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2016-08-23
Soil CO2 emissions under different slope
gradients and positions
in semiarid Loess Plateau of China
Shengli Guo, Zhiqi Wang, Rui Wang,Yaxian Hu
Institute of Soil and Water Conservation, CAS and MWR
Northwest Agriculture and Forestry University
THIRD CONFERENCE OF WORLD ASSOCIATION FOR SOIL AND WATER CONSERVATION
Soil CO2 emissions, as a linkage, can
have significant effects both on the
atmospheric CO2 concentration and soil
organic carbon stock.
Substantial research dedicated to soil
CO2 emissions, but mostly on flat field.
Nearly no investigation on CO2
emissions on sloping land.
Background
More than 60% of the global land areas
are slopes of gradients > 8o.
Variations in slope steepness potentially
affect soil water and heat distribution,
change soil properties and vegetation
growth, which all possibly influence soil
CO2 emissions.
(Kirkels et al., Geomorphology, 2014, 226, 94–105)
(IPCC，2013)
Background
While generally regulated by soil moisture, SOC and fine root
biomass, CO2 emissions in sloping land are particularly affected by
their spatial distribution on different slope gradients and positions.
Soil moisture significantly lower than on plains, mostly because of the
increase of runoff loss and a corresponding reduction in infiltration;
Soil moisture can also be spatially different along the slope.
SOC, as the main substrate for microbial organism, can also differ
spatially along slopes due to selective or non-selective erosion effects.
The knowledge of the effect of slope land on soil CO2 emissions is
essential for a better understanding of the global atmospheric CO2
budget and climate change.
Background
Chinese Loess Plateau:
• 640,000 km2, 80 Million population, 1.3 million
cropland.
• Ancient region of Chinese farming.
• Fragile and complex landform
• Severe soil erosion.
Objectives
In this study:
the magnitude of CO2 emissions at different slope gradients were
related to erosion induced variations of water, crop growth and SOC
across slope gradients and positions.
With the aim to investigate:
1) to compare the differences of CO2 emissions across slope gradients
and positions;
2) to evaluate the potential effects of slope differentiated water, crop
growth and SOC on CO2 emissions at an eroded slope.
Material & Methods – Exp. Design
Six slope gradients：
• 0.5° (S0.5)
• 1° (S1)
• 3° (S3)
• 5° (S5)
• 10° (S10)
• 20° (S20)
Date (yyyy/mm/dd)
2013
/10/
1
2013
/11/
1
2013
/12/
1
2014
/1/1
2014
/2/1
2014
/3/1
2014
/4/1
2014
/5/1
2014
/6/1
2014
/7/1
2014
/8/1
2014
/9/1
2014
/10/
1
2014
/11/
1
2014
/12/
1
2015
/1/1
2015
/2/1
2015
/3/1
2015
/4/1
2015
/5/1
2015
/6/1
2015
/7/1
2015
/8/1
2015
/9/1
2015
/10/
1
0
1
2
3
4
5
6
S0.5
S1.0
S3.0
S5.0
S10
S20
So
il r
es (
μm
ol
m-2
s-1)
Results – soil CO2 emission rates from six slopes
• Temporal variations over seasons
• Soil CO2 emission rates decreased with slope gradients
Results – soil annual CO2 emissions from six slopes
Slope gradients
S0.5 S1.0 S3.0 S5.0 S10 S20
An
nu
al
so
il C
O2 e
mis
sio
n (
g C
m2 y
r-1)
0
200
400
600
800
1000
Results – soil CO2 emissions on three slope positions
Upper < Middle < Bottom
S20
Date
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
S5.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Up
Middle
Bottom
Soil
CO
2 e
mis
sio
n r
ate
(μ
mo
l m
-2s-1
)
S5.0
Accu
mu
lati
ve C
O2
em
issi
on
0
10
20
30
40
50
60
S20
Date
0
10
20
30
40
50
60
Up
Middle
Bottom
Slope
S0.5 S1.0 S3.0 S5 S10 S20
Ru
no
ff (
m3
)
0
2
4
6
8
10
12
14R1
R2 R3
R4
R5
Results – soil CO2 emissions and soil moisture
• Soil annual CO2 emissions linearly
increased with soil moisture
• Soil water differentiated among
six slopes, and also spatially
redistributed across three slope
positions
More runoff,
thus less water on
steeper slopes
Water tends to
accumulate at
lower positions
Soil moisture (WFPS %)
26 28 30 32 34 36 38 40 42 44 46
200
300
400
500
600
700
800
900
1000
An
nu
al
so
il C
O2 e
mis
sio
n (
g C
m2 y
r-1)
S20
S10
S5
S3
S1.0
S0.5
S20
S10S5
S3
S1.0 S0.5
S20
S10S5
S3
S1.0
S0.5
Up
Middle
Bottom
Slope
S0.5 S1.0 S3.0 S5 S10 S20
25
30
35
40
45
50
Up
Middle
Down
So
il m
ois
ture
(W
FP
S %
)
Up
Middle
Bottom
Results – soil CO2 emissions and SOC redistribution
SOC loss (kg/ha)
0 2 4 6 8 10 12 14 16
300
400
500
600
700
800
900
An
nu
al
so
il C
O2 e
mis
sio
n (
g C
m2 y
r-1)
More runoff, thus more SOC loss
on steeper slopes
• Soil annual CO2 emissions
exponentially decreased with
SOC loss
• SOC loss differentiated among
six slopes, and also spatially
redistributed across three slope
positions
SOC (g/kg)
6 7 8 9 10 11
An
nu
al
So
il C
O2 e
mis
sio
ns
200
300
400
500
600
700
800
900
1000
Up
Middle
Bottom
Results – soil CO2 emissions and root biomass
Greater root biomass at lower positions,
potentially contributing higher CO2 emissions
Root biomass (g m-2
)
0.28 0.30 0.32 0.34 0.36 0.38
200
300
400
500
600
700
800
900
1000
Up
Middle
Bottom
An
nu
al
so
il C
O2
em
iss
ion
(g
C m
2 y
r-1)
Implications – Slope index?
On the sloping land, differences in soil CO2 emissions related to soil water, SOC and
root biomass, which resulted from runoff, SOC loss by sediments, and crop growth.
1) Clearly, y0 means the minimum soil CO2 emissions. That is to say, even at
extremely steep slope, any soils would still have a minimum soil CO2 emission.
2) Technically, by changing α and β, clearly see what they really mean.
Slope
0 5 10 15 20 25
0
200
400
600
800
1000
An
nu
al
so
il C
O2 e
mis
sio
n (
g C
m2 y
r-1)
0 5 10 15 20 25R
un
off
(m
3)
0
2
4
6
8
10
12
14
16
y=y0+αe(-βx)
α=467
β=0.29
r2=0.98
Slope
0 5 10 15 20 25
An
nu
al s
oil C
O2 e
mis
sio
n
200
400
600
800
1000
1200
Implications – Slope index?
Slope
0 5 10 15 20 25
An
nu
al s
oil C
O2 e
mis
sio
n
200
400
600
800
1000
1200
α=467
β=0.15
α=200
α=800
β=0.29
β=0.60
Does this mean, we somehow find a slope coefficient? For each soil,
is it possible to have a certain slope coefficient, such as β, to estimate
its potential CO2 emissions?
While limited by many other factors, such as soil water, temperature
and vegetation, our results still cast a new light into CO2 emissions on
sloping land. They are definitely not the same as on flat land. They
certainly have something to do with the slope gradients!
3) When α changes from 200 to 800, CO2 emissions shift up and
down, without changing the shape. That may suggest, for the same
erosion events, inherent soil properties may decide the maximum
potential of soil CO2 emissions at different slope gradients.
4) When β changes from 0.15 to 0.60, the maximum and minimum
of CO2 emissions does not change, but the decreasing rate of CO2
emissions are much greater. This may imply, for the same soil,
erosion amounts or soil loss may decide the sensitivity of soil CO2
emissions at every unit increase of slope gradient.
Acknowledgements
This work is supported by NSFC (No. 41371279), and the "Strategic Priority Research
Program-Climate Change: Carbon Budget and Related Issues" of the Chinese Academy of Sciences
(Grant No. XDA05050504).
Thank you for the attention!
Shengli Guo: [email protected]
Institute of Soil and Water Conservation, CAS and MWR
Northwest A&F University
