Introduction to the science of agricultural emissions and sinks

This presentation provides participants with a basic understanding of the soil, plant and animal carbon and nitrogen cycles and explains the science behind agriculture emissions and how these affect the atmosphere and the role of sinks in the global carbon budget

Source:University of Melbourne, UoM June 2013

The Carbon cycle

http://www.davidsuzuki.org/files/CCcarboncycle.GIF

C-stocks in Pg (Gt), C-fluxes in Pg yr-1; Pg = 1015g = 1 Gt (gigatonne)

• More carbon emissions than carbon uptake fossil fuel emissions

• Increase carbon sinks – increase terrestrial plant or soil sinks

• Large carbon pools, relatively small fluxes between pools

Global forest distribution

www.fao.org/forestry

Sources of global CO2 emissions

12% of total anthropogenic

emissionsLe Quéré et al. 2009, Nature-Geoscience; Data: CDIAC, FAO, Woods Hole Research Center 2009

Slide courtesy of J. Canadell, Global Carbon Project

Fossil fuel

Land use change

10

8

6

4

2

1960 20101970 1990 20001980

CO2 e

miss

ions

(PgC

y-1)

Carbon stocks and sequestration

Carbon stock/pools

Carbon sequestration

How much C at one point in time

Change of C stock over time

Aboveground biomassLeaves, stem, branches

Below ground biomassCoarse roots, fine roots, microbes

Soil carbonStable and labile fractions

Litter & coarse woody debris

NPP

Net primary productivity

Tropical forest are the most productive

Forests produce most of the terrestrial carbon

Saugier (2001) IN: Terrestrial Global Productivity

NP

P (

t C

ha-1

yr-1

)

-2

0

2

4

6

8

10

12

14

NPP blowground

NPP aboveground

0.5 2.6 8.1 21.9 14.9 7.0 3.5 4.1 NPP Pg C yr -1

Crops produce mainly aboveground NPP consequences for soil C

belowgroundaboveground

The Carbon cycle

• Human activity greatly influences the global C cycle

• The sink capacity of natural CO2 sinks is decreasing, leading to increased atmospheric CO2

• Forest ecosystems are the greatest carbon sink in the

terrestrial biosphere

• Globally, soils store more C than biomass

• The capacity of an ecosystem to store C is determined by the balance of C uptake (photosynthesis) and C loss (respiration)

Grazing system carbon cycle

• The carbon cycle in animal production systems and the various pools of carbon

• Fluxes between carbon pools • Methane, carbohydrate, carbon dioxide

Grazing system carbon cycle

• Terms used:– Hydrogen (H2), Carbon (C), Oxygen (O2),

Nitrogen (N)– Methane (CH4), carbohydrate (CHO),

carbon dioxide (CO2), water (H2O), bicarbonate (HCO3)

– Nitrate (NO3-), Nitrite (NO2

-), Ammonia (NH3), Ammonium (NH4

+), nitrous oxide (N2O)

Grazing system carbon cycle

• Atmosphere• Animal• Plant• Soil

Photosynthesis

CO2 in atmosphere

Consumption

Respiration

Mineralisation Crop and animal Residues

Carbon in soilOrganic matter

Decomposition inmicrobes

Humus and aggregate

formulation

Eckard 2011

Grazing system carbon cycle

• Carbon into plants– Photosynthesis

• CO2 + H2O + chlorophyll + solar energy -> CHO + O2

• Structural carbohydrate– Cellulose, hemi-cellulose, lignin

• Non-structural carbohydrate– Sugars

• Fats/ Lipids• Protein/amino acids

• Carbon from plants– Respiration

• Burning sugars - energy for growth• O2 + CHO -> H2O + CO2 + energy

– Decay of plant residues• Mineralisation to CO2

Photosynthesis

CO2 in atmosphere

Consumption

Respiration

Mineralisation Crop and animal Residues

Carbon in soilOrganic matter

Decomposition inmicrobes

Humus and aggregate

formulation

Grazing system carbon cycle

Photosynthesis Respiration

Produces sugars from energy Burns sugars for energy

Energy is stored Energy is released

Occurs only in cells with chloroplasts Occurs in most cells

Oxygen is produced Oxygen is used

Water is used Water is produced

Carbon dioxide is used Carbon dioxide produced

Requires light Occurs in dark and light

Comparison of Photosynthesis & Respiration

Grazing system carbon cycle

• Carbon into the animal• Structural carbohydrate

– Cellulose, hemi-cellulose, lignin

• Non-structural carbohydrate– Sugars

• Fats/ Lipids• Protein/amino acids

• Carbon from the animal• As above in products (meat, milk, fibre)• CH4 and CO2 – microbial digestion and

respiration

Photosynthesis

CO2 in atmosphere

Consumption

Respiration

Mineralisation Crop and animal Residues

Carbon in soilOrganic matter

Decomposition inmicrobes

Humus and aggregate

formulation

Grazing system carbon cycle

The Rumen Simple stomach

Grazing system carbon cycle

• Fluxes between carbon pools– Into soil

• Plant and animal residues• Microbes

– From soil• Microbial decomposition• Organic Carbon to CO2 and CH4

Baldock 2009

Grazing system carbon cycle

Humus (~stable)<0.053 mm

RespirationPhotosynthesis

CHO

Plant ResiduesLitter > 2mm

POC (~labile)> 0.053 mm

Carbon dioxide (CO2)

DecompositionMicrobial Biomass

MineralisationMethane (CH4)Nitrous Oxide

(N2O)

Baldock et al. 2009; Eckard, 2009

Nutrients

In established pastures Photosynthesis ≈ Respiration + Mineralisation Photosynthesis ≠ Respiration + Mineralisation + Methane

Decay

The nitrogen cycle in agricultural systems

• The nitrogen cycle in agricultural systems• Fluxes between nitrogen pools• Forms of nitrogen and their fate

The nitrogen cycle

Whitehead 1995

The nitrogen cycle

• Atmosphere– N2 (nitrogen)

• 78% of the atmosphere– O2 = 21%, CO2 = 0.04%

• Fixed by legumes into plants and soil

– N2O (nitrous oxide)• 0.32 ppm (0.00003%)

Eckard 2011

The nitrogen cycle

• Main forms of soil & plant N– NH3 – Ammonia

• Organic matter• Fertilisers

– Urea, DAP, UAN etc.

• Major source of plant N

Eckard 2011

oxygen

amide ionamide ion

The nitrogen cycle

• Main forms of soil & plant N– NH4

+ – Ammonium• Soil solution• Loosely bound on cation exchange

– Positive charge attached to clay» Exchangeable » Clay-fixed (non-exchangeable)» Does not readily leach

• Major source of plant N (nitrogen)– Preferential uptake in colder, wetter soils

• Rapidly converts to NO3- (nitrate ion)

– In warm, well-drained soils

Eckard 2011

The nitrogen cycle

• Main forms of soil & plant N– NO3 (nitrate ion)

• Major source of plant nutrition– Drier soils

• Accumulates in some plants– e.g. Brassicas, annual ryegrass, kikuyu, cereal grains– Breaks down to NO2 in rumen – toxicity

• Soluble in water – leaches

– NO2 (nitrite ion)• Transient in plants and soils• Main form of toxicity in ruminants

Eckard 2011

The nitrogen cycle

• Main forms of soil & plant N– Soil organic matter N

• Decomposed residues– Amides, proteins etc

• Microorganisms (microbial biomass)• C: N ratio

– Usually 10:1 to 40:1

• Major source of plant N– Through mineralisation

Eckard 2011

The nitrogen cycle

• Mineralisation– Microbial breakdown of soil organic matter to ammonium– The main mechanism for supplying N to plants

• Nitrification– Microbial conversion of ammonium to nitrate

• Ammonia sources– Urine, decaying organic matter, fertiliser

• Warm, moist (not waterlogged) soils• Denitrification

– Microbial conversion of nitrate to N2 and N2O gasses• Warm, waterlogged soils• N2O is a powerful greenhouse gas

• Immobilisation– Microbial assimilation of soil nitrogen into OM

OM

NH4+

NO3-

N2 N2O

Eckard 2011

organic matter

ammonium ion

nitrous oxidenitrogen

nitrate ion

The nitrogen cycle

• Nitrogen balances in Agricultural systems – Biological efficiency

• Less than 50%

N Inputs Dairy Grains

N fertiliser 150 90N2 fixation 80 0Atmosphere 8 8Feed 45 0Total Input 283 98N output in productMilk 80Meat 8Grain   40Total Outputs 88 40N Surplus 195 58Efficiency (%) 31 41

Eckard et al 2007

© Copyright 2013 The University of Melbourne, The Carbon Market Institute and the Department of Agriculture, Fisheries and Forestry, Carbon Farming Futures, Extension and Outreach Program