Abating Climate Change and Feeding the World Through Soil Carbon Sequestration

Abstract

Two degrees Celsius was accepted by the Copenhagen Accord and the G-8 Summit as an acceptable upper limit of increase in global temperature. This requires identification and implementation of viable options to reduce emissions of CO₂ and other greenhouse gases and sequester carbon from the atmosphere: business as usual will mean a drastic increase in atmospheric CO₂ with dire consequences for the environment, ecosystem services and human well-being. However, net emissions can be reduced by enhancing terrestrial C pools: the soil (4,000 Pg to 3 m depth) and the biotic (620 Pg). The soil C pool is ~5 times the atmospheric pool (780 Pg) and 6.5 times the biotic pool. Most agroecosystems have severely depleted their soil organic carbon (SOC). The magnitude of depletion (30–40 MgC/ha, i.e. 25–75 % of the antecedent) depends on climate, soil type, land use history, farming systems and management.

In the long term, extractive farming practices can severely deplete SOC, exacerbate degradation and adversely affect agronomic productivity. Nonetheless, depleted and degraded soils have a large carbon sink capacity, and the SOC pool can be restored by restorative land use and adoption of management practices that create a positive soil carbon budget, reduce emissions from farming operations like tillage, and minimize risks of soil erosion and nutrient and SOC depletion. These practices include conservation agriculture with mulch farming and cover cropping, complex rotations including agroforestry, integrated nutrient management in conjunction with biological N fixation and recycling of plant nutrients fortified by rhizobial and mycorrhizal inoculations, biochar, fertigation with drip subirrigation, and creating disease-suppressive soils through improvement of rhizospheric processes. The SOC pool should be enhanced to above a threshold level of 1.5–2.0 % in the surface layer of most cultivated soils. Increase in SOC pool in the root zone by 1 Mg/ha can enhance total food production in developing countries by 30–50 million Mg/year. The rate of SOC sequestration in most cropland soils ranges from 100 to 1,000 kgC/ha/year with a total global sequestration potential of 0.4–1.2 PgC over 50–100 years. The potential of C sequestration in the terrestrial biosphere is estimated to be equivalent to a drawdown of 50 ppm of atmospheric CO₂ over a century.