Land – Weathering, Soils and the P cycle

AerosolsPrecipitation

Transformations in soilOrganic matter decomposition

(produces gases, soluble organics, etc)

Mineral weathering(produces secondary minerals (clays),

soluble ions, bicarbonate ion

Water and soluble ion

loss

CO2, NO, N2O

)Internal verticaltransfers

(form horizons)

Gas losses

Deposition Erosion

)Plant uptake, Dead plant material

Rock-derived elements Atmospherically derived elements

• Calcium (Ca), Magnesium (Mg),Potassium (K), Phosphorus (P)

• Important constituents of minerals

• Enter through:- Chemical weathering ( partial

or complete dissolution of these minerals)

• Carbon (C) & Nitrogen (N)

• Important gas phase

• Enter through: - Biological processes:

(photosynthesis, N2 fixation)- Deposition from the atmosphere

(dissolved in precipitation or by dry deposition of particles and gasses)

LOST WITHOUT REPLACEMENT REPLENISHED CONTINUOUSLY

Chadwick et al.

Atmospherically derived: Chemistry of rainfallION Marine source Land Source Pollution Source

Na+ (sodium ion) Sea salt Soil dust Biomass burning

Mg++ (magnesium) Sea salt Soil dust Biomass burning

K+ (potassium) Sea salt Soil dust, Biogenic aerosol

Biomass burning, fertilizer

Ca++ (calcium) Sea salt Soil dust, Biogenic aerosol

Biomass burning, fertilizer

H+ (hydrogen ion) Gas reactions Gas reactions Fuel burning

Cl- (chloride ) Sea salt - Biomass burning

SO42- (sulfate) Sea salt,

BiogenicGases

Biogenicgases(H2S, DMS)

Volcanoes, dust

Biomass burning

NO3- (nitrate) Lightning (NO) NOx from microbial processes

(nitrification, denitrificaiton)Fuel burning

Fertilizer , Biomass burning

NH4+ (ammonium) Biological

activity (NH3)Biogenic gases (NH3) + and gases from

microbial decay processesFertilizers

Human/animal waste decomposition

PO43- (phosphate) Soil dust Biomass burning

Fe, Al, SiO2 Soil dust Increased dust sources

HCO3- and CO3

2- CO2 CO2

Why is rain water naturally acidic?

CO2 (g)

CO2 (aq) + H2O « H2CO3 *(aq)

CO2 (aq) = 3.38x10-2 (mol/L*atm-1)* 0.000339 atm

= 1.146*10-5 (mol/L)

air

water

CO2 (g) = 0.035%. Vapor pressure of water: PH2O =0.031 atm

Partial pressure of CO2 (g) in air PCO2:PCO2 = (1 atm - 0.031 atm) x 0.035% = 0.000339 atm.

Henry’s law solubility constant

now 0.000397 atm

Now 1.342*10-5 (mol/L)

è now .041% (410 ppm)

H2CO3 ó H+ + HCO3- ó 2H+ + CO3

2-

0

0.2

0.4

0.6

0.8

1

4 5 6 7 8 9 10 11 12

pH

Frac

tion

as d

esig

nate

d sp

ecie

s

CO2 HCO3- CO32-

Carbonic acid Bicarbonate ion Carbonate ion

Seawater pH ~8

pKa1 = 6.35 pKa2 = 10.33

H2O + CO2

rain

pH of rain, continuedH2CO3 + H2O <=> HCO3

- + H+

Ka1 = [H+][HCO3-]/[H2CO3] Ka1 = 4.45x10-7

For electroneutrality, we know:[H+] ~ [HCO3

-] (at low pH we can ignore [CO32-])

then [H+]2 ~ (Ka1x [H2CO3])1/2

={(4.45x10-7) x (1.146*10-5) }1/2

= 2.25 x 10-6 (M)

pH = -log [H+] = -log (2.25 x 10-6) = 5.65

now 2.44 x 10-6 (M)

now 5.61

What causes ‘acid rain’? Addition of strong acids (sulfuric and nitric)

Atmosphere and water meet rockWeathering

• Unloading/exfoliation• Ice freeze/thaw• Fauna –

burial/movement of soil by animals and plants (root cracking of rocks)

Physical weathering

Transport-Limited Slopes: Rates of weathering are more rapid that rates of transport

Weathering-Limited Slopes: Rates of soil/regolith production are less that rates of erosion

Exfoliating granite – Joshua Tree; weathering limited

Transport limited – thick soils, chemical weathering dominant

Chemical weathering (dissolving rocks)

Rocks + H2O + CO2 (carbonic acid) èsecondary minerals + dissolved ions + HCO3

-

(bicarbonate ion) + amorphous silica

Removes CO2 from the atmosphere and puts it in soluble form (where it can be transported eventually to the ocean)

Releases “base” cations that provide necessary nutrients for life.

Weathering happens because primary minerals in rocks are thermodynamically unstable compared to secondary products

carbonic acid carbonates CaCO3 + H2CO3 = Ca++ + 2HCO3

-

Congruent dissolution

Left behind:“regolith” = accumulation of

fine rock material“soil” = regolith plus organic

matter - often vertically stratified

O-Layer:Organic Debris

C-Layer:Actively WeatheringRock (“Saprolite”)

FreshRock

1) resistant minerals (eg, quartz) don’t dissolve

2) aluminosilicates alter to clays

3) soluble elements removed in waters

4) some precipitate lower in soil or in sediments (Fe-oxides, carbonate)

Soil Development is a result of long interactions of water with rock

A-Layer:OrganicsIon-Depleted ClaysResistant Minerals

B-Layers:Primary &Secondary Minerals

Soils as‘bioreactors’

Chemical transformations fueled by energy from (1) decomposing

nonliving organic matter (NOM)

(1) thermodynamicinstability of minerals at the earth’s surface

Properties vary with rock, plants, climate, time

Photos M. Schrumpf

Primary mineral + CO2 + H2O è secondary mineral + ions (Na, Ca) + bicarbonate + amorphous silica

Exactly which secondary mineral is formed depends on temperature, moisture, composition of primary mineral

Plagioclase è smectite under low leaching conditions

Plagioclase è kaolinite under high leaching conditions

Final weathering products – Al and Fe oxyhydroxides (no more Si) – e.g. in

tropical soils

In the Al-octaheron, one Al (+3) is surrounded by 6 O (-2) for a net negative charge of –3. Surface has permanent negative charge

CaSi

Na

Mg K

Ca

NaMg

K

SiCa

Fe

Al

KMg

Na

Rivers

Rain

Rocks

River chemistryDifferent from rain - increase in Si; much more concentrated (>5 times more total ions)

Must be explained by input from weathering to rivers

Rivers also very different from rocks (e.g. extremely low Al, Fe)

Must be explained by incongruent weathering processes (secondary minerals retain these elements in soil)

average stream chemistry versus precipitation

Surface Charge and Cation/Anion Exchange capacity

Kaolinite 1:1Smectite 2:1Two silica tetrahedral sheetsOne aluminum octahedral sheet

One silica tetrahedral sheetOne aluminum octahedral sheet

The Oxygens (blue) on the outside have permanent negative charge; these are what are partly responsible for holding onto cations in soils (edges of broken crystals). 2:1 clays have greater negative charge density than 1:1 clays.

Clay mineral surfaces – Permanent negative charge

Al3+> H+ >Ca2+ > Mg2+ >K+ > NH4+ > Na+Strongly

sorbedWeakly sorbed

Soil buffering capacity

• Buffer – if the pH of the soil solution is acid, a buffer will tend to keep the pH of soil solution the same.

• This is accomplished in soils by exchanging positively charged ‘base cations’ like Ca+2

that are sorbed to mineral surfaces for H+ ions in solution.

• The degree to which a soil contains this exchangeable cation complex is expressed as its ‘cation exchange capacity’ (CEC)

Figure 9.3 Soils become acid when H+ ions added to the soil solution exchange with nonacid Ca2+, Mg2+, K+, and Na+ ions held on humus and clay colloids. The nonacid cations can then be exported in leaching water along with accompanying anions. As a result, the exchange complex (and therefore also the soil solution) becomes increasingly dominated by acid cations (H+ and Al3+). Because of this sequence of events, H+ ion–producing processes acidify soils in humid regions where leaching is extensive, but cause little long-term soil acidification in arid regions where the Ca2+, Mg2+, K+, and Na+ are mostly not removed by leaching. In the latter case, the Ca2+, Mg2+, K+, and Na+ remain in the soil and re-exchange with the acid cations, preventing a drop in pH level.

In general, cations are held and displace one another in the sequence:Al3+ > H+ > Ca2+ > Mg2+ > K+ ~= NH4

+ > Na+

Assuming that they have an equal molar abundanceBuffering also depends on the size of the exchangeable cation complex

Other surfaces –oxy/hydroxides and organic matter

oxy/hydroxides; charge depends on pH – very important for tropical soils that are base cation poor; tropical soils can adsorb anions under the right pH conditions.

Organic matter-carboxyl (COO-) or alcohol (O-) groups-Can be responsible for a lot of CEC in soils with slow organic matter decomposition rates-- also pH dependent; under acid conditions COOH and limited CEC

PO42-> SO4

2- >Cl- > NO3-Strongly

sorbedWeakly sorbed

Increase pH of tropical soils to release P

If we start adding H+ to soil, the pH would drop slowly – this is the degree of buffering or acid neutralizing capacity of the soil

Dissolve carbonates

Phosphorous cycling on land – largest fluxes are recycling between dead organic matter and vegetation

Bound P is associated withFe, Al (as iron/aluminum phospates) or adsorbed onto clay surfaces

Optimal pH for P availability: pH 6-6.5higher pH ® precepitation as Ca-phosphatelower pH ® more sorption to Fe oxides

(more positve charge), precepitiation as Al / Fe-phospate

Coupling P and Fe cycles

Colloidal Fe(III)oxyhydroxides scavenge PReductive dissolution of Fe(OH)3 releases P (Al-oxyhdroxides also scavenge P but have no redox chemistry)

Walker and Syers model of evolution of the forms of P in soils over time. ‘nonoccluded’ P is sorbed to the surfaces of Al, Fe oxides/hydroxides and carbonates (extractable P)‘occluded’ P is in the matrix of secondary minerals (more difficult to extract)

Progressive dissolution of mineral P (apatite)

Over time,P is lost by leaching

Ecosystems on young soils tend to be N limited

Ecosystems on young soils tend to be P limited

“occluded” = bound by Al, Fe oxides e..g. FePO4“non-occluded” = held on surface (often anion adsorption in tropical soil)

Soil P forms vary over time

apatite

Parent material = basaltFlat sites (little erosion)Same rainfall, temperatureSame vegetation (though properties of the trees vary)

Chronosequence

Chadwick et al.

Old soils are low in both cations and P

% = total mass of an element in soil profile/ total mass of that element in the amount of lava that produced that profile

Assumes 100% retention of an ‘immobile’ element like titanium

Ecological changes and substrate age

Soils: High concentration of cations (Ca, Mg, K) & little N and P in youngest sites, which reverses with age.

Net Primary Production (NPP) & limitations:~pattern similar to N & P availability ~ N limiting in youngest site; P limiting in oldest site

http://en.wikipedia.org/wiki/File:Metrosideros_polymorpha.jpg

Chadwick et al.

Chadwick et al. 1999

87Sr produced by radioactive decay of 87Rb, which is enriched in continental rocks and depleted in mantle rocks.

Basalt has low 87Sr/86Sr, Asian dust/seawater have high 87Sr/86Sr

Young soils get their Sr from basalt weathering, old soils from dust/sea salt

basalt

Dust orocean

37

Baitaille and Bowen, 2012

150 ky Climosequence

Stewart et al., 2001

Time is not the only factorImportance of dust inputs with rainfall at constant age

Rainfall/Pore space <1Weathering products remain in soil

Rainfall/Pore space >1Weathering products leached from soil

Chadwick et al.

Oldest sites are also using dust-derived P

• Transportation of dust through the troposphere from Asia

• Refractory trace elements and isotopes of Neodymium (Nd)

• Using 143Nd/144Nd, Eu/Eu*, Hf/Th =>

0.9 ± 0.3 mgP/m2yr input value of P added by dust

Older surfaces are more eroded

http://geopubs.wr.usgs.gov/open-file/of00-124/images/Fig2KauaiShadedRelief.jpg

Where we would sample our chronosequence

But more of the land area is like this

Extrapolating to Landscapes – ‘rejuvination’ of soils by erosion

Increased basalt Sr

Increased atmospheric Sr

S. Porder et al. PNAS 2005

Okin et al. GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 18, GB2005, doi:10.1029/2003GB002145, 2004

The Amazon forests might be less P limited than they would be without Saharan dust fluxes

Dust transport of Phosphorous –important for fertility in old, highly weathered soils?

Human mining of P

Guano mine -Peru

Whenever any citizen of the United States discovers a deposit of guano on any island, rock, key, not within the lawful jurisdiction of any other government, and not occupied by the citizens of any other government, and takes peaceable possession thereof, and occupies the same, such island, rock, or key may, at the discretion of the President, be considered as appertaining to the United States. - Guano Island Act of 1856

P as a limiting nutrient in old soils

Control Dry

Control Wet

BurnedDry

BurnedWet

SoyDry

SoyWet

Averages binned per hour

See poster in this session….

Low Rnet

Low ET

High Rnet

HIgh ET

P balance

~50% harvest~ 50% remains in soil(why?)

Current questions in P cycles

Links between Fe and P cycles; redox conditions play a large role in P-bioavailability in soils and sediments

Light-oxidative pathways for recycling of Fe and P in lakes and seawater (reduction of iron without anoxia)

Role of dissolved organic phosporous (can organisms take this up directly?) Impact on ‘Redfield ratio’ based analyses.

What is the nature of organic P in sediments/soils? Why – given that the source is plant/animal matter and the need of organisms for P – is there any left at all to accumulate in sediments?