Soil organic carbon accumulation in Conservation Agriculture: a review of literatureSandra Corsi a, bMichele Pisante aAmir Kassam bTheodor Friedrich b a University of Teramo, Agronomy ad crop sciences research and education center b Food and Agriculture Organization of the UN
The challenges for agriculture HIGHER FOOD CONSUMPTION WATERResources outlook FUTURE CLIMATE irregular water availability, extreme weather events, GENERATIONS LAND CHANGE higher normal temperatures GENETIC RESOURCES GROWING WORLD POPULATION
Traditional agricultural systems Vicious circle
Sustainable agricultural systems Virtuous circle
CA is based on three principles applied simultaneously (FAO, 2009):Conservation agriculture systems 1 2 3 Minimum mechanical Permanent organic Diversified crop rotations including soil disturbance soil cover cover crops (the minimum soil (retention of adequate (to help moderate possible disturbance necessary to levels of crop residues on weed, disease and pest problems) sow the seed) the soil surface)
Factors influencing C sequestration • agronomic management • rotation patternC sequestration • input rates of organic matter • chemical composition of organic matter inputs • soil type and texture • previous land use • climatic conditions
LITERATURE SURVEY No C accumulation in the soil associated with: • soil disturbance •C sequestration poor management of crop residues • monoculture • rotations that do not guarantee a positive N balance • soil sampling in deep soil layers
LITERATURE SURVEY Where CA principles and methods are not followed • soil disturbance SOC accumulation is a reversible process: with even a single tillage event, sequestered soil carbon and years of soil restoration may be lost (Grandy et al., 2006)C sequestration Formation of stable micro-aggregates within macro-aggregates is inhibited under TA (Six et al., 1998) In TA soils the mixing of the litter favours bacteria, hence quick degradation processes (Beare et al., 1992; Guggenberger et al., 1999) The mouldboard plough disturbs the highest soil volume, produces the maximum CO2 flux and uses the most energy. NT the least (Reicosky et al., 2005) Tillage erosion is the major cause of the severe soil carbon loss on upper slope positions of pland landscapes (Lobb et al., 1995; Lobb and Lindstrom, 1999; Reicosky et al., 2005)
LITERATURE SURVEY Where CA principles and methods are not followed • removal of crop residues • mixing of crop residuesC sequestration Residues mixed into the soil decay more rapidly (Magdoff and Weil, 2004) Mixing readily decomposable carbon into native soils induces a priming effect; the composition of crop residues not mixed does not affect the decay of the native SOM (Chadwick et al., 1998; Flessa and Beese, 2000; Kuzyakov et al., 2000; Chantigny et al., 2001; Bol et al., 2003; Fontaine et al., 2004; Sisti et al., 2004; Fontaine, 2007) In a soil not tilled for many years SOM active fractions increases (Franzluebbers et al., 1995; Stockﬁsch et al., 1999; Tebrügge and During, 1999; Horáček et al., 2001) Soil enzymes activities higher in cropping systems with cover crops or organic residues (Dick, 1994; Karlen et al. 1994; Bandick and Dick 1999; Kandeler et al., 1999; Dilly et al., 2003; Balota et al. 2004; Nurbekov, 2008)
LITERATURE SURVEY Where CA principles and methods are not followed • monocropping in NT systems Monoculture is in itself a reason for exclusion from CA systemsC sequestration Changing monocrop to multicrop rotation results in positive influence on SOC concentration (Havlin et al., 1990; Entry et al., 1996; Mitchell et al., 1996; Robinson et al., 1996; Robinson et al., 1996; Buyanovsky and Wagner, 1998; Gregorich et al., 2001; Lopez-Fando and Pardo, 2001) • fallow-based crop ‘rotations’ These should not be associated with the concept of CA
LITERATURE SURVEY Rotations that do not allow C sequestration • barley - wheat - soybean (Angers et al., 1997) Barley is a versatile species often cultivated where growing conditions are less favourableC sequestration • maize - wheat - soybean (Yang and Kay, 2001; VandenBygaart et al., 2002) These experiments are based on too few soil profiles sampled and the previous land use is not mentioned • soybean as the only legume in the rotation (Machado and Silva, 2001; Freixo et al., 2002) When a green-manure crop with high annual aboveground biomass production is included, carbon stocks are significantly greater under CA than under TA (Diekow et al., 2005) C accumulates in the soil when the N balance of the rotation is positive (Sidiras and Pavan, 1985; Bayer and Mielniczuck, 1997; Boddey, 1997; Alves et al., 2002, 2003, 2006; Sisti et al., 2004; Bayer and Bertol, 1999; de Maria et al., 1999; Amado et al., 1999, 2001; Bayer et al., 2000 a,b).
LITERATURE SURVEY Mechanisms for deep C sequestration In the medium term, C concentration in deep soil layers is higher under TA when the C-enriched top layer is turned upside-down (Baker et al., 2007) recalcitrant C from deeper layers becomes exposed to rapid mineralization at the surfaceC sequestration SOC accumulated ceases and regresses as soon as the external carbon input is interrupted In the long run, in CA system the depth of the O horizon increases translocation of soluble carbon compounds from surface residues (Eusterhues et al., 2005; Wright et al., 2007) roots, due to their chemical recalcitrance, contribute twice the C than surface residues (Hussain et al., 1999; Wilts et al., 2004; Johnson et al. 2006).
LITERATURE SURVEY Correctly assessing C sequestration potentials • rates should be referred to specific C pools, as each C category hasC sequestration different turnover rates • undisturbed soils under natural vegetation should be used as benchmark and compared to soils disturbed by human activities • data analysis should be carried out, at the most, at the level of agro-ecological zones
Is the carbon budget for CA higher than for TA? • mechanical equipment farm power requirements number of passes across the field tractor lifeC sequestration fuel consumption depreciation rates of equipment • fertilization quantity of nitrate leached soil protection function of mineralization of organic N in post-harvest N immobilisation • GHGs emissions methane emissions from rice fields nitrous oxide emissions
Benefits of CA over TA • higher biological activity • improved capture and use of rainfall • less peak runoffConclusions • energy savings • lower production costs • incorporation of new areas into production • reduced CO2 emissions But: introduction of CA takes time before all advantages become apparent
Constraints to CA implementation • Cultural background (tradition, prejudice) • Lack of knowledge on how to implement CA (know-how)Conclusions • Inadequate policies • Lack of experience of the tractor driver • Lack of adequate seeding equipment • Poor weed control • Poor management of residues