Using a bioenergy crop to partition soil respiration fluxes through stable isotope measurements over 4 years.

1. ANDY ROBERTSON1,2,3, PETE SMITH2, CHRISTIAN DAVIES3, ANDY STOTT1,
HELEN GRANT1 AND NIALL MCNAMARA1
1 - NERC CENTRE FOR ECOLOGY & HYDROLOGY, LANCASTER, UK
2 - SCHOOL OF BIOLOGICAL SCIENCES, UNIVERSITY OF ABERDEEN, UK
3 - SHELL INTERNATIONAL EXPLORATION AND PRODUCTION INC.
Partitioning soil respiration fluxes
The relative importance of temperature,
moisture content and plant phenology

2.  Soils contain huge stores of carbon and every year
~60 billion tonnes is cycled through this pool
Why care about soil respiration?

3.  Soils contain huge stores of carbon and every year
~60 billion tonnes is cycled through this pool
 Climatic conditions (mainly temperature and
moisture) are primary drivers of soil respiration
Why care about soil respiration?
Wood et al., 2013. PlosOne 8(12): e80965Lloyd and Taylor, 1994. Functional Ecology 8: 315-323

4.  Soils contain huge stores of carbon and every year
~60 billion tonnes is cycled through this pool
 Climatic conditions (mainly temperature and
moisture) are primary drivers of soil respiration
 Increases in atmospheric CO2 concentrations are the
primary cause of climate change
 Changes in climatic conditions may cause a positive feedback
effect, driving ever-increasing soil respiration rates
Why care about soil respiration?

5.  Soils also have the potential to store huge amounts of
carbon, not just emit it!
Should we give up and move to Mars?
DOE(1999) Carbon
Sequestration
Research and
Development

6.  Soils also have the potential to store huge amounts of
carbon, not just emit it!
 If we add more carbon to soils than is emitted
through soil respiration we have net sequestration
Should we give up and move to Mars?

7.  Soils also have the potential to store huge amounts of
carbon, not just emit it!
 If we add more carbon to soils than is emitted
through soil respiration we have net sequestration
 Not all climate change scenarios will necessarily
increase soil respiration
Should we give up and move to Mars?

8.  Soils also have the potential to store huge amounts of
carbon, not just emit it!
 If we add more carbon to soils than is emitted
through soil respiration we have net sequestration
 Not all climate change scenarios will necessarily
increase soil respiration
 If chosen correctly, land use (i.e. vegetation type) can
help limit soil respiration
Should we give up and move to Mars?

9.  How will soil respiration respond to changes in
climatic conditions?
 Do emissions come primarily from soil, or from
living and dying vegetation?
 What vegetation choice is best…and where?
What do we want to know?

10.  Soil respiration has considerable seasonal variation
 Emissions vary a lot (particularly in temperate regions)
What do we need to know?
Raich et al., 2003. Interannual Variability in Global Soil Respiration on a 0.5 Degree Grid Cell (1980-1994)

11.  Soil respiration has considerable seasonal variation
 Emissions vary a lot (particularly in temperate regions)
 Soil respiration comes from three main sources
 Each source has different responses to climatic conditions
What do we need to know?

12.  Soil respiration has considerable seasonal variation
 Emissions vary a lot (particularly in temperate regions)
 Soil respiration comes from three main sources
 Each source has different responses to climatic conditions
 Splitting soil respiration into discrete sources is hard
 Physically separating sources in a lab removes important
interactions and in-situ effects
What do we need to know?
Measure CO2, temperature
and moisture throughout
the year – all in situ
Measure soil respiration
efflux in a way that can
partition it into sources
Use experimental design
that can associate temp and
moist with discrete sources

13. Monthly CO2
measurements
(Mar’09 – Mar’13)
Monthly temperature and
moisture measurements
(Mar’09 – Mar’13)
Regular estimates of litter
and soil carbon pools
(Mar’09 – Mar’13)
 Miscanthus – bioenergy crop
 Perennial – C4 – 10 t/ha/yr – 20 yrs high yields– no fertiliser
The experimental system

14.  Miscanthus – bioenergy crop
 Perennial – C4 – 10 t/ha/yr – 20 yrs high yields– no fertiliser
 Input manipulation experiment to isolate three
sources – roots, aboveground litter and soil
The experimental system

15.  Miscanthus – bioenergy crop
 Perennial – C4 – 10 t/ha/yr – 20 yrs high yields– no fertiliser
 Input manipulation experiment to isolate three
sources – roots, aboveground litter and soil
The experimental system

16. Monthly CO2
measurements
(Mar’09 – Mar’13)
Monthly temperature and
moisture measurements
(Mar’09 – Mar’13)
Regular estimates of litter
and soil carbon pools
(Mar’09 – Mar’13)
 Miscanthus – bioenergy crop
 Perennial – C4 – 10 t/ha/yr – 20 yrs high yields– no fertiliser
 Input manipulation experiment to isolate three
sources – roots, aboveground litter and soil
 Miscanthus planted in 2006, treatments from 2008
The experimental system

17. Soil respiration by source
Soil = 1.19 tC · ha-1 · yr-1
Roots = 0.93 tC · ha-1 · yr-1
Litter = 0.79 tC · ha-1 · yr-1
Emissions from soil dominates total soil respiration but litter/roots still emit lots

18. Carbon pools and fluxes
Dormant phase

19. Carbon pools and fluxes
Emergent phase

20. Carbon pools and fluxes
Growth phase

21. Relationships to soil temperature
Higher temperatures, higher soil respiration – all treatments show same trends
Dormant Emergent Growth
Soil temperature (°C)

22. Relationships to soil moisture
Lower soil moisture, higher soil respiration – no real change for plots with no roots
Dormant Emergent Growth
Volumetric soil moisture (%)

23. Source Temperature Moisture
Total respiration 52.4 % 28.3 %
Soil only 52.7 % 14.4 %
Litter only 14.8 % 7.3 %
Roots only 25.8 % 32.7 %
Variance explained by key drivers
 How much of the variation in soil, root and litter
respiration can be explained by soil temperature and
soil moisture?
 Use linear mixed effects models and determine r2

24. Variance explained by key drivers
 How much of the variation in soil, root and litter
respiration can be explained by soil temperature and
soil moisture?
 Use linear mixed effects models and determine r2
 What about when plant impact is lessened?
Source Temp Moist Temp Moist Temp Moist
Total
respiration
29.7 8.4 19.2 2.8 14.4 18.1
Soil only 29.9 2.0 27.0 1.8 31.8 0.2
Litter only 3.5 0.7 4.8 0.7 5.2 5.7
Roots only 0.6 18.3 17.3 9.2 2.8 22.9
Dormant Emergent Growth

25. What have we learnt?
 How will soil respiration respond to changes in climatic
conditions?
 Different sources of soil respiration will respond differently
 Do emissions come primarily from soil, or from living and
dying vegetation?
 Soil is the main source but litter and roots also emit quite a lot
 What vegetation choice is best…and where?
 Not sure – use models to predict what land use would emit lease CO2
 Are there management options that can increase
sequestration and decrease soil respiration?

26. The big picture again!
 If relative contributions of soil, roots and litter to
total soil respiration are typical:
 Emissions from litter account for very large efflux
 Models need to relate different sources to temp and moist
 Crop phenology interacts with temperature and moisture to
play a big role in root respiration

27. THANK YOU!
Thank you to everyone who has helped
throughout my PhD
And thank you for listening
Any questions?

28. Carbon pools and fluxes
Soil carbon dominates the total stock in top 15 cm

29. Carbon pools and fluxes
Belowground biomass (roots) and plant litter accumulate over time

30. Miscanthus lifecycle
 Miscanthus – bioenergy crop in EU and N.America
 Perennial – C4 – 20 yrs high yields – 10 t/ha/yr – no fertiliser
April
June August
March
December
October
February
20 yrs of high productivity

31. What about all the variables together?
 Use linear mixed effects model to get an idea of how
much variance is explained by all measured drivers
 Global model:
 CO2 = (Temp * Moist * Phase ) + sampling time + radiation
Source Variance explained (%)
Total respiration 67.6
Soil only 69.6
Litter only 12.0
Roots only 48.4

32. Tang et al., 2006. J. Integrative Plant Biology 48(6): 654-663
IPCC (2013) AR5:
The Physical Science
Basis