land management impact on soil organic carbon stocks – what do we really know?
TRANSCRIPT
GLOBAL SYMPOSIUM ON SOCMarch 2017
Land management impact on soil organic carbon stocks – what do we really know?Axel Don
Thünen Institute of Climate-Smart Agriculture, Braunschweig, Germany
Axel Don
73% of the land surface is managed land
Luyssaert et al. 2014, Nature climate change
Fraction of plant cover conversion as measure for land-use change and land-management intensity
Axel Don
Land-use change vs. land management
Grassland WetlandForestCroplandLand-use
Land management
Land-use change
pasture hay meadow
conv. tillage
no tillage
Axel Don
Land-use changes in Europe Land-use changes: 25% of EU since 1900. >15% of EU since 1950.
Fuchs et al. 2013, Biogeosciences
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0 20 40 60 80 100 120
0
50
100
150
200
age
chan
ge
0 50 100 150 200
-80
-60
-40
-20
0
age
chan
ge
Cropland to grasslandGrassland to cropland
N = 89N = 176
Cropland to grassland and vice versa
Slow in fast out
Conversion of cropland to grassland has long-term potential to sequester carbon
Time since conversion [years] Time since conversion [years]
Rela
tive
SOC
stoc
k ch
ange
[%]
Rela
tive
SOC
stoc
k ch
ange
[%]
Poeplau, Don et al. 2011, GCB
Axel Don
50 100 150 200
-20
0
20
40
60
80
100
age
chan
ge
0 50 100 150 200
-50
0
50
100
age
chan
ge
N = 100
N = 45
with forest floor
Rela
tive
SOC
stoc
k ch
ange
[%]
Rela
tive
SOC
stoc
k ch
ange
[%]
Time since conversion [years] Time since conversion [years]
Afforestation of grasslands causes a transitional C loss
SOC sequestration only in the forest floor Poeplau, Don et al. 2011, GCB
Afforestation of grassland
only mineral soil
Axel Don
0 50 100 150
0
50
100
150
200
age
chan
ge
0 20 40 60 80 100 120
0
50
100
150
200
age
chan
ge
N = 70 N = 59
Rela
tive
SOC
stoc
k ch
ange
[%]
Rela
tive
SOC
stoc
k ch
ange
[%]
Time since conversion [years] Time since conversion [years]Poeplau, Don et al. 2011, GCB
Afforestation of cropland
30 % of soil carbon is sequestered in the forest floor
Axel Don
C-in
put
C-ou
tputor
NPP,Harvest,Fire
DecompostionMircrobial activityC-stabilisation
Controls on soil organic carbon
Axel Don
Sampling depth was often too shallow
Missing data on bulk density
Missing mass correction
Ploughing depth
No-tillageConventionaltillage Temperate zone:
61% North American studies35% European studies
In total 116 studies
Minimum duration of tillage treatment: 5 years
Reducing C-mineralisation with no tillage ?
Soil carbon
Soil depth
Axel Don
0 10 20 30 40
-4-2
02
x
y
0 10 20 30 40
-0.5
0.0
0.5
1.0
1.5
2.0
x
y
On average +0.15 ± 0.11 Mg C ha-1 yr-1
N=86, not significant
No-tillage Minimum tillage
On average +0.16 ± 0.10 Mg C ha-1 yr-1
N=36, not significant
SO
C se
ques
trati
on [M
g C
ha-1
yr-1
]
SO
C se
ques
trati
on [M
g C
ha-1
yr-1
]Duration of treatment [years] Duration of treatment [years]
Soil carbon sequestration with low tillage
Axel Don
Gree
nhou
se g
as b
alan
ce [k
g CO
2 eq
. ha-1
yr-1
]
-1500
-1250
-1000
-750
-500
-250
0
250
500
750
1000
No-tillage
Minimum tillage
SOC
N2ODiesel
HerbicideTotal
Field greenhouse gas balance low tillage
Soil carbon effect is offset by more N2O emissions
More C-input, e.g. with cover crops
Cover crops are grown during winter season
Grown at around 5% of all croplands in the EU/North America
We estimated, that 25 % of the agricultural area worldwide could be used for cover crop cultivation.
Aiming at reducing nitrate leaching, less erosion
Meta-analysis of 37 world wide studies (139 pairs of cover crops vs. no cover crops).
Poeplau and Don 2015, Agric. Ecosys. & Environ.
Cover crops increased SOC stocks by 0.32±0.08 Mg C ha 1‐ yr 1‐ .
We predicted an average SOC stock accumulation at equilibrium of 16.7 ±1.5 Mg C ha 1‐ .
Cover crop effect on soil carbon stocks
Axel Don
Human appropriated fraction of NPP
Haberl et al. 2007, PNAS
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Conclusions
Soil carbon accumulation is slow – carbon loss can be fast
No tillage in the temperate zone seems to be no climate mitigation measure
More C input is the main control on soil carbon
More NPP (e.g. via cover crops) can enhance soil carbon
Thank you for your attention!
Axel Don
0 10 20 30 40
-20
020
4060
80
x
y
0 10 20 30 40
-40
-20
020
x
y
+6.5 ± 2 % relative SOC stock change
No-tillage Minimum tillage
+5.6 ± 3 % relative SOC stock change
Rela
tive
SOC
chan
ge [%
]
Rela
tive
SOC
chan
ge [%
]Duration of treatment [years] Duration of treatment [years]
IPCC default
IPCC default
Low tillage effects on soil carbon
Don and Poeplau, in prep.
Axel Don
0
20
40
60
80
100
0
0.2
0.4
0.6
0.8
1.2
1.0
No-tillage Minimum tillage
addi
tiona
lN2O
[kg
N 2O-N
ha-1] N
2 Oem
issionincrease
N=35 sites
+86% ± 26 +63% ± 31
N=9 sites
44 sites65% from North America27% from Europe
Average measurement duration: 9 months
Average measurement frequency: every 10th day
Nitrous oxide (N2O) emissions
Don and Poeplau, in prep.