Organic carbon stocks in rubber (Hevea brasiliensis) growing soils of Sri Lanka and strategies to increase: A Review

Soil plays a pivotal role in global carbon stocks and climate change due to green house gasses. Atmospheric CO₂ could be sequestered in soils by converting degraded lands into rubber plantations and by adopting better agro-management practices in existing rubber plantations. This study reviews the information available on soil organic carbon (SOC) stocks in rubber growing soils of Sri Lanka and strategies to increase carbon sequestration in rubber plantations. To date, data generated under the SRICANSOL project were the only available information to estimate SOC stocks down to 1 m depth in rubber growing areas in Sri Lanka. Calculations made using that data set revealed that the mean SOC stocks in rubber growing soils of Wet, Intermediate and Dry Zone were 105, 85 and 78 Mg ha^-1, respectively. In mature rubber plantations, SOC stocks had ranged from 40 to 133 Mg ha-1 and this as a proportion of ecosystem C stock could be varied from 25 to 65% depending on the soil depth, type, and agro-management practices. Establishment of rubber based agro-forestry systems using perennial crops that incorporate large quantities of organic matter into soil (e.g. cocoa, banana) is the best land use option to sequester atmospheric CO₂ in rubber growing soils. Erosion control by establishing a good Mucuna bracteata ground cover since land preparation, establishment of deep rooted vetivar grass or gliricidia as hedgerows and mulching with their loppings, return part of the fuelwood back into rubber field as biochar, adopting favourable agricultural practices that increase growth of the rubber plants were among the key strategies to increase SOC stocks in rubber plantations. The information generated here could be useful when drafting a project design document when tapping into carbon markets.