Nutrient Cycling in Soils:The Big 3 and Carbon

Gurpal ToorDepartment of Environmental Science & Technology

University of Maryland–College Park

Fundamentals of Nutrient Management

@ToorUMD

Nutrients: Plant food; essential for crop production

P deficiency in Corn

N deficiency in Corn

K deficiency in Corn

Images from Better Crops (1997), Vol. 81. No. 3

Nitrogen: NPhosphorus: PPotassium: K

What is Nutrient Cycling: Use, movement, and recycling of nutrients

Image Credit: USGS

Carbon Cycle: Plants use (photosynthesis) carbon

dioxide (CO2) to grow. When plants decompose, CO2 is returned to atmosphere via respiration.

Nitrogen

Most complicated & fascinating Many reactions mediated by soil organisms

Wonderful oxidation and reduction reactions

N has gaseous forms

Atmosphere contains 78% N gas

Hu

man

s (b

illio

ns)

Hab

er-B

osc

h (

Tera

gram

of

N)

1 Tera Gram = 1 billion kg or 2.54 billion pounds

Image Credit: Galloway and Cowling (2002). Ambio.

Nitrogen Cycle

Inputs:

Crop & animal residues

Biosolids

Compost

Fertilizers

N fixation

Nitrogen Cycle

N Fixation Mechanisms

Natural fixation

• Atmospheric additions

– electrical discharge of lightning

• Biological processes

– symbiotic – for example between Rhizobiumbacteria and leguminous plants

– non-symbiotic – carried out by free living bacteria and blue-green algae

N2 Gas

Photo credits: Harold Evans, OSU

Soybean Nodules

Biological N Fixation

Calculating Legume Credits

Crop lbs N/Ac

Perennial Crops

Alfalfa 100 to 150

Red Clover 40

Winter Annual Crops

Hairy Vetch 75 to 150

Crimson Clover 50 to 100

Summer Annual Crops

Lespedeza 20

Soybeans 15 to 40

Photo credit: Ray Weil

Synthetic or industrial processes

Industrial fixation – for fertilizer manufacturing

Haber-Bosch process: N2 + 3H2 2NH3 (g)

High temperature fixation – internal combustion engines

N Fixation Mechanisms

Transformations of Organic N

NH4+

NO3-

NO2-

2-5% of total soil N

ProteinsAmino sugarsAmino acids

Mineralization (a transformation)

Conversion of organic N to inorganic N,

ammonium (NH4+)

Mediated by bacteria and fungi

Immobilization (a transformation)

Conversion of inorganic N (NH4+ & NO3

-) to organic N Soil organisms assimilate nutrients into biomass

Soil organisms are extremely numerous and very well distributed in the soil Outcompete plants for available nutrients under

many circumstances

Nitrification (a transformation)

Biological transformation of ammonium (NH4

+) to nitrate (NO3- )

Aerobic conditions and moderate pH suppressed below pH 5.5

Soil bacteria Nitrosomonas and Nitrobacter carry out nitrification

Nitrification: A Two-Step Process

Nitrosomonas

2 NH4+ + 3 O2 2 NO2

- + 2 H2O + 4 H+ + eAmmonium Nitrite Hydrogen

Nitrobacter

2 NO2- + O2 2 NO3

- + e

Nitrite Nitrate

Soil N Cycle: A Biological Phenomenon

Influenced by: pH: most bacteria suppressed at low pH

Temperature: is the key

Moisture: dictates oxidation or reduction

oxygen status (aeration)

Effect of Temperature on N Cycling:

Water freezes

“Biological” zero

Optimum

for

nitrification

Optimum

for

mineralization

Most

microbes

killed

Optimum for growth of corn, cotton, potatoes

Effect of Moisture & Oxygen Status on N Cycling:

Fate of N in Organic Additions

C:N ratio is the amount of C relative to the amount of N in a given material

A high C:N ratio (>25:1) N immobilized

A low C:N ratio (<20:1) N mineralized

a Cornell On-Farm Composting Handbook, Rynk et al, 1992b The Nature and Properties of Soils, Brady and Weil, 1999

Changes in NO3- levels when organic additions have wide C/N

Adapted from F.J. Stevenson, 1986

Managing C:N Ratio of Inputs

Manage cover crops Incorporate cover crops while in vegetative

state

Leave mature cover crops on surface

Monitor soil N and plant growth when incorporating straw, sawdust, and other high C:N materials

Plant-available N in soil solution

Ammonium (NH4+)

Nitrate (NO3-)

Small organic molecules present in dissolved organic N (DON) pool

Days/wks

Weeks/months

Years

Decades

Nitrate leaching

(J.J. Meisinger, USDA)

3. Denitrification:

waterlogged soils

1. Volatilization:

hot, dry weather

2. Erosion/runoff:

slopes; heavy rain

5. Leaching:

soil texture; rain amount

Losses:

4. Crop uptake

1. Leaching (a liquid loss)

Primarily as Nitrate (NO3-)

Moves freely in soil profile• Transported by drainage water, especially important in

sandy soils

can lead to pollution of groundwater

An economic loss with environmental consequences

Nitrate leaching

Minimal under natural vegetation (forests)

Greater under modern row-crop productions

A risk if N management is sub-optimal

Reasons for Excessive NO3- Leaching

Inefficient N management

Heavy one-time applications

Improper timing

Over-application of manure/sludge

Enhanced by periods of heavy rainfall

Enhancing Synchrony: Minimize Vulnerable Nitrogen

If the N is not yet applied, it can not be lost

Importance of split applications (Penn State Agronomy 12)

2. Ammonia Volatilization (a gaseous loss)

Loss of ammonia-N to the atmosphere

Ammonium in the presence of hydroxyl (OH-) can produce ammonia gas

NH4+ + OH-

H2O + NH3

Affects all surface-applied N sources−Urea, ammonium nitrate, manure

Enhanced by warm, dry atmospheric conditions

Diurnal variations

(Pote & Meisinger, 2014. J. Soil & Water Cons.)

Managing Ammonia Losses

Know where and when ammonia loss occursFirst day of application

Sunny, warm, low humidity, breezy conditions

Incorporation is required under many circumstances in Maryland

Spread and incorporate manures in the early morning or evening (when dew still present)

3. Denitrification (a gaseous loss)

NO3- N2O N2

nitrate ions nitrous oxide gas dinitrogen gas

Biological reduction of nitrate to N2 gas

NO3- transformed to gaseous compounds

Favored by saturation of the soil (anaerobic

conditions)

A Visual on Denitrification in Field

Photo credit: www. Agleader.com

Principles of N Management

Maintain soil pH appropriate for crop

Reduce runoff/erosion losses

Apply N fertilizers and manure when plants need it, where they need it

Timely incorporation of manures and sewage sludge when practical

Use cover crops to scavenge residual N

Practice Question #1

In which form is nitrogen most likely to be lost in the greatest quantity from soils in Maryland?

a) dinitrogen gas (N2)

b) nitrate (NO3- )

c) ammonium (NH4+)

d) organic N

Phosphorus (P) Cycle

Simpler cycle than N No gaseous

forms

Soluble P can be “fixed” to less available forms

Image Credit: Jaisi and Blake (2014). Advances in Agronomy

Transformation of Organic P

P mineralization and immobilization

occur simultaneously in the soil

ORGANIC PHOSPHORUS

POOL

INORGANIC PHOSPHORUS

POOL

HPO4-2

H2PO4-

Plant-available P

Orthophosphate (water-soluble phosphate) H2PO4

- at pH 5 to 7.2 HPO4

2- at pH > 7.2

Phosphate moves slowly in soil by diffusion

Image Credits: Utah State and University of Minnesota

P Fixation (a transformation)

A set of processes through which P is converted to less available formsPrecipitation

Adsorption

P fixation

Adsorption or Sorption

In acid soils, P is adsorbed to surfaces of Al/Fe oxides and clay minerals

In neutral and calcareous soils, P is adsorbed to surfaces of CaCO3 and clay minerals

P Fixation

Precipitation as secondary P compounds In acid soils, P combines with

iron (Fe) and aluminum (Al) to form insoluble compounds

In neutral and calcareous soils, P combines with (Ca) to form insoluble compounds

N.C. Brady,

1974

Mn

Availability of Fixed P

P relatively available when first fixed dissolution from newly formed minerals

desorption from sorption sites

Availability decreases as time passes

Occluded P

‘Sorbed P’ tends to become ‘occluded P’ as time passes

Sorbed P covered by coatings of iron, aluminum, or manganese oxides

Unavailable

Summary of different P pools in soils

Image Credit: Frossard et al. (2011).

(tiles)

(ditches)

Phosphate leaching

1. Eroded and runoff P:

slopes; heavy rain

3. Leaching losses:

tile drains; ditches, coarse textured soils; high P fertilization

P Loss Pathways:

2. Crop uptake

Principles of P Management

Maintain soil pH for desired crop

Apply P fertilizers when needed, where most efficiently utilized

Band starter fertilizer

When practical, incorporate manure

Utilize practices that reduce soil erosion and runoff

Practice Question #2

What is the optimum pH range for maximizing plant-available P in soils?

a) 5.0-5.5

b) 5.5-6.8

c) 6.5-7.7

d) >7.5

Potassium (K) Cycle

Simpler cycle than N no oxidation and reduction

no gaseous forms

Soluble K can be “fixed” to less available forms in some soils different mechanism than P

minimal water quality issues

Maryland Cooperative Extension, 2000

SourcesLosses

PLANT RESIDUES

Plant-available K

K+ dissolved in soil solution

Exchangeable K+

Moves primarily by diffusion along moist soil pores

Soil solution K

Only 0.1 – 0.2% of

total K in soils

Readily available for

plant uptake

Exchangeable K

K+ at exchange sites on soil colloids in dynamic equilibrium

with K+ in soil solution replenishes the soil

solution

Readily available for plant uptakeUsually less than 1% of

total K in soils

Fixed K

Fixed as K+

trapped in interlayers of clay minerals

Non-exchangeable

Minimally available by weathering

About 1-10% of total K in soil

N.C. Brady, 1974

Mineral (structural) K

K is a structural component of primary minerals such as feldspars and micas

90 – 98% of total K in soil

Relatively unavailable

K Loss Pathways

Leaching coarse soils/high

rainfall

Erosion

Crop removal luxury

consumption

N.C. Brady, 1974

Luxury consumption of potassium by plants. If excess amounts of potash fertilizers are applied to soil, plants will absorb potassium in quantities exceeding that required for optimum yields. This may be wasteful if crops are completely removed from the soil.

Principles of K Management

Maintain soil pH for desired crop

Utilize practices that reduce soil erosion

Split application

reduce losses to luxury consumption

Practice Question #3

Potassium is lost from the soils by:

a) Microbial transformation to gaseous K+

b) Leaching through clayey, fine-textured soils

c) Luxury consumption by plants

Summary: Nutrient Cycling in Soils

Questions?

Email: gstoor@umd.edu

@ToorUMD