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The potential value of legumes in farming systems
What role for grazing livestock in a world of climate change and diet-related disease?
Food Trust. Bristol, 3 February 2015
Bob Rees
222
Legumes and farming
ForageCover cropRotationalIntercrop
333
Background and context ….
• Lack of forage protein (big imports)
• Increasing costs for energy and N-fertiliser
• Emissions from agriculture
• Restrictions in resource input
• Growing demand for meat and milk
444
Global grain legume production
FAOSTAT 2013
555
Internationally traded reactivenitrogen
A
B
C
Fertilizer (31
Tg)
Grain (12 Tg)
Meat (0.8 Tg)Galloway, Erisman, Sutton et al.
Science (2008)
666
Estimated BNF in EU27 in 2009
Stoddard et al, 2014
777
The Totals
BNF (kt)
Grasslands
Temporary 173
Extensive 114
Intensive 305
Total Grassland 592
Grain legumes 247
Total EU27 839
Fertiliser value at 0.9€ kgN-1 €755M
Synthetic fertiliser application (2000) 11,200 kt N
Stoddard et al, 2014
888
Soil nitrogen
pools
Grass-
clover
ley
Spring
cereal
(undersown
)Root
crop
Spring
cereal
Managing soil fertility?
•More cultivation
•Fewer worms
•Less dense root systems
•Less cultivation
•More worms
•Denser root systems
999
N fixation by grass/clover leys
N Fixation
kg/ha
0
5
10
15
20
25
22 May -
June 29
19 June -
July 27
17 July -
Aug 24
Aug 14 -
Sept 21
Sept 11 -
Oct 19
Period of measurement
1-yr-old (T)
2-yr-old (T)
3-yr-old (T)
4-yr-old (T)
1-yr-old (A)
Rees et al, 2004
101010
Legumes and nitrous oxide emissions
Root and
nodule turnover
N2O
emission
• Rhizobium sp can
produce N2O
directly
• Decomposition of
plant residues can
increase N2O
emissions
Two mechanisms:
111111
Legumes produce lower nitrous oxide emissions?
•Lower emissions of N2O
where legumes form a part or
all of the rotation
•Legume crops receive lower
fertiliser N inputs
•But residues can increase
emissions
Stehfest and Bouwman 2006
121212
Nitrous oxide emissions from grasslands; Crichton, SW Scotland
Recommended fertiliser rate in red
Bell et al, 2015
0
1
2
3
4
5
6
7
8
9
10
Control AN 80 AN 160 AN 240 AN 320 AN 400
An
nu
al
Cu
mu
lati
ve f
lux
(kg
N2O
-N h
a-1
)
0.0
0.5
1.0
1.5
2.0
2.5
AN 80 AN 160 AN 240 AN 320 AN 400
EF
(%
)
0
2
4
6
8
10
12
14
Control AN 80 AN 160 AN 240 AN 320AN 400T
ota
l D
M y
ield
(t
ha
-1)
131313
Why do legumes release less N2O than non legumes?
141414
Nodules and N2O
Itakura et al 2012, Nature Climate Change
151515
N2O emission from nosZ++ and nosZ+:
lower than native, throughout the cultivation
period
Affects of Rhizobium species on N2O emissions
Itakura et al 2012, Nature Climate Change
161616
Crop residues
• A highly uncertain component of the agricultural
greenhouse gas inventory
• Emissions assumed to represent 1% of N
contained in residue inputs
• Difficult to assess inputs and emissions associated
with them
• Likely to be opportunities for mitigation
171717
N-fertiliser replacing potential
• Well balanced mixtures at N50 can be as productive as grass monocultures at N450
N50
N150
N450
Nyfeler et al. (2009)
181818
Industry, Haber-Bosch Symbiosis, Sun-Energy
For production of
1 kg fertiliser-N- 2 l oil
- 2.25 kg CO2
- 9.8 g N2O
=> 8.6 kg CO2-equivalents
Symbiotic N2 Fixation (the) key of legumes
N2
Nitrogen
(Amino acids)
Energy
(Carbohydrates)
Luscher,A., Mueller-Harvey,I., Soussana,J.F., Rees,R.M. & Peyraud,J.L. 2014.
Potential of legume-based grassland-livestock systems in Europe. Grass and Forage Science, 69, 206-228.
191919
Summary
• Legumes play an important role in supporting
agricultural production
• CO2e emissions per unit of N input associated with
biological fixation are lower as a result of avoided
manufacturing and more efficient utilisation
• Optimised design offers opportunities to improve N
use efficiency in forage systems
• Uncertainties remain regarding the contribution of
residues to N2O emission and the role of N fixation
in controlling C sequestration
202020
Acknowledgements
Funding provided by Scottish Government, DEFRA
and the EU
