Pedogenesis Flashcards
Soil Genesis: Pedogenesis
Factors of Soil Formation (Hans Jenny 1941)
Soil f (Cl, O, R, P, T,…)
Cl Climate O Organisms R Relief P Parent material T Time …. Stochastic factors e.g. fire, flood, humans
High rainfall increases soil acidity through element leaching
Biomes and soil types show strongly linked distributions associated with climate.
There are 12 soil orders
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
Plants and associated microbiota can alter soil - making it more or less favourable for their own growth.
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.
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.
First ‘trees’
~385 Ma
First roots & vascular system ~ 407 Ma
First roots & vascular system ~ 407 Ma
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*
*Both global biome / soil type relationships, evolutionary and geological time effects and possible deployment of rock for enhanced weathering.
The life-cycle of soils
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.
Water + Rocks + Plants + Time
Gives organic matter, clays and sequioxides (Fe and Al hydrous oxides)
How plants drive soil development-
evidence from chronosequences
(LOOK UP Crocker & Major, 1955).
Retreating glacier at Glacier Bay, Alaska- leaving moraines of known age- ground up rock fragments of the same kinds eroded from the surrounding mountains
Soil development can be different under different species.
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.
Apatite (a form of calcium phosphate found in basalt and other rocks) is the primary source of P in most ecosystems and P fertilizer.
Stones
provide
nutrients…
for a time
Plants promote chemical weathering on silicate rocks to a greater extent than microbial communities alone.
(Cochran & Berner 1996)
(Lal, 2008) (doesn’t include carbonate in rocks)
global organic and inorganic C pools, oceanic and pedologic
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?
Enhanced weathering by increasing: plant productivity rooting depth nutrient demand photosynthate allocation to mycorrhiza mycorrhiza hyphal lengths
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).
Earth’s long-term silica cycle is intimately linked to weathering rates and biogenic uptake
To what extent has mycorrhiza co-evolution with land plants enhanced mineral weathering and altered global biogeochemical cycles and atmospheric chemistry?.
Atmospheric CO2 fell by 90% coincident with the rise of land plants with increasingly deep roots (Algeo & Scheckler 1998)
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).
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
The mycorrhizosphere- effect of root and mycorrhizal fungal interactions with soil and biota
Carbon flows
Interactions with bacteria etc.
Uptake and transport of weathering products
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
The evolution of deep-rooted trees increased biotic weathering rates by more than an order of magnitude compared to liverworts
Remineralization is the utilization of natural broad elemental spectrum rock dust
materials for the purpose of renewing the mineral content of soils through weathering.
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.
Sugar cane- the main feedstock for bioethanol in Brazil
- 23.4 billion liters @ 61% less greenhouse gas emissions than oil in 2014
is mainly grown on oxisols and in the Cerrado using large amounts of agricultural lime to reduce soil acidity
Taylor et al 2015
Enhanced weathering strategies for stabilising climate and averting ocean acidification
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
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
DeVilliers (1961) Int. Sugar J. 63, 363-364.
29% increase
Sugarcane yield over
5 successive crops with basalt additions (1952)
Calcium silicate rock dust application to soil pathogen and insect attacks on sugar cane to a greater extent than adding lime or limerstone
Keeping et al., (2014), Frontiers in Plant Science doi: 10.3389/fpls.2014.00289
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
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.
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.
Potential
co-benefits of enhanced weathering from adding basalt to soil in agri-ecosystems
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