sustainable phosphorus use in agroecosystems: a story of global imbalance and resource recycling...
TRANSCRIPT
Sustainable phosphorus use in agroecosystems:A story of global imbalance and resource recycling
Thomas NESME & Elena BENNETT
5th Phosphorus in Soils and Plants congress,August 2014
Key messages1. At the global scale, trade of agricultural products
improves P resource use efficiency
2. But at regional scale, current P management in agroecosystems exhibits major imbalances
3. These imbalances are often due to crop and livestock segregation
4. This segregation drives major P flows and P resource displacement
Phosphorus is a key factor for crop production
Moderate P fertilisation
No P fertilisationHigh P fertilisation
At the global scale, a significant fraction of croplands is nutrient limited
Maize
Wheat
(Mueller et al., 2012)
In heavily P limited soils, small addition of P can boost crop yields
• Small addition of 10 kg P/ha/yr could increase maize yields by 12% in South America and 26% in Africa
• With N addition, this would save 29 millions ha from cropland expansion and provide food for +200 millions people
(van der Velde et al., 2013)
P losses from agricultural lands trigger algal blooms, hypoxia and water eutrophication
'Dead zones' are observed worldwide and their number has doubled since the 1960's
(Diaz and Rosenberg., 2008)
?
Toward rock phosphate depletion?
(Cordell et al., 2009)
Although controversies exist, reports converge to– Peak in global P extraction by mid-21st century– Depletion of phosphate resources before mid-22nd century– And, as a result, to predicted increase of mineral P fertiliser price
(Peñuelas et al., 2013)
As a consequence, there is a need to:
– Draw a picture of the current management of P resources in agroecosystems at the global scale
– Understand the effects of crop / livestock segregation on P resource use
– Assess the effects of agricultural product trade
Trade of agricultural products has increased dramatically over the last decades
• International trade represents nowadays ~20% of global crop production
• Trade connects countries with different P use practices
Imports Domestic
PUE
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Trade improves P resource use efficiency globally
• Crop imports are often sourced from countries with higher PUE
• At the global scale, trade of crop products may improve the use efficiency of limited P fertiliser resources
PUE = P in harvested crops /
P fertiliser applied
2007
n=8
(From Schipanski and Bennett, 2011)
But at regional scale, P budgeting exhibits major imbalances across the world croplands
(MacDonald et al., 2011)
•At the global scale, annual inputs of P fertilizer (14 Tg P) and manure (10 Tg P) exceed P removal by crops (12 Tg P), resulting in a 12 Tg P surplus in croplands
•10% of the croplands receive over 50% of the global use of both fertiliser and manure
•However, 15% of the cropland area has major deficits while 35% has major P surplus
Cumulative imbalances led to major residual soil P
• At the global scale, from 1965-2007, half of the total applied P (550 kg P/ha) was taken up by crops (225 kg P/ha)
• This resulted in massive accumulation of residual P in highly fertilised soils (e.g. in Brittany in France)
(Lemercier et al., 2008)
Residual soil P could help to reduce fertiliser P demand
• In regions with strong P accumulation, residual soil P could play a critical role to meet crop requirements
• In those regions, P application could be reduced
• Innovations are needed to better mine this residual soil P (e.g., intercropping, enhanced microbial activity)
(Sattari et al., 2012)
• In contrast, in regions with limited accumulated past P supply, residual soil P will play a small role
• In those regions, additional inputs will be required to meet crop requirements
(Sattari et al., 2012)
In regions with massive P supply, soil P is mainly anthropogenic
• Massive use of P fertiliser has increased the contribution of anthropogenic P (i.e. inherited from mineral fertiliser) vs natural origin of soil P stocks
• Case-study: modelling of the natural vs anthropogenic soil P pools for France, accounting for mineral P fertiliser use and crop-livestock recycling loop, from 1948 to 2010
Labile PNat
Stable PNat
Labile PAnt
Stable PAnt
Livestock
Fertilisers
Soil P pools
Food
Feed Manure
By 2010, ~80% of France's soil P originated from mineral fertiliser!
(Ringeval et al., 2014)
Anthropogenic signature of French
soil P pools
LP: Labile PSP: Stable P
Years
The uneven distribution of mineral fertilisers explains part of the soil P imbalances
Fertiliser inputs exceed crop P requirements in 45% of the world croplands
(Potter et al., 2010)
But manure supply also drives soil P imbalances
Manure inputs exceed crop P requirements in 25% of the world croplands
(Potter et al., 2010)
Manure P surpluses result from the uneven distribution of livestock animals
Cattle
Chickens
Pigs
(Robinson et al., 2014)
Crop P fertiliser use efficiency = 66%
Livestock feed P use efficiency = 8%
Livestock feed P100 g
Livestock product P8 g
92 g
Fertiliser P100 g Crop P
66 g
34 g
Manure P surpluses also result from the low P use efficiency of livestock production
Crop and livestock segregation is a key driver of soil P imbalances
• Livestock production systems are increasingly specialised and spatially segregated from arable production systems
• This segregation generates– Large feed imports and soil P
surplus in regions of livestock production
– Limited manure supply and large mineral P fertiliser use in regions of arable production
(Gaigné et al., 2012)
Pig density in France in 2010
Urban areaArable landPermanent cropGrasslandMixed cropsForestNatural pasturePeatlandWater
Legend
Livestock district1.2 LU/ha(n=21)
Arable district0.2 LU/ha(n=25)
Mixed district0.6 LU/ha (n=17)
Crop / livestock segregation limits the P resource recycling
Specialisedarable
Mixed Specialisedlivestock
Surveyed farm Other farm Material flow Cycling pattern
(Nowak et al., subm)
Local autonomy (%)
Cycling index (%)
Specialised arable 39 0
Mixed 52 20
Specialised livestock 13 0
Material exchanges are more important in mixed districts
Crop / livestock segregation structures P flows at regional scale
Centre region•Livestock density: 0.3 LU/ha•Arable crops: 65% of UAA
• Balanced soil P inputs and outputs (+1 kg P/ha/yr)
• Large use of mineral P fertiliser (13 kg P/ha/yr)
(Senthilkumar et al., 2012)
Other inputs1.9
Fertilizer12.9
Erosion1.8
Animal products1
Feed2.3
Crop products13.4
Fodder2.8
Crop uptake
20.2
Crop residue4
Animal Excretion
4.2
Animals 5.1 (-0.6)
Crops20.2 (0)
Soils23.1 (1.1)
Brittany region•Livestock density: 2.1 LU/ha•Arable crops: 6% of UAA
(Gaigné et al., 2012)
Dairy cows
PoultryPigs
• Soil inputs >> outputs highly positive soil P budget (+ 19 kg P/ha/yr)
• Animal feed represents 75% of total P inputs. Even without mineral P fertiliser, the soil P budget would remain highly positive
• Animal manure spreading on soils can hardly be qualified as P recycling
(Senthilkumar et al., 2012)
Other inputs2
Fertilizer7.9
Erosion1.7
Animal products12.3
Feed28.9
Crop products7.1
Fodder23.5
Crop uptake
21.8
Crop residue3.3
Animal Excretion
29.1
Animals 40.2 (-1.2)
Crops21.8 (0)
Soils42.3 (18.9)
Crop / livestock segregation drives mineral P fertiliser use in arable regions
(Nesme et al., subm)
Proxy of crop / livestock segregationVariation coefficient of the stocking rate at department scale (%)
Similar patterns of soil P accumulation in livestock regions exist worldwide…
(Gerber et al., 2005)
Poultry density Soil P balance
The crop / livestock segregation drives global P resource displacement
International food/feed trade among countries increased dramatically in the past decades– P trade flows increased from 0.4 Tg in 1961 to 3.0 Tg
in 2011 (x7 increase)
– In 2011, 20% of the global crop production was traded
– In 2011, P trade flows were equivalent to 17% of global P fertiliser use
International P flows are driven by soybean and cereal trade
Trade P flows(Tg P/yr)
Years (Nesme et al., in prep)
For some countries, P imports through trade provide large amounts of P resources
P import through trade as % of domestic P fertiliser use(Nesme et al., in prep)
P flows among world regions in 2011 (in Tg P/yr)
Trade P flows interconnect world
regions
(Nesme et al., in prep)
Conclusion
Take home message1. At the global scale, trade of agricultural products
improves P resource use efficiency
2. But at regional scale, current P management in agroecosystems exhibits major imbalances
3. These imbalances are often due to crop and livestock segregation
4. This segregation drives major P resource displacement at the global scale
Solutions?
• The multi-faceted P issues call for solutions adapted to different contexts– Increased mineral P inputs in soils with low P status– Reduced P losses to water bodies from soils with high
P status– Increased P resource recycling everywhere
• The global interconnections and regional inefficiencies call for integrated approaches across the world
However, in the long term P resource recycling in agroecosystems should be a priority
A range of different options should be explored– P mining from residual soil P– Reduced P losses from agricultural soils– P recovery from rich streams (e.g. struvite production
from urban wastes)– Agriculture redesign towards more integrated crop-
livestock farming systems… with synergies for other environmental issues (e.g., biodiversity, soil erosion, animal diseases)
A 5R strategy should be deployed and adapted to the different P contexts
(from Withers et al., subm)
Struvite production?• Which sources for struvite production at the global scale?
– Total annual P production in manure = 20-30 Mt P/yr (of which a large fraction is probably already recycled)
– Total annual P production in waste-water = 3-5 Mt P/yr (of which 30-40% is already recycled to Ag soils)
– Compared to total annual use of mineral P fertiliser = 15-20 Mt/yr
• Some technical issues to be overcome– Organic effluents have low (<10 mg P/L) and variable P content– Struvite production exhibits high energy and economic costs
• Struvite production costs: 6800 US $/t P• Mineral P fertiliser price: 2000 US $/t P
– Most countries lack of proper regulation framework
• Struvite production could solve part of the P problem but does not account for the other consequences of crop/livestock segregation (e.g., short crop rotations, pest and disease propagation, etc.)