Carbon is one of Earth’s most common and essential elements and is one of the life’s building blocks. Carbon constitutes approximately 58% of the soil organic matter, and soils with more carbon content tend to be darker in color and much more resilient to precipitation extremes, improving water infiltration and retention. Soil organic carbon above-ground biomass density comprises the carbon stored in living plant tissues above the Earth’s surface.
We can conceptualize carbon storage by imagining it as a bank account, with several deposits and withdrawals of carbon. Both above-ground and below-ground crop biomass left in the field after harvest can be considered ‘deposits,’ as can manure and cover crop biomass, while soil erosion, crop harvest (grain, chopping, baling), tillage, burning residue, and microbial respiration can all be considered ‘withdrawals.’
Source: Freepik
How is carbon lost from the soil?
Carbon can be lost through physical or chemical means. Physical loss occurs when water, wind, or other erosive forces carry soil, carbon, and other nutrients offsite. A historical example of this loss was the rapid topsoil erosion from northeastern hill farms in the 19th century during a mass conversion of forests into pasture. The widespread removal of trees altered water patterns and reduced soil stability, resulting in losses of soil carbon from the landscape during wind and rain events.
Chemical loss of soil carbon results from the combustion of organic matter during a forest fire or from the metabolic processes of the soil biota, which we call respiration. As soil organisms break down and consume organic matter, they release carbon gas into the atmosphere. Most respiration is in the form of CO2. However, when oxygen levels are low in the soil, which occurs when the soil is saturated with water, the soil organisms emit methane (CH4), a potent greenhouse gas.
Living roots also release CO2 into the soil during metabolic processes associated with maintenance and growth. The release of carbon gas through the metabolism of living microbes, fauna, and roots is called soil respiration. It’s important to note that the living organisms in the soil are respiring, not the soil itself.
Feeding your soil takes 90% of added plant biomass:
At least 90% of crop biomass inputs added to soils are consumed by soil organisms (mostly microbes) and returned to the atmosphere as carbon dioxide. Put another way, for every ten lb., tons, or kg of crop biomass going into your soil, you get one lb., ton, or kilograms of soil organic matter, a 10:1 ratio. About half of this loss occurs in the first year, increasing to 80% after seven years and reaching the 90% loss after 30 years. However, it can happen much faster depending on several factors. Warmer, wetter conditions and soil disturbance all increase the speed of these losses.
Scientists believe this is a conservative estimate, most relevant when the soil has a large capacity for holding organic matter. As SOM levels build, it becomes increasingly difficult to increase SOM, thus requiring more than the 10:1 ratio.
How will continued climate change affect soil carbon?
The affect of climate change on soil organic carbon above-ground biomass is uncertain. Warming may accelerate decomposition and soil respiration rates, leading to significant losses of soil carbon, as is occurring in the Arctic, where permafrost is thawing. Heavy rainfall events caused by a warmer atmosphere could increase soil erosion. Higher water tables could also increase methane emissions if soils are saturated for more extended periods. Climate change may also influence tree growth and health, influencing live root exudates, root turnover, and organic matter inputs.
Scientists project that over time, the effects of climate change are likely to change the distribution and composition of vegetation, which may result in changes in the amount of carbon stored in the soil. And we may see more incidents of wildfire that result in emissions of stored carbon. While the impact of climate change on soil carbon is complex and context-dependent, forest soils should be protected because they store a large amount of carbon.
How can we manage soil carbon?
Forest managers and landowners should seek to minimize disturbances caused by water runoff, equipment, vehicles, tilling, and excavation to prevent soil carbon losses. Maintaining vegetation adjacent to wetlands, seeps, streams, vernal pools, and other water bodies can reduce soil erosion from water runoff. Water diversion structures on roads and trails, such as dips and water bars, help to minimize erosion and keep water on site by redirecting water into depressions where the soil can slowly absorb it.
Strategies to reduce soil carbon losses include laying out roads and trails to avoid steep slopes. Restricting equipment and vehicle use when the soils are frozen or dry, avoiding wet soils, and using structures such as bridges, or branches spread across the traveled surface will reduce soil rutting and compaction. Because soil disturbance intensity is directly proportional to soil carbon reduction, disturbances should be limited to as small an area as possible.
In conclusion:
Soil is more than just a bucket for C storage. It is instead a valve controlling the flow of C. Depending on our management, that flow can be increased or decreased, and timing can be adjusted. Given the difficulty of increasing soil organic matter, and climate change will make it increasingly complex, we should focus on maintaining current SOM levels and C/energy flow through the soil. That may be all we can hope to do.