Biochar suppression of N2O
emissions from an agricultural soil:
effects and potential mechanisms
Sean Case
Dr Jeanette Whitaker, Centre for Ecology and Hydrology
Dr Niall McNamara, Centre for Ecology and Hydrology
Dr David Reay, University of Edinburgh
Overview
1. Introduction – soil N2O emissions and biochar
2. Observed effect of biochar addition on soil N2O
emissions
3. Investigating the mechanisms
4. 15N stable isotope experiment
5. Conclusions
Dynamotive Energy Systems, (2009)
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
Soil N2O production
Nitrifying bacteria
N2O
NH4+
(Ammonium)
2. Biochar
N2O
NO3(Nitrate)
3. Effect
N2
Denitrifying
bacteria
NO2-
• Use NH4+ and O2
• Activity peaks at mid
range (50 – 60 %) of
water filled pore space
1. Soil N2O
NO
• Use NO3- and C
• Anaerobic conditions
• Activity increases as soil
approaches saturation
4. Mechanisms
5. 15N Exp.
6. Conclusions
Biochar addition and soil N2O
emissions – a (very) brief history
• 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 mechanisms
Yanai et al., (2007), Soil Sci. and Plant Nut.,
53, 181-188
Soil aeration
pH change
N immobilisation
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
Substances on
biochar (e.g.
ethylene, α-pinene)
5. 15N Exp.
6. Conclusions
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
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
Laboratory experiment
• Soil cores wetted to high
gravimetric water
content
• Cumulative N2O emission
reduction of at least 49 %
within 48 hours
Miscanthus crop soil
n=4
Wetting event
Arable soil – recently N fertilised
Wetting event
1. Soil N2O
2. Biochar
Adapted from Case et al. (2012), Soil
biology and Biochemistry, in press
n=5
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
1. Increased soil aeration ?
• Biochar has a higher water holding capacity
(WHC) than soil
• May increase soil aeration, inhibiting
denitrification
n=4
Miscanthus crop soil
97 % suppression
• All treatments wetted
to same % of
WHC/WFPS
• Still suppression with
increasing biochar
content
• Increased soil aeration
is not the key
mechanism
Wetting event
Adapted from Case et al. (2012), Soil biology and
Biochemistry, in press
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
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
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
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
n=4
n=4
n=5
n=5
Miscanthus
crop soil
(initially
low-N)
Arable
crop soil
(initially
high-N)
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
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
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
•
•
•
•
•
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
15N
stable isotope
experiment
‘Mirrored’ 15N addition treatments
1
2
3
4
1
2
3
4
Soil
15NH 14NO
4
3
Label:
•
Ammonium
14NH 15NO
4
3
Soil +
biochar
Nitrate
Analysed with Monte Carlo (Müller et al., 2007) or FLUAZ methods (Mary et al., 1998)
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
The N cycle
N2O
N2
Ammonia (NH3)
Volatilisation
Humus
Denitrification
Nitrification
Ammonium
(NH4+)
Gross
mineralisation
Plant residue (+
biochar)
Nitrate
(NO3-)
Immobilisation
Remineralisation
Adapted from Mary
et al., (1998)
Microbial
biomass
Direct microbial
assimilation of
plant residue N
Additional analyses: pH, total CN, qPCR genetic analyses
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
The N cycle
Directly measured
N2O
N2
Ammonia (NH3)
Calculated
Denitrification
Volatilisation
Humus
Nitrification
Ammonium
(NH4+)
Gross
mineralisation
Plant residue (+
biochar)
Nitrate
(NO3-)
Immobilisation
Remineralisation
Adapted from Mary
et al., (1998)
Microbial
biomass
Direct microbial
assimilation of
plant residue N
Organic-N
Additional analyses: pH, total CN, qPCR genetic analyses
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
Preliminary results
• N2O emissions
suppressed with biochar
n=8
• CO2 emissions increased
with biochar
n=8
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
Conclusions
1. 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
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
Thank you
Acknowledgements:
Jeanette Whitaker, Niall McNamara, David Reay,
Emily Bottoms, Mike Whitfield, Simon Oakley,
Andy Robertson
1. Soil N2O
2. Biochar
3. Effect
4. Mechanisms
5. 15N Exp.
6. Conclusions
References
1. 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 Nfertilizer: 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