Understanding soil organic matter

So when does

crop residue

become SOM?

Grain yields

have increased

dramatically

since WWII

Crop residue

production has

also increased

but not as

dramatically as

grain yields

*200 bu/a

in 2014

IL corn yields have doubled

during my lifetime!

How much residue does a 200 bushel corn

crop return to the soil?

General rule of

thumb for corn

grain, stover

and roots each

comprise

~1/3rd of the

total biomass 33%

200 bushels*~50 lbs/bu * 2 = 20,000 lbs residue/a/yr!

So how much of the ~10 tons/acre of dry residue

left by a 200 bu/a corn crop turns into

soil organic matter?

Why not 56? Why multiply by 2?

Estimating C inputs retained as soil organic matter from corn

Plant and Soil Volume 215, Issue 1, pp 85-91

M.A. Bolinder, D.A. Angers, M. Giroux, M.R. Laverdière

Abstract

In agroecosystems, the annual C inputs to soil are a major factor controlling

soil organic matter (SOM) dynamics. However, the ability to predict soil C

balance for agroecosystems is limited because of difficulties in estimating C

inputs and in particular from the below-ground part. The objective of this

paper was to estimate the proportion of corn residue retained as SOM. For

that purpose, the results of a 13C long-term (15 yr) field study conducted on

continuous silage corn and two silage corn rotations along with data from the

existing literature were analyzed. The total amount of corn-derived C (0–30

cm) was about 2.5 to 3.0 times higher for the continuous corn treatment (445

g m-2), compared to the two rotational treatments (175 and 133 g m-2 for the

corn-barley-barley-wheat and corn-underseeded barley hay-hay rotations,

respectively). Assuming that the C inputs to the soil from silage-corn was

mainly roots and would have been similar across treatments on an annual

basis, the total amount of corn-derived C for the two rotational treatments was

approximately proportional to the number of years the silage-corn was

The results from this study indicate that ~ 17%

of corn root-derived C was retained long-term as SOM.

This is almost twice the ~ 10 % retention reported in the

literature for shoot-derived C and is in agreement with

many studies showing that

more root C is retained than shoot C.

The standard conversion from

SOM to C is %SOM / 1.72 = %C.

For soils or soil horizons containing large

amounts of relatively undecomposed plant

materials, a factor of 2 may be more

accurate (i.e., %SOM / 2 =%C).

crop

residue

carbon

CO2

Living organisms Microbial

compounds

often > 75%

After

1

year

Most of the C in

crop residues

quickly returns

to the

atmosphere

Plant compounds

SO

M

The current OM level in a soil is the

result of the long-term balance

between organic inputs and outputs

So… shouldn’t yield enhancing

practices build SOM?

“The microherd”

Phil Brookes

Practices that enhance crop yield

also impact the soil stomach!!

When there is more

grass, I eat more!!

”But with the removal of water through furrows, ditches,

and tiles, and the aeration of the soil by cultivation, what

the pioneers did in effect was to fan the former simmering

fires… into a blaze of bacterial oxidation and more

complete combustion. The combustion of the accumulated

organic matter began to take place at a rate far greater

than its annual accumulation. Along with the increased rate

of destruction of the supply accumulated from the past, the

removal of crops lessened the chance for annual additions.

The age-old process was reversed and the supply of

organic matter in the soil began to decrease instead of

accumulating.”

William Albrecht – 1938 Yearbook of Agriculture

Tillage + Lime + Drainage + N fertilizer =>

higher crop yields & higher decomposition rates

Mollisols (prairie soils) under long-term

agricultural management have ~ 50% less

organic C in their top 8” than native prairie soils.

Most of the loss of organic C is likely to have

occurred within 25 years of the original plowing

(as opposed to the last 50-100 years).

Organic C levels in IL soils have been relatively

stable over the past 50 years.

Soil Changes After Sixty Years of Land Use in IowaJessica Veenstra, Iowa State University, 1126 Agronomy Hall, Iowa State

University, Ames, IA 50010

Soils form slowly, thus on human time scales, soil is essentially a non-renewable

resource. Therefore in order to maintain and manage our limited soil resources

sustainably, we must try to document, monitor and understand human induced

changes in soil properties. By comparing current soil properties to an archived

database of soil properties, this study assesses some of the changes that have

occurred over the last 60 years, and attempts to link those changes to natural and

human induced processes. This study was conducted across Iowa where the

primary land use has been row crop agriculture and pasture. We looked at

changes in A horizon depth, color, texture, structure, organic C content and pH.

Hill top and backslope landscape positions

have been significantly degraded w/ less organic C

but

catchment areas have deeper topsoil w/ more organic C.

Are crop

residues

different than

the residues

from native

vegetation ?

perennial roots > annual roots!

http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html

This pie chart represents

organic matter in soil with native

vegetation. Notice the large

active fraction.

When native vegetation is

converted to agriculture, the

active OM fraction normally

quickly declines. The stable

OM fraction changes much less.

From the U of MN bulletin on SOM

http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html

From the U of MN bulletin on SOM

After

long-term

agriculture

Why does old OM become dominant?

Smaller pie and different slices

< 1 year

decades

centuries

What is

the

average

age of

organic C

in soil ??Janzen (2006)

Old C is most

abundant!

depleted in

most ag

soils

C dynamics

There is very clear evidence that atmospheric levels of CO2

are increasing and that the majority of the CO2 added to the

atmosphere in the last 3 decades has come from fossil fuels

Why do CO2

levels go up

and down

annually?

Prior to ~1980,

majority of CO2↑

came from

loss of SOM

Agricultural Production Affects Annual CO2 Cycle

Each year in the Northern Hemisphere, levels of atmospheric carbon

dioxide drop in the summer as plants grow, and then climb again as they

decompose. Over the past five decades, the size of this seasonal swing

has increased by ~50%, for reasons that aren’t fully understood.

Scientists recently evaluated global production statistics for 4 leading

crops – corn, wheat, rice and soybeans and found that production of

these crops in the Northern Hemisphere has more than doubled since

1960. This translates to ~ 1 billion metric tons of C captured and

released each year!

According to Dr. Josh Gray of Boston U:

“Croplands are ecosystems on steroids. They occupy about 6

percent of the vegetated land area in the Northern Hemisphere but

are responsible for up to a quarter of the total increase in seasonal

exchange of atmospheric carbon dioxide, and possibly more…that’s

a very large, significant contribution, and 2/3 of that contribution is

attributed to corn.“

http://www.grida.no/climate/vital/graphics/large/12.jpg

Global

C cycleAll# = GT

Gt = 109 t = 1 billion metric tons

Soil C > Atmosphere C + Vegetation C

2400

Why is SOM

important ??

What Does Soil Organic Matter Do (for you)?

Nutrient cycling

Increases the nutrient holding capacity of soil (CEC).

Serves as a slow release form of nutrients for plants.

Chelates nutrients increasing their availability to plants.

Feeds soil organisms from bacteria to worms that excrete available nutrients

Water dynamics

Improves water infiltration.

Decreases evaporation.

Increases water holding capacity, especially in sandy soils.

Structure

Reduces crusting, especially in fine-textured soils.

Encourages root development.

Improves aggregation, preventing erosion and reducing compaction.

From the U of MN bulletin:

Fertilizer is not

a substitute

for SOM

SOM

Most (but not all)

soil organisms

eat SOM

Some bacteria are CHEMOAUTOTROPHS

Chemoautotrophic bacteria obtain energy

through the oxidation of electron donors

other than C.

For example, the bacteria that oxidize ammonium

into nitrate, a important process called nitrification,

do NOT eat SOM

Many bacteria and all fungi

(as well as all other soil organisms)

are HETEROTROPHS

(which means that they eat organic matter).

SOM is the fuel

that energizes

most biological

processes in soil

(Watts and Dexter, 1997)

Structural

damageSoils with high OM

are more resistant to

structural damage !

Soils with more OM have less strength when dry

and more strength when moist!

SOM increases plant available H20

Adapted from Brady and Weil (2002)

SOM is a very important adsorbent in soil

Adapted from Brady and Weil (2002)

Humus gives soil a darker color

Is this beneficial?

Biologically

active

SOM

SOM is a complex mixture of

living, dead and very dead OM

Living organisms

Recent residues

Stabilized

SOM

Adapted from Magdoff and Weil (2003)

Historically often

called HUMUS

What is humus ???

Humus is organic matter that has been

transformed such that its original source

is no longer apparent… The diverse

products of “humification” have many

common characteristics:

Resistance to further decomposition

Complexation with fine mineral materials

High specific surface and negative charge

Dark color

HUMMUS

HUMUSis NOT

The traditional concept of

highly complex humus

macro-molecules distinctly

different from bio-molecules has

been rejected by most scientists

Recent research has demonstrated that chemical

structure alone does not control SOM stability:

in fact, environmental and biological controls are

more important…

Nature, October 2011

Have you

ever heard of

any humate

products?

Hydra-hume

Descriptions of humate products often

refer to humic and fulvic acid content

Fulvic

acid Humic

acid

Fulvic acid = soluble in strong base and still soluble when pH => 7

Humic acid = soluble in strong base but precipitates when pH => 7

HA & FA are

solubility

fractions

NOT

specific

compounds

TIDIC acid production system ☺

Leonardite

source material for

commercial

humate products

Products differ

with respect to

the amount

and type of

processing

There is growing evidence that humate products can enhance

crop growth **BUT** are not the same as natural SOM

There are lots of humate products on the market.

Ask for research results and clear explanations.

Reputable companies should be able

to provide analytical results obtained

using this new standardized method

Humate products are not a substitute for good soil organic matter management

accumulate in

soil?

why does

matorganic ter

So…

Understanding biochemical recalcitrance

(Giller, 2000)

aka digestibility

C:N ~ 25

Faster

SlowerDecomposition

LIGNIN is the main molecule that makes

plants stiff – it decomposes much more

slowly than cellulose but research shows

that it eventually decomposes and does

not accumulate significantly in soil

Field-Grown Bt and non-Bt Corn:

Yield, Chemical Composition, and Decomposability Sandra F. Yanni, Joann K. Whalen and Bao-Luo Ma

Abstract

Bt (Bacillus thuringiensis) corn (Zea mays L.) accounted for 74.5% of the

corn acreage in eastern Canada in 2009. Reports that Bt corn has

greater yield and lignin concentrations than unmodified corn have raised

questions about its effect on the soil ecosystem. Our objectives were to

evaluate the biomass of field-grown Bt and non-Bt corn, the chemical

composition of different corn components that remain as residues in the

field after harvest, and the effect of the Bt modification on residue

decomposition. Nine Bt corn hybrids and their near isolines were field-

grown in 2008 and 2009. Grain and stover yields were measured and

leaves, stems, and roots were collected and analyzed for lignin, C, and N

concentrations. Stem sections from a Bt/non-Bt corn pair were buried in

the field and sampled periodically during 1 yr. No difference in yield or

lignin concentrations due to the Bt gene was noted; however, N

concentration in Bt stems was significantly greater than in non-Bt stems

in 1 yr of the 2-yr study. Leaves had less lignin and a lower C/N ratio

than stems and roots in both years. In buried field litterbags, the decline

in C/N ratio and mass loss suggests that Bt stems were decomposing

We conclude that the Bt gene does NOT affect the

chemical composition of corn residues in fields

without herbivory, and that Bt corn residue may

actually be MORE susceptible to decomposition than

non-Bt corn residue.

What about with herbivory???

SOME SURPRISING RESULTS

Charcoal is highly

resistant to

decomposition and

full of porosity

Terra Preta

soils contain

lots of ancient

charcoal

Typical

acid infertile

rain forest

soil

Can we make

new Terra Preta

soils by

amending

infertile soils

with BIOCHAR?

Scientists are trying to figure out…

Understanding Mineral Protection

Magdoff and Weil (2004)

Soil C content

is often

closely related

to fine mineral

content

Weak relationship between clay content and SOC

for 1261 agricultural soils in England and Wales

Webb et al.(2003)

Clay is clearly

not the only

factor controlling

C content in

these soils

Understanding physical protection

Adapted from Carter (2002)

Mineral protected OM

Intra-

aggregate

OM

Free OM

Over time SOM

becomes more and

more intimately

connected

to soil

mineral

particles

Soil microaggregates

Soil macroaggregate

OM

OM

Is there organic matter inside aggregates?

Soil macro-

aggregates

form around

fresh organic

residues

Tillage

disrupts

aggregates

and

accelerates

decomposition

Tillage OM inputs

What is POM??

Mineral protected

Sand sized Silt and clay sized

http://www.grdc.com.au/growers/res_summ/pdfs/cso00029.pdf

Particulate OM = POM

Geographic distribution of SOM

What Determines Soil Organic Matter Levels?

The amount of organic matter in soil is the result of two processes: the addition of

organic matter (roots, surface residue, manure, etc.), and the loss of organic

matter through decomposition. 5 main factors affect both additions and losses.

Soil texture - Fine-textured soils can hold much more organic matter than sandy

soils for two reasons. First, clay particles form electrochemical bonds that hold

organic compounds. Second, decomposition occurs faster in well-aerated sandy

soils. Sandy loams rarely have more than 2% organic matter.

Historical vegetation - In prairies, much of the organic matter that dies and is

added to the soil each year comes from grass roots that extend deep into the soil.

In forests, the organic matter comes from leaves that are dropped on the surface

of the soil. Thus, farmland that was once prairie will have higher amounts of

organic matter deep in the soil than land that was previously forest.

Climate - High temperatures speed up the degradation of organic matter. In

areas of high precipitation (or irrigation) there is more plant growth and therefore

more roots and residues entering the soil.

Landscape position - Low, poorly-drained areas have higher organic matter

levels, because less oxygen is available in the soil for decomposition. Low spots

also accumulate organic matter that erodes off hill tops and steep slopes.

So what is the 5th factor?

MANAGEMENT

Interstream

divide

SOIL

DRAINAGE

CLASSES

Poorly

drained

Somewhat

poorly

drained

Moderately

well drained

Poorly

drained

Well

drained

Interfluve

Valley floor

Backslope

Shoulder

LANDSCAPE

POSITIONS

Landscape position affects SOM dynamics

Where does the most OM accumulate?

“flat black”

soils

Temperature affects OM production and decomposition

Brady and Weil (2002)

70 F

Organic matter

synthesis by plants

Illinois in 50 yrs?

How

much

is

enough

??

Have we learned anything in the last 77 years ?

Janzen (2006)

Hydroelectric dam metaphor

OM forms and

dynamics are more

important than total

quantity

but many soils

currently have OM

levels that limit

soil function

SOM is a very important source of nutrients in modern production systems but

we are less dependent on SOM to supply nutrients than we were in 1938.

There are many ways to “measure” SOM

Adapted from Strek and Weber (1985)

Total organic matter

by “loss on ignition”

Total C

by several wet and

dry oxidation

methods

Humic matter

by alkali extraction

OM/1.72 = C

% OM

Permanganate oxidizable C

a routine test for “active” soil C ??

“Our analysis demonstrates the

usefulness of POXC in quickly and

inexpensively assessing

changes in the labile soil C pool.”

Soil from a

long term

experiment in

Beltsville, MD

After

adding

water

1.4 % C1.0% C

Relatively small differences in C

48 bu/a 140 bu/a

Large differences in soil function

Aggregation changes much more rapidly than total C

Jastrow (1996)

Years since PRAIRIE RESTORATION

Aggregation

Total C

16 % clay 39 % 49%

More OM is needed to stabilize fine textured soils

Adapted from Russell (1973)

16 % clay39 % 49%

Comparison of soil from fields with the same soil type but

different OM levels can help identify sites with the most

potential for building SOM and improving soil function

Managing SOM

well mixed vs.

stratified

Conventional tillage Conservation tillage

Adapted from House and Parmelee (1985)

It is widely believed that tillage was the main cause of soil C loss when

natural ecosystems were converted to agriculture, and that substantial C

sequestration can be accomplished by changing from conventional tillage to

no-till. This is based on lots of experiments (and on-farm observations) where

soil C increased under no-till. However, sampling methods may have biased

the results. In essentially all cases where no-till was found to sequester C, soils

were only sampled to a depth of 1 foot or less…

What is meant by

the term

CARBON

SEQUESTRATION?

CARBON

SEQUESTRATION

in soil

CO2 -> SOM

Very few tillage studies have been sampled deeper than 1’

Many studies were only sampled 6” deep!

In the Upper Midwest, tillage reduces OM levels

near the soil surface but has much less impact

on SOM in the whole profile.

In warmer climates, tillage more negatively

effects OM levels in the whole profile.

Effect of tillage on microbial activity

Havlin et al. (1999)

Tillage

Which tillage system has

more total microbial

activity ?

Conventional tillage

Which system releases

more CO2 when crops need

CO2 ?

Elevated OM levels at the soil surface are beneficial

even if no greater OM accumulates at depth

Artificial drainage has greatly increased the number of

days when soils in the Upper Midwest are suitable for

field operations

but has also

contributed

to some

environmental

problems

Pollution of

water resources?Loss of SOM??

Original soil surface of a Histosol (muck soil) in FL

Adapted from Bailey and Lazarovits (2003)

A systems approach

to SOM management

Well adapted crop

Nutrient

ManagementWater

Management

SOM

Crop

residue

management

Increase residue production

(especially roots) and minimize

soil disturbance

Crop Rotation

High residue crops

Cover crops

Forages

Erosion Control Practices

Erosion is a major cause of reduced SOM levels at the soil surface

http://tucson.ars.ag.gov/isco/isco10/SustainingTheGlobalFarm/P241-Kimble.pdf

Erosion status

On-farm recycling of OM

Most IL

fields never

receive

manure

Off-farm sources of OM

~ 60% of the

biosolids generated

in the US are land

applied. Farmers get

nutrients and OM

for free!

Innovative cover cropping

Its possible to ~double the months

of active plant growth in IL

4 8

A good way

to grow

more roots!

Actual C

Practically

attainable C

Potential C

(Dick and Gregorich, 2004)

Input factors

Many

factors

control

SOM

content

Residue yield

Saturation deficit

Saturation of capacity

Actual C

Practically

attainable C

Potential C

(Dick and Gregorich, 2004)

Disturbance factors

Input factors

capacity factors

man

ag

em

en

t

= opportunity

Residue yield

Fencerows are often a good place to check

a soil’s capacity for C accumulation

Some

effects of

higher OM

quantity

and/or

quality

occur

relatively

quickly

Other

effects

take

longer

Fields or parts of fields with the lowest OM content

(relative to their potential) will benefit the most

from practices that build SOM.