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

Sources of global CO 2 emissions 12% of total anthropogenic emissions Le 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 CO 2 emissions (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 biomass Leaves, stem, branches Below ground biomass Coarse roots, fine roots, microbes Soil carbon Stable 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 NPP Pg C yr -1 Crops produce mainly aboveground NPP  consequences for soil C belowground aboveground

The Carbon cycle Human activity greatly influences the global C cycle The sink capacity of natural CO 2 sinks is decreasing, leading to increased atmospheric CO 2 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 (H 2 ), Carbon (C), Oxygen (O 2 ), Nitrogen (N) –Methane (CH 4 ), carbohydrate (CHO), carbon dioxide (CO 2 ), water (H 2 O), bicarbonate (HCO 3 ) –Nitrate (NO 3 - ), Nitrite (NO 2 - ), Ammonia (NH 3 ), Ammonium (NH 4 + ), nitrous oxide (N 2 O)

Grazing system carbon cycle Atmosphere Animal Plant Soil Eckard 2011

Grazing system carbon cycle Carbon into plants –Photosynthesis CO 2 + H 2 O + chlorophyll + solar energy -> CHO + O 2 Structural carbohydrate –Cellulose, hemi-cellulose, lignin Non-structural carbohydrate –Sugars Fats/ Lipids Protein/amino acids Carbon from plants –Respiration Burning sugars - energy for growth O 2 + CHO -> H 2 O + CO 2 + energy –Decay of plant residues Mineralisation to CO 2

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) CH 4 and CO 2 – microbial digestion and respiration

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 CO 2 and CH 4 Baldock 2009

Grazing system carbon cycle Humus (~stable) <0.053 mm Respiration Photosynthesis CHO Plant Residues Litter > 2mm POC (~labile) > mm Carbon dioxide (CO 2 ) Decomposition Microbial Biomass Mineralisation Methane (CH 4 ) Nitrous Oxide (N 2 O) Baldock et al. 2009; Eckard, 2009 Nutrients In established pastures Photosynthesis ≈ Respiration + Mineralisation Photosynthesis ≠ Respiration + Mineralisation + Methane Decay

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 –N 2 (nitrogen) 78% of the atmosphere –O 2 = 21%, CO 2 = 0.04% Fixed by legumes into plants and soil –N 2 O (nitrous oxide) 0.32 ppm ( %) Eckard 2011

The nitrogen cycle Main forms of soil & plant N –NH 3 – Ammonia Organic matter Fertilisers –Urea, DAP, UAN etc. Major source of plant N Eckard 2011 oxygen amide ion

The nitrogen cycle Main forms of soil & plant N –NH 4 + – 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 NO 3 - (nitrate ion) –In warm, well-drained soils Eckard 2011

The nitrogen cycle Main forms of soil & plant N –NO 3 (nitrate ion) Major source of plant nutrition –Drier soils Accumulates in some plants –e.g. Brassicas, annual ryegrass, kikuyu, cereal grains –Breaks down to NO 2 in rumen – toxicity Soluble in water – leaches –NO 2 (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 N 2 and N 2 O gasses Warm, waterlogged soils N 2 O is a powerful greenhouse gas Immobilisation –Microbial assimilation of soil nitrogen into OM OM NH 4 + NO 3 - N2N2 N2ON2O Eckard 2011 organic matter ammonium ion nitrous oxidenitrogen nitrate ion

The nitrogen cycle Nitrogen balances in Agricultural systems –Biological efficiency Less than 50% N InputsDairyGrains N fertiliser15090 N 2 fixation800 Atmosphere88 Feed450 Total Input28398 N output in product Milk80 Meat8 Grain 40 Total Outputs8840 N Surplus19558 Efficiency (%)3141 Eckard et al 2007

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