Carbon sequestration in soils

Current estimates are that 2,500 gigatons of carbon reside in the earth’s soils. This number compares with 800 gigatons of carbon in the atmosphere and 560 gigatons in the biosphere (plant and animal life). So soils hold almost twice as much carbon as the atmosphere and biosphere combined. Soils provide a valuable but poorly understood pathway for removing carbon dioxide (CO2) from the atmosphere, thus slowing the rate of global warming.

Carbon sequestration is a vital part of the earth’s carbon cycle. Hence, discussions on climate change inevitably include CO2 sequestration as a partial solution to global warming. Accordingly, carbon sequestration in soils is one of the more significant pathways available for the removal of greenhouse gases from the atmosphere.

Mechanisms of carbon sequestration in soils

The pathway for sequestration of carbon into soils starts with plant photosynthesis. Plants remove carbon (primarily CO2) from the atmosphere and then use sunlight to transform it into the carbon compounds they need for growth. Excess carbon from the plant’s photosynthesis exudes from the roots where it then feeds micro-organisms in the soil. Through this process, the excess carbon transforms into humus.

The dark, rich, texture of high-quality soils is a reflection of the amount of humus available. Humus is also valuable from an agricultural standpoint, since it provides the soil with fertility, structure, and water retention capacity.

Soil-based carbon-storage can be maintained or destroyed under the influence of human agriculture. Farming methods that revitalize the carbon in the soil, also build its sequestration capacity. However, not all farming has historically cared for the land in this way. Worldwide, cultivated soils have lost an estimated 50 to 70 percent of their stored carbon since the original agricultural revolution.

Most of this soil-carbon loss occurred after the industrial revolution when large-scale farming became possible. Then many of the practices adopted, as agriculture became an industry, were carbon-reducing in nature. Not only did these practices eliminate carbon-storage capacity, but they released excess carbon into the atmosphere since exposed humus oxidizes into carbon dioxide (CO2). Between 50 and 100 gigatons of CO2 are estimated to have been released into the atmosphere during this process.

Soil maintenance and recovery

Soil is what farmers desire for growing crops. If you remove the humus and nutrients from the soil, then you just have dirt. The development, restoration, and maintenance of healthy soils is a win/win proposition. Maintaining existing soils in a healthy state preserves high-quality cropland for agriculture, and it also retains the carbon storage capacity of those soils. Development or restoration of depleted soils creates new carbon storage opportunities and potentially valuable cropland. If crops are not the goal, then restoration still adds value by enriching the local ecosystem.

Future management of soils includes efforts to restore forest and replant grasslands. The vast traditional grasslands of North America and China were once massive carbon sinks. But today only three percent of North America’s grass-prairies remain.

As soil science progresses, more strategic approaches to reforestation become possible. Carbon storage partitioning between soil and vegetation varies between warm tropical climates, where more carbon is stored above ground, and cooler climates with massive below-ground carbon storage. So, the development of long-term carbon storage in higher latitude ecosystems will maximize below ground carbon fixing in the soils.

Society has both incentives and viable mechanisms to slow down climate change through reforestation, restoration of grasslands, and sustainable farming practices. It is a partial solution to a larger climate change problem, but never-the-less government policy should support movement in this direction.