erosion: loss of organic matter, clay, and sesquioxides

Vegetation lost: cultivation, landslide, fire, ice-age etc.

Bare rock and broken rock.

Question 8

Water + Rocks + Plants + Time

Answer 8

Gives organic matter, clays and sequioxides (Fe and Al hydrous oxides)

Question 9

How plants drive soil development-
evidence from chronosequences
(LOOK UP Crocker & Major, 1955).

Answer 9

Retreating glacier at Glacier Bay, Alaska- leaving moraines of known age- ground up rock fragments of the same kinds eroded from the surrounding mountains

Question 10

Soil development can be different under different species.

Answer 10

Leaching of base cations is greater under conifers than many broadleaved trees - the latter having higher rates of Ca and Mg uptake and recycling back into the surface soil.

Question 11

Apatite (a form of calcium phosphate found in basalt and other rocks) is the primary source of P in most ecosystems and P fertilizer.

Answer 11

Stones
provide
nutrients…
for a time

Question 12

Plants promote chemical weathering on silicate rocks to a greater extent than microbial communities alone.

Answer 12

(Cochran & Berner 1996)

Question 13

(Lal, 2008) (doesn’t include carbonate in rocks)

Answer 13

global organic and inorganic C pools, oceanic and pedologic

Question 14

Plants convert solar energy into chemical energy that can accelerate rates of mineral weathering. As land plants evolved over 470 Ma from root-less liverworts to deep-rooted trees has their effect on soil formation and weathering increased?

Answer 14

Enhanced weathering by increasing:
plant productivity
rooting depth
nutrient demand
photosynthate allocation to mycorrhiza
mycorrhiza hyphal lengths

Question 15

The rise of land plants that form biogenic silica from Si in the soil is thought to have increased the soluble Si flux to the oceans, and the increasing importance of diatoms – which now contribute nearly half the ocean primary production (26 Gt C y-1).

Answer 15

Earth’s long-term silica cycle is intimately linked to weathering rates and biogenic uptake

Question 16

To what extent has mycorrhiza co-evolution with land plants enhanced mineral weathering and altered global biogeochemical cycles and atmospheric chemistry?.

Answer 16

Atmospheric CO2 fell by 90% coincident with the rise of land plants with increasingly deep roots (Algeo & Scheckler 1998)

Question 17

Why did land plants form symbioses with soil fungi?

Weathering of the calcium phosphate apatite provides > 95% of the primary P source in soils
Soil solution [P ] - typically 100-1000 x less than the other major plant and fungal essential elements (Marschner, 1995).
The C cost per unit absorptive area is up to 100 times less for mycorrhizal hyphae than roots (Read 1990).

Answer 17

Stoichiometry of element abundance in the Earth’s crust and in plants
Crustal abundance   
Si = 293 x P 
Al = 90 x P   
Fe = 27 x P   
Ca = 27 x P

Question 18

The mycorrhizosphere- effect of root and mycorrhizal fungal interactions with soil and biota

Answer 18

Carbon flows
Interactions with bacteria etc.
Uptake and transport of weathering products

Question 19

Minerals buried under Marchantia paleacea liverworts grown with and without glomeralean AM fungal symbionts under 200, 450 and 1200 ppm CO2 atmospheres for 1 year

Answer 19

The evolution of deep-rooted trees increased biotic weathering rates by more than an order of magnitude compared to liverworts

Question 20

Remineralization is the utilization of natural broad elemental spectrum rock dust
materials for the purpose of renewing the mineral content of soils through weathering.

Answer 20

Intensively weathered tropical soils like oxisols and ultisols are acidic, depleted of Ca, Mg and other base cations and silica and rich in residual iron and aluminium oxides –giving bright red colours and strong binding of the tiny remaining P reserves.

Question 21

Sugar cane- the main feedstock for bioethanol in Brazil

- 23.4 billion liters @ 61% less greenhouse gas emissions than oil in 2014

Answer 21

is mainly grown on oxisols and in the Cerrado using large amounts of agricultural lime to reduce soil acidity

Question 22

Taylor et al 2015

Answer 22

Enhanced weathering strategies for stabilising climate and averting ocean acidification

Question 23

Australia: highly weathered oxisol amended with ground basalt
74% of the > 1200 mm rainfall zone is strongly acid, and the remainder is medium acid or acid. Sugar growing areas occur in this zone and requires regular liming or other alkaline inputs for long term sustainability

Answer 23

Basalt increases available P 
Up to 8.7 fold increase
Basalt reduces P sorption
Basalt reduces soil acidity
Gillman et al., 2002 Applied Geochemistry 17: 987-1001

Question 24

DeVilliers (1961) Int. Sugar J. 63, 363-364.

Answer 24

29% increase
Sugarcane yield over
5 successive crops with basalt additions (1952)

Question 25

Calcium silicate rock dust application to soil pathogen and insect attacks on sugar cane to a greater extent than adding lime or limerstone

Answer 25

Keeping et al., (2014), Frontiers in Plant Science doi: 10.3389/fpls.2014.00289

Question 26

Basalt rock dust enhances the formation of plant hydrous amorphous silica – phytoliths- involved in plant resistance to herbivores and biogeochemical Si cycle.

Basalt powder effect on rice
Guo et al., 2015 Sci. Bull. 60: 591-597

Answer 26

Do phytoliths sequester and stabilize carbon in soils?
Carbon in crop phytoliths suggests that 26 ± 10 Tg CO2 sequestered per year (Anthropogenic emissions were 35,900 Tg CO2 in 2014).
Evidence that phytoliths trap ‘old’ soil carbon that is already stable, and shows widely used phytolith-C measurements are flawed.

Question 27

Note that forest soils are much richer in silica than grasslands or arable land growing cereals- forest soils are “clay mineral factories” but play a limited role in formation of biogenic silica.
Croplands and grasslands generate biogenic silica but deplete clays.

Answer 27

Potential
co-benefits of enhanced weathering from adding basalt to soil in agri-ecosystems
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