- A case study on volcanic soils of Reunion island under sugarcane crops -

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Long-term diachronic series for soil carbon saturation evidence

- A case study on volcanic soils of Reunion island under sugarcane crops -

Myriam Allo, Alain Albrecht, Patrick Légier, Fabien Gogé, Magali Jameux, Laurent Thuriès and Pierre Todoroff

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Sugarcane crops in Reunion Island

 Saccharum officinarum, a tropical, perennial C4 grass  60% of agricultural area in Reunion

 Cultivated for 2 centuries on the island

PLANTATION

every 10 years

Milling (July to December)

HARVEST REGROWTH GROWTH Time B io mass MULCH HARVEST GROWTH GROWTH HARVEST HARVEST GROWTH Sugarcane cycle

Yield : from 70 to 120 t.ha-1

SOIL TILLAGE + PLANTATION SOIL TILLAGE +

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AlexiaGobrecht©February2017 3

Reunion, a young volcanic island in the Indian Ocean

Evolution of the oceanic plaque on the

hot spot of Reunion Simplified geological map of Reunion (made from BRGM 1/50000 and 1/100000).

Wageningen, Netherlands

Reunion, France

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Volcanic soils of Reunion

Annual rainfall (mm) Mean temperature (°C)

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Volcanic soils of Reunion

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A HUGE SOIL ANALYSIS DATABASE IN REUNION Since 1993, 20 000 geolocated samples from

sugarcane plots on Reunion island :

 SOC content in 0-30 cm depth

 High variability of SOC content

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Long-term diachronic series of SOC content

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Soil map of Reunion Mean : 24.3 g.kg

-1 RSD : 27.1 % Mean : 24.0 g.kg-1 RSD : 30.4% Mean : 21.3 g.kg-1 RSD : 35.4 % Mean : 27.8 g.kg-1 RSD : 37.7% Mean : 78.5g.kg-1 RSD : 33.0 %

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Long-term diachronic series of SOC content

Soil map of Reunion

STABILITY of SOC content over time

 At soil unit level  At field level

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Huge amount of Organic Matter returned to the soil :

OM = (6.5*(1-0.81_)+12*(1-0.92₎

= 2.5 tDM. ha-1_.y-1

1_{mineralisation rate of aerial biomass} 2_{mineralisation rate of root biomass}

Carbon = 58% Organic Matter 1.5 Mg C. ha-1_.y-1 Roots 6.5tDM.ha-1 Mulch 12 tDM.ha-1 Stalks 27.2tDM.ha-1

Yield = 100t.ha-1 _{of fresh matter}

Is SOC saturation reached under sugarcane in Reunion?

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Conservative agricultural practices in

Reunion :

no burning => mulching plantation cycle of 10 years => only 1 tillage every 10 years

STABILITY of SOC content over time + huge amount of Carbon returned to the soil without any increase in SOC content => SOC saturation reached in soils under sugarcane crops in Reunion ?

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Perspectives

Important things to remember :

 Stability of SOC content over time

 Huge amount of organic carbon returned to the soil (conservative agricultural practices)  SOC saturation reached in soils under sugarcane crops in Reunion …

But still some work to do :

 On SOC saturation:

Specific Surface Area (SSA) estimation through the proxy of water content of air-dried samples

(see Beare et al., 2014)

 On SOC stock calculation :

Using NIR and MIR spectroscopy to calculate bulk density and C stocks of volcanic soils in Reunion island

Beare, M. H., McNeill, S. J., Curtin, D., Parfitt, R. L., Jones, H. S., Dodd, M. B. and Sharp, J., 2014. Estimating the organic carbon stabilisation capacity and saturation deficit of soils: a New Zealand case study", Biogeochemistry, vol. 120, 71-87, doi=10.1007/s10533-014-9982-1

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