You want to have your carbon cake and eat it too??? For .../media/Files... · Dilemma •Soil C:N...
Transcript of You want to have your carbon cake and eat it too??? For .../media/Files... · Dilemma •Soil C:N...
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You want to have your carbon
cake and eat it too???
For nitrogens sake!
Dr Susan Orgill and Dr Mark Conyers
Wagga Wagga Agricultural Institute
NSW Department of Primary Industries
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Sacred benefits of pasture rotations
• Fix N and build soil C (= Soil Organic Matter)
• Improve soil structure
• Enhance retention of soil nutrients
• Sequester C
• Good crops need lots of N
• This comes from the soil N pool and fertiliser
It is inevitable that soil N mineralises
But
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Dilemma
• Soil C:N varies only ~10:1 to 14:1 in surface soil
• The provision of soil N comes with a loss of soil C
• Therefore you cannot sequester C and have cheap N
too
Solution
There is a build up of C&N during the pasture phase and
a run down in the cropping phase.
Best we can do is:
1. Avoid a net rundown in SOM (C&N) overtime
2. Build enough C&N to minimise the need for N
fertiliser during the cropping phase
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Lets look at some examples
Pasture phase
Cropping phase
Pasture phasePasture phase
Cropping phase Cropping phase
SOM
(Carbon &
Nitrogen)
Time
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KohnRot 1963-1980 (0-30 cm depth; 540 mm)
• WWAI, Kandosol
• Initial SOC was low 0.8% (0–10 cm W&B)
• Pastures dominated by subclover and annual ryegrass
• Long (6 years) or short (2 or 3 years) rotations
• Short-fallow cultivation (multiple passes), stubble burnt
in autumn
• P was applied, 12 kg/ha/yr
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Rotation C kg/ha/yr
33% pasture 67%wheat +21 6
50% 50% +253
67% 33% +403
(1 00%) (0%) (+570)
KohnRot 1963-1980 (0-30 cm depth; 540 mm)
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Source: Helyar et al AJAR 1997
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SATWAGL 1979-2004(Sustainable Ag Through Wheat and Grain Legumes)
0-30 cm depth (540 mm)
• WWAI, Kandosol, ‘high’ initial SOC
• 3 rotations i.e. 1:1 lupin/wheat (L/W), 1:1 subclover/wheat
(S/W) and continuous wheat (W/W)
• Treatments:
• Stubble retained (SR) / burnt (SB)
• No-till (NT) and conventional cultivation (CC; 3 passes)
• Each plot received P at a sowing at rate of 20 kg/ha/year
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SATWAGL 1979-2004(Sustainable Ag Through Wheat and Grain Legumes)
0-30 cm depth (540 mm)
Rotation Stubble Tillage C kg/ha/yr
L W Retain nil -52
L W Burn nil -98
LW Retain CC -1 74
LW Burn CC -1 76
WW +N Burn CC -1 93
WW Burn CC -278
S W Retain nil +257
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Source: Heenan et al Soil & Tillage Research 2004
L/W – SR - NT L/W – SB - NTL/W – SR - CC
L/W – SB - CC W/W+N – SB - CCW/W – SB - CC
S/W – SR - CC S/W – SR - CCS/W – SR - NT
Grazed Mulched Mulched
Circles: Carbon Triangles: Nitrogen
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C run down…
Source: Heenan et al Soil & Tillage Research 2004
Treatment C kg /ha/yr.30cm Standard error t-value
L/W – SR - NT - 8 50 ns
L/W – SR - CC - 199 50 ***
L/W – SB - NT - 138 50 **
L/W – SB - CC - 284 49 ***
W/W – SB - CC - 389 48 ***
W/W+N – SB - CC - 311 49 ***
S/W – SR – CC (G) -72 51 ns
S/W – SR – NT (M) 185 50 ***
S/W – SR – CC (M) - 4 50 ns
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And the N run down…
Source: Heenan et al Soil & Tillage Research 2004
Treatment N kg /ha/yr.30cm Standard error t-value
L/W – SR - NT - 13 7.3 ns
L/W – SR - CC - 29 7.3 ***
L/W – SB - NT - 28 7.3 ***
L/W – SB - CC - 42 7.3 ***
W/W – SB - CC - 51 7.2 ***
W/W+N – SB - CC - 42 7.3 ***
S/W – SR – CC (G) -13 7.4 ns
S/W – SR – NT (M) 9 7.3 ns
S/W – SR – CC (M) - 6 7.3 ns
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MASTER 1992-2010(Managing Acid Soils through Efficient Rotations)
0-30 cm depth
• Book Book, Acidic Sodosol
• Pastures estab 1992; perennial (phalaris and cocksfoot)
and annual (annual rye and subclover)
• With (incorporated) or without lime
• Plots limed 6-yearly (target pHCa 5.5, 0-10cm)
• Maintenance lime top-dressed
• Fertiliser applied annually
• Site was grazed
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MASTER 1992-20100-30 cm depth
Rotation C kg/ha/yr
Perennial pasture - no lime +499
Annual pasture - no lime +496
Perennials - limed +552
Annuals - limed +462
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Source: Chan et al Soil Research 2011
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EverCrop Carbon Plus
• Yerong Creek, Red Kandosol
• Treatments:
• Rotation: 3yrs pasture phase then 3yrs crop (wheat,
canola, barley)
• Pasture type: Phalaris/subclover,
Cocksfoot/subclover, Lucerne/subclover and
Chicory/subclover vs continuous crop
• Sowing year: 2008, 2009 and 2010
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EverCrop Carbon Plus
CC - 7 years crop
P - 4 years crop
CC - 6 years crop
P - 3 years crop
CC - 6 years crop
P - 2 year crop
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Tota
l C
%
Depth (cm)
Carbon
Black: 2013; Red: 2014
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Nitrogen
Black: 2013; Red: 2014
Depth (cm)
Tota
l N
%
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Take home messages
• C sequestration is slow in our soils (-300 to +550 kg C/ha.30cm/yr)
• Where soil is already at >1% SOC, max rates under cropping are
about zero kg C/ha.30cm/yr
• Crop residue retention does not provide increases in SOM (C&N) in
such soils (but pastures may)
• Productivity and environmental benefits from SOM accumulation are
sacred
• But benefits to soil fertility stem from SOM being dynamic in the soil,
not from continuously accumulating.
• Need to either use SOM-N during the cropping phase or use
fertiliser-N
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New upper limit
defined by WLY, clay
content etc
Pasture phase
Cropping phase
Pasture phasePasture phase
Cropping phase Cropping phase
SOM
(Carbon &
Nitrogen)
Time
You want to have your carbon cake and eat it
too??? For nitrogens sake!
Dr Susan Orgill
Wagga Wagga Agricultural Institute
M: 0428 424 566