The author is an assistant professor at The Ohio State University.
Global warming is the rise in heat retention within the atmosphere, caused by the buildup of greenhouse gas (GHG). The consequence of global warming is climate change, which in practice results in the higher frequency and intensity of extreme weather events, such as drought, storms, flooding, and others. These extreme weather events can have a direct impact on production agriculture.
Nearly two-thirds of atmospheric carbon dioxide (CO2) is the result of burning fossil fuels. The remainder comes from deforestation or food production. Other important agricultural GHG are methane (CH4) and nitrous oxide (N2O). Methane is primarily emitted from ruminant animals’ digestive systems, during manure application, and when manure is in storage. Nitrous oxide is emitted mainly from fertilizer application and urine deposition in soils.
The solution to climate change involves reducing the emissions of GHG and improving carbon (C) sequestration. Reducing emissions from CH4 and N2O is more difficult than reducing CO2 because both CH4 and N2O result from complex biological processes in agricultural soils. Carbon sequestration represents uptake of carbon from the atmosphere into the soil, creating a negative flow of emissions that can be used to balance out CH4 and N2O emissions in reducing the total carbon footprint.
Carbon sequestration is the process of capture, via photosynthesis, and long-term storage of atmospheric CO2. Photosynthesis and plant growth draw carbon into plant cells, releasing oxygen. Once plants die, plant residues are decomposed by soil organisms (such as bacteria, fungi, and earth worms), transforming the plant material into organic matter. Carbon is also added to the soil system by plant roots through root death, root exudates, and root respiration.
As organic matter in the topsoil layers increases over time, it slowly moves down into the soil profile and becomes more stable. Eventually, the soil organic matter is further decomposed into a more stable form of organic matter known as humus. The accumulation of organic carbon in stable organic matter at deeper soil layers represent carbon stock. The carbon stock in the soil prevents it from being emitted to the atmosphere again.
The second important aspect of carbon sequestration is that soil organic carbon (or soil organic matter) improves soil health by promoting water and nutrient retention, which are essential to plant and soil microbes’ growth. Better soil health will, then, enhance farm productivity, as well as reduce CO2 in the atmosphere.
Carbon addition via roots
Plant roots provide soil organic carbon primarily in the form of root litter and the release of organic material, including exudates, dead cells, and mycorrhizal biomass. Roots can also contribute to organic carbon input by forming soil aggregates and protecting organic carbon from the act of microbial decomposition. The region between plant roots and the soil in which plants grow is often referred to as the rhizosphere, and the process of carbon addition directly from roots is called rhizodeposition.
There are two main processes that add carbon to the soil:
1. Accumulation of organic matter and conversion of organic matter in humus.
2. Root exudates and other root-borne organic substances released into the rhizosphere during plant growth and sloughing of root hairs and fine roots.
The carbon inputs directly via roots result in organic carbon gain, particularly when the input is of stable organic matter. It has been reported that photosynthetically fixed carbon in cereals and grasses is rapidly transported toward the root, reaching the soil within one hour. During one vegetative season, cereals and grasses can allocate about 1,400 to 2,000 pounds of carbon per acre below the ground. This accounts for nearly 5% to 20% of all photosynthetically fixed carbon transferred to soils through root exudates.
Taking a closer look
Recently, the importance and role of plant roots to carbon sequestration has been deeply investigated. It has become clear that organic matter inputs from roots contribute to organic carbon stabilization much more than aboveground plant inputs. More than 200 carbon compounds released from plant roots through rhizodeposition are reported.
The compounds released by plant roots that deposit in the root sphere are many and complex, ranging from mucilage, root border cells, extracellular enzymes, simple and complex sugars, amino acids, and vitamins. Belowground inputs are up to five times more efficient on stabilizing soil organic matter than aboveground residues. The carbon storage capacity of the soil is twice compared to the atmospheric carbon.
The rise in soil organic matter via roots results in more atmospheric carbon storage. More soil organic matter also leads to greater soil fertility, better soil tilth, water-holding capacity, and reduced soil erosion. It makes plants, and the entire agricultural field, more resistant to stress and better able to withstand the climatic fluctuations that are expected to happen because of climate change.
Strategies for sequestration
The carbon content of most agricultural soils is now about one-third less than its native condition of either forest or grassland. Fortunately, modern agriculture has changed this scenario, reduced the net loss of soil carbon, and provided multiple options of strategies to restore soil carbon.
Practices that increase root growth and amount will intensify the carbon addition by roots to soils. Crop species with greater roots can deposit carbon in deeper layers — where it is protected from tillage and erosion — and contribute to carbon stocks. Introduction of perennial crops elevate carbon sequestration through root growth and cut down in soil disturbance. The use of cover crops during fallow periods provides year-round carbon sequestration and reduces soil loss via erosion. By reducing tillage, soil exposure to air is reduced, slowing down the decomposition of organic matter, which releases CO2 back to the atmosphere.
The addition of legumes into grass pastures has positive effects on both plant yield and root production. Legumes, due to their capacity of biologically fixing nitrogen (N), provide additional nitrogen supply to the grass growing in the mix. The grass will uptake nitrogen, reducing its content in the soil, favoring legumes to continue nitrogen fixation. The positive interaction between these different plant types results in greater yield and stability of production.
Higher levels of biomass production, the return of greater proportions of crop residue to the soil, and diversification (via cover crops and mix of species, such as grass-legumes in pastures) provide soil protection from loss. They also result in greater inputs of carbon sequestration.
This article appeared in the February 2023 issue of Journal of Nutrient Management on pages 22-23. Not a subscriber? Click to get the print magazine.