Biochar suppression of N 2 O emissions from an agricultural soil: effects and potential mechanisms Sean Case Dr Jeanette Whitaker, Centre for Ecology and.

Biochar suppression of N2O emissions from an agricultural soil: effects and potential mechanisms

Sean Case

Dr Jeanette Whitaker, Centre for Ecology and HydrologyDr Niall McNamara, Centre for Ecology and HydrologyDr David Reay, University of Edinburgh

Overview1. Introduction – soil N2O emissions and biochar

2. Observed effect of biochar addition on soil N2O emissions

3. Investigating the mechanisms4. 15N stable isotope experiment5. Conclusions

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Dynamotive Energy Systems, (2009)

Soil N2O production

• Use NH4+ and O2

• Activity peaks at mid range (50 – 60 %) of water filled pore space

NH4+(Ammonium) NO2

-

NO3-

(Nitrate)

NO N2O N2

Nitrifying bacteria

Denitrifying bacteria

• Use NO3- and C• Anaerobic conditions• Activity increases as soil

approaches saturation

N2O

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Biochar addition and soil N2O emissions – a (very) brief history

Yanai et al., (2007), Soil Sci. and Plant Nut., 53, 181-188

• Biochar addition observed to suppress soil N2O emissions in laboratory

• Very few published papers showing consistent N2O suppression following biochar application to field

• N2O suppression put down to several different mechanismsSoil aeration

pH changeSubstances on biochar (e.g. ethylene, α-pinene)N immobilisation

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Field experiment:Field experiment:• Biochar – Hardwood biochar (400°C), 50

tonnes ha-1

• Crop – Miscanthus Giganteus X (Elephant grass)

• Soil – Sandy loam, low inorganic-N• Seasonal greenhouse gas measurements

• Significant suppression in first measurement

• Sporadic measurements? Not timed with rain events?

n = 5

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Laboratory experimentLaboratory experiment• Soil cores wetted to high

gravimetric water content

• Cumulative N2O emission reduction of at least 49 % within 48 hours

Adapted from Case et al. (2012), Soil biology and Biochemistry, in press

Wetting event n = 4

Miscanthus crop soil

Wetting event n = 5

Arable soil – recently N fertilised

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

1. Increased soil aeration ?

• All treatments wetted to same % of WHC/WFPS

• Still suppression with increasing biochar content

• Increased soil aeration is not the key mechanism

• Biochar has a higher water holding capacity (WHC) than soil

• May increase soil aeration, inhibiting denitrification

97 % suppression

Wetting event

n = 4

Adapted from Case et al. (2012), Soil biology and Biochemistry, in press

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Miscanthus crop soil

2. pH change• Biochar is a high pH material (8.8)• Soil pH increase - decreases activity of soil

nitrifying/denitrifying bacteria ? • Or increases reduction of N2O to N2 during

denitrification ?• Soil pH increases with biochar content in

field and lab• pH shift not strongly correlated with N2O

production• Suspect not the ‘key’ mechanism

n = 15

Soil incubated with biochar in the field one year after application

3. Ethylene• Our biochar not found to produce ethylene

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

4. Nitrogen immobilisation ?• Lower N2O evolution - sorption of ammonium or nitrate to the surface of biochar?

(Spokas et al., 2011)• Especially in biochar created at low temperatures with high concentrations of

surface oxygen groups

Miscanthus crop soil (initially low-N)

Arable crop soil (initially high-N)

n = 4 n = 4

n = 5 n = 5

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

4. Nitrogen immobilisation ?• Lower N2O evolution - sorption of ammonium or nitrate to the surface of biochar?

(Spokas et al., 2011)• Especially in biochar created at low temperatures with high concentrations of

surface oxygen groups

What is the effect of biochar addition on the soil N cycle?

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

• Arable soil – rapeseed/winter wheat rotation• 2 % Biochar added to half of cores• Water added to reach 70% Water Filled Pore

Space• Ammonium nitrate fertiliser added (0.1 mg g-1

N), 10% 15N enrichment• Measurements for 6 days

1 2 3 4

15NH414NO3

Soil + biochar

1 2 3 4

14NH415NO3

Soil

15N stable isotope experiment

‘Mirrored’ 15N addition treatments

Label: Ammonium Nitrate

• Analysed with Monte Carlo (Müller et al., 2007) or FLUAZ methods (Mary et al., 1998)

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Humus

Plant residue (+ biochar)

Ammonium (NH4

+)Nitrate (NO3

-)

Microbial biomass

Ammonia (NH3)N2ON2

Gross mineralisation

Volatilisation

Nitrification

Immobilisation

Remineralisation

Denitrification

Additional analyses: pH, total CN, qPCR genetic analyses

Direct microbial assimilation of plant residue N

Adapted from Mary et al., (1998)

The N cycle

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Humus

Plant residue (+ biochar)

Ammonium (NH4

+)Nitrate (NO3

-)

Microbial biomass

Ammonia (NH3)N2ON2

Gross mineralisation

Volatilisation

Nitrification

Immobilisation

Remineralisation

Denitrification

Directly measured

Calculated

Additional analyses: pH, total CN, qPCR genetic analyses

Direct microbial assimilation of plant residue N

Adapted from Mary et al., (1998)

Organic-N

The N cycle

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Preliminary results

• N2O emissions suppressed with biochar

• CO2 emissions increased with biochar

n = 8

n = 8

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Conclusions1. Addition of this biochar suppresses N2O emissions in

the lab following wetting events – limited evidence in the field

2. Increased soil aeration, pH change or ethylene production are not the key mechanisms to explain N2O suppression in this case

3. Biochar addition may immobilise soil inorganic-N due to physical/chemical nature of biochar surfaces, could this explain N2O suppression in this case?

4. Stable isotope studies are needed to investigate this

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

Acknowledgements:Jeanette Whitaker, Niall McNamara, David Reay,

Emily Bottoms, Mike Whitfield, Simon Oakley, Andy Robertson

Thank you

1. Soil N2O 3. Effect 4. Mechanisms 5. 15N Exp. 6. Conclusions2. Biochar

References1. Case S. D. C., McNamara N. P., Reay D. S., Whitaker J., 2012, The effect of biochar

addition on N2O and CO2 emissions from a sandy loam soil - The role of soil aeration. Soil biology and Biochemistry, in press

2. Mary B. , Recous S. and Robin D., 1998. A model for calculating nitrogen fluxes in soil using 15 N tracing. Soil Biology and Biochemistry 30, 1963-1979.

3. Müller, C., Rütting, T., Kattge, J., Laughlin, R.J., Stevens, R.J., 2007. Estimation of parameters in complex 15N tracing models by Monte Carlo sampling. Soil Biology and Biochemistry 39, 715–726.

4. Spokas K. A. , Novak J. M., Venterea R. T., 2012. Biochar’s role as an alternative N-fertilizer: ammonia capture. Plant and Soil 350, 35-42.

5. Yanai Y., Toyota K., Okazaki M., 2007. Effects of charcoal addition on N2O emissions from soil resulting from rewetting air-dried soil in short-term laboratory experiments. Soil Science and Plant Nutrition 53, 181.

Future work

1. Inducing high N2O emissions in the field via water addition

2. Ongoing biological work (qPCR looking at NirK and AmO enzymes)

3. 15N stable isotope experiment – finish analyses

and develop further hypotheses