kemanian carbon and nitrous oxide v2bioenergy.psu.edu/shortcourses/2011lifecycle/... · e7 k(1 --e...

8/1/2011

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Armen R. KemanianDept. Crop & Soil Sciences

Penn State University

Life Cycle Analysis for Bioenergy

University Park, PA

26-27 July, 2011

Carbon and Nitrous Oxide in LCA

Why is this important?Introduction

� In grain, forage, and biomass production systems both

the net C balance and the emission of N2O are the

main factors affecting the farm-gate LCA outcome

� Agriculture is responsible for approximately 75% of the

total GHG attributable to N2O emissions in the US

� EPA 2010: to qualify as “renewable”, advanced biofuels

GHG emissions must be 50% of those from petroleum

based fuel over the fuel lifecycle

� Management for improving the C balance or to reduce

N2O emissions may involve optimizing the outcome for

multiple criteria

� These are two pieces of a complicated puzzle! The rest

of the analyses focuses on these two pieces

8/1/2011

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The C and N cyclesIntroduction

Inputs of organic Carbon through photosynthesis

Losses of CO2 or CH4 through respiration or fermentation

GRAIN

FORAGE

RESIDUES

Exported from farm for animal or human consumption.

Most C respired or fermented, a fraction returned as

manure or biosolid.

Exported or not, ~50% respired or fermented (CH4), the

rest returned to the soil as manure

Most returned to the soil, with a large fraction (>80%)

respired as CO2

� The net balance is usually accounted for in the soil organic

carbon pool, but as we can see, the overall LCA is more

evolved. I will focus on the crop and soil aspects

The C and N cyclesIntroduction

N2O

N2O

Emission during

Nitrification

Emission during

Nitrification

Emission during

Denitrification, N2 and N2O

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The carbon balance equationCarbon

This equation states that the change in storage is equal the gains of C

minus the losses of C for a given time interval

At equilibrium:

Sc = soil organic carbon

Rc = residue input

hx = humification coefficient

k = soil organic carbon decomposition (apparent respiration) coeff.

= Inputs - Outputs

The carbon balance equationCarbon

Soil carbon increases through

higher inputs:

Increase residue inputs!

Limitation is soil C saturation,

unlikely in most soils in temperate

conditions

Soil carbon increases through

reduction in losses:

Reduce k, the decomposition rate,

by maintaining the soil drier (with

crops that use the water, possible)

or cooler (more difficult) or flooded

(not the point obviously), or with

less mechanical disturbance,

and with minimal erosion

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Carbon balance and bioenergy cropsCarbon

Increase residue inputs

Simplistic proposition:

Biofuel production entails removing

biomass, not returning it to the soil

But…

This biofuel offsets emissions from

fossil fuel, therefore a neutral C

balance is possibly a net gain

More sophisticated proposition:

Capture more radiation and water

by intensifying the cropping

sequence (cover crops?)

Increase inputs through the roots of

perennials (depth)

Reduction in C losses

Tillage: reduce tillage type or

directly the frequency of tillage by

using perennial crops

Soil moisture: more cropping or

perennial crops minimize the

period of wet soils (e.g. after

harvest)

Erosion: perennial crops reduce

erosion, and so does the use of no-

till in most circumstances

Carbon balance and bioenergy cropsCarbon

There are too many factors to consider, how do we evaluate them?

(1) Experimentally (long term, limited number of scenarios)

(2) Using simulation models

DPM SPM RPM

Biomass

Labile

Meta-

stable

Stable

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DPM SPM RPM

Biomass

Labile

Meta-

stable

Stable

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COCOCOCO2222

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COCOCOCO2222

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Carbon Input

Soil Carbon

8/1/2011

5

The sources of nitrous oxideNitrous Oxide

Nitrous oxide is produced by several processes in the soil, the most

important of which are microbial denitrification and nitrification

Denitrification consists on the sequential reduction of nitrate (NO3) to

NO, N2O, and N2

Nitrification is the process by which NH4 is oxidized to NO3; N2O is a

byproduct (~0.3%). The process is fast in aerobic conditions.

In terms of the N mass balance, the N2O losses are low. In GHG terms,

however, a loss of 1 kg of N2O-N equates to ~54 kg of C

So, a small flux that in GHG terms is too important. Quantifying it is as

challenging as it gets for LCA

Factors that promote the losses Nitrous Oxide

The whole process is somewhat perverse:

NH4 NO3

N2O

nitrificationdenitrification

N2, N2O

FertilizerMineralization

DepositionUrine / Manure

Fertilizer

MineralizationDeposition

NH3

� N entering as NH4 has two chances to be emitted as N2O � When residues decompose, a fraction of the N is recycled back through

NH4!� To uptake sufficient N a non-legume crop needs available NO3

(perennials somewhat bypass it by internal recycling; the extent to which it can be coupled to high biomass removal is unknown)

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Factors that promote the losses Nitrous Oxide

NH4 NO3

N2O

nitrificationdenitrification

N2, N2O

FertilizerMineralizationDepositionUrine / Manure

FertilizerMineralizationDeposition

NH3

Available C for heterotrophic respiration (residues, roots, organic matter)Low oxygen (< 10% of the absolute porosity filled with air)Fully anoxic conditions drive almost all of the denitrified N to N2; maximum N2O rates are shifted with respect to maximum denitrification rates

Typical rates: Natural environments < 1 kg N2O-N ha-1 yr-1

Ag systems ~ 2 to 4 kg N2O-N ha-1 yr-1

Higher losses reported > 40 kg N2O-N ha-1 yr-1

ManagementNitrous Oxide

By keeping NO3 low

Control of fertilization rates

Use of non-nitrate sources

Use of nitrification-inhibitors

Use of perennial crops?

Use of switchgrass / poplar or

willow as buffer strips?

By controlling factors affecting the rate other than the NO3 level

Place fertilizer away from C source

Manage soils to provide good

drainage, e.g. avoiding compaction

Minimize N inputs when soil is

moist and prone to higher moisture

(snowmelt; marginal lands)

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Nitrous oxide and bioenergy cropsNitrous oxide

There are too many factors to consider, how do we evaluate them?

(1) Experimentally (long term, limited number of scenarios)

(2) Using simulation models

(3) Yes, the same as slide 8 for C!

DPM SPM RPM

Biomass

Labile

Meta-

stable

Stable

COCOCOCO2222

COCOCOCO2222

COCOCOCO2222

COCOCOCO2222

KKKK1111EEEE1111

KKKK1111(1(1(1(1----EEEE1111

)))) KKKK2222(1(1(1(1----EEEE2222

))))COCOCOCO2222

KKKK3333(1(1(1(1 ----EEEE3333

))))

KK KK22 22 EE EE

22 22

KKKK3333EEEE3333

KKKK5555(1(1(1(1----EEEE5555

))))

KKKK9999EEEE9999

KKKK8888EEEE8888

KKKK6666(1(1(1(1 ----EEEE6666

))))

KK KK4m

4m

4m4mEE EE44 44

KK KK4l

4l

4l4lEE EE

44 44


KKKK7777(1(1(1(1----EEEE7777

))))

KK KK55 55 EE EE

55 55


DPM SPM RPM

Biomass

Labile

Meta-

stable

Stable

COCOCOCO2222

COCOCOCO2222

COCOCOCO2222

COCOCOCO2222

KKKK1111EEEE1111

KKKK1111(1(1(1(1----EEEE1111

)))) KKKK2222(1(1(1(1----EEEE2222

))))COCOCOCO2222

KKKK3333(1(1(1(1 ----EEEE3333

))))

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22 22

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Carbon Input

Soil Carbon

A commentary on modelsModels

“… why do this? … The answer I’d give is that models are an enormously important tool for clarifying your thought. You don’t have to literally believe your model — in

fact, you’re a fool if you do — to believe that putting

together a simplified but complete account of how things

work … helps you gain a much more sophisticated

understanding of the real situation. People who don’t use

models end up relying on slogans that are much more

simplistic than the models — [fill in with your favorite

slogan] all of which are just wrong some of the time”.

Paul Krugman

November 18, 2010 http://krugman.blogs.nytimes.com/2010/11/18/debt-deleveraging-and-the-liquidity-trap/

8/1/2011

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A simple model for a simple systemExample

Disclaimer: This is a hypothetical situation. The crop yield

and soil properties are fictional and any similarity with a

real situation is mere coincidence

Soil: 50 Mg C ha-1 in topsoil (0.3 m)

Crop: Maize producing

3 Mg ha-1 of root (~1.3 Mg of C)

8 Mg ha-1 of residue (~3.5 Mg of C)

8 Mg ha-1 grain, removed from the field

Fertilization: 150 kg N as ammonium nitrate


Assume:

�The k or soil apparent respiration is 1.5% per year

� The humification is about 16% (i.e. stabilization of residue

inputs)

� About 0.75% of the fertilizer is lost as N2O

Then:

Soil C respired: 0.015 x 50 = 0.75 Mg ha-1 yr-1

Residue C humified: 0.16 x (1.3 + 3.5) = 0.77 Mg ha-1 yr-1

Therefore soil C is approximately in steady state

8/1/2011

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What about nitrous oxide?

N2O-N lost 0.0075 x 150 = 1.1 kg ha-1 yr-1

This is, approximately, equivalent to 0.06 Mg ha-1 yr-1 of C

lost.

Therefore, the GHG balance is slightly negative. It is worth

noting that nitrous oxide losses can be much larger


We can conclude that:

Further removal of C by harvesting the residue may tilt the

balance towards soil C losses (and erosion).

However, removal coupled with the incorporation of a

cover crop may restore the equilibrium, effectively

intensifying the system. And if that cover crop includes a

legume, it may reduce the need of external N inputs.

Once again, models become extremely important to help

think through the impact of different management options

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There are limitations!Notes

�Quantitatively, most controls of soil carbon dynamics

have been incorporated in simulation models, yet we are

still unable to use these models without much

supervision

�Soil carbon is rarely uniform across the landscape

1 m 40 m 600 m

The Palouse as study caseNotes

8/1/2011

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Frequency distribution of CsNotes

Frequency distribution of soil organic carbon in the profile (left panel), the top 0.3-m of the profile (middle

panel) and between 0.3 and 1.5 m in the Cook Agronomy Farm in eastern Washington (n = 177).

Huggins et al., unpublished

Profile SubsoilTopsoil

Soil Carbon and Carbon InputsNotes


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Topsoil C in the landscapeCART

CART for soil organic carbon in the topsoil (A.depth = thickness of the A horizon, curv.pln = plan curvature, ems00 =

electromagnetic conductivity in spring of 2000, Bw.depth = depth of the Bw horizon, flod = flow direction).


Soil carbon in the Palouse regionNotes

Fraction of cases in the upper, middle or lower third of

soil productivity and topsoil organic carbon

If productivity is stable:

25% of area could gain soil carbon

47% of area is likely at equilibrium with inputs

28% of area could lose soil carbon

Soil Carbon Low Medium High

Productivity

Low .175 .124^ 0.03^

Medium .102�� .119 .119^

High .062�� .096�� .175

8/1/2011

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Soil carbon in Texas, modeling studyNotes

Change in soil carbon when moving a system from till

(CT) to reduced till (RT) or no-till (NT), and viceversa.

SD=0.5 Mg ha-1

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0 20 40 60 80 100

Ch

an

ge i

n S

OC

(M

g h

a-1

)

Years

Change from CT to RT

Change from CT to NT

Change from NT to RT

Change from NT to CT

Meki et al., unpublished

Soil carbon in Texas , modeling studyNotes

Change in soil carbon when moving a system from till

(CT) to reduced till (RT) or no-till (NT), and viceversa.

Meki et al., unpublished

8/1/2011

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Nitrous oxide, hypothetical landscapeNotes

Concluding Remarks

� Tools are available that compute the carbon balance and

nitrous oxide emission of a system

� Variation in the landscape is known but difficult to

quantify and manage. Advances in this area are rapid.

� To provide useful outputs, simulation models need

adequate inputs

� Pairing biofuel production with landscape management

appears as a strategy that can greatly enhance the

appeal of bioenergy crops and have a favorable impact in

the LCA of biofuels

� As a side note, I will be happy to show the simulation

model Cycles to those interested in the C and N2O angles

8/1/2011

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Contact information: [email protected]

Questions?

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