It is not usual to think of soils in the context of climate change. Policy is usually focussed on reducing greenhouse gas (GHG) emissions from the electricity sector, transport and industry. There has, however, been a renewed interest in understanding how soils can serve as a sink for carbon dioxide since atmospheric concentrations of carbon dioxide have crossed 410 parts per million and oceans are already turning acidic. Besides, increasing soil carbon offers a range of co-benefits and this would buy us time before other technologies can help us transition to a zero-carbon lifestyle.
Significant carbon pools on earth are found in the earths crust, oceans, atmosphere and land-based ecosystems. Soils contain roughly 2,344 Gt (1 gigatonne = 1 billion tonnes) of organic carbon, making this the largest terrestrial pool. Soil organic carbon (SOC) comes from plants, animals, microbes, leaves and wood, mostly found in the first metre or so. There are many conditions and processes that determine changes to SOC content including temperature, rainfall, vegetation, soil management and land-use change.
Approaches to increase SOC include reducing soil erosion, no-till-farming, use of cover crops, nutrient management, applying manure and sludge, water harvesting and conservation, and agroforestry practices. Rattan Lal from Ohio State University estimates that an increase of just 1 tonne of soil carbon pool of degraded cropland soils can increase crop yield by several kilograms per hectare. Moreover, carbon sequestration in soils has the potential to offset GHG emissions from fossil fuels by up to 15% annually. In contrast, it has been estimated that SOC in India has reduced from 30% to 60% in cultivated soils compared with soils that are not disturbed.