Written by Sabrina Kleinman
Decomposition is the breakdown of dead organic material into smaller particles, which helps release nutrients and carbon for plant and microbial production. Decomposition converts these dead materials into organic materials, inorganic nutrients, and CO2. The process involves a series of integrated physical and chemical reactions that are performed by a vast array of soil organisms, enzymes, chemicals, and physical processes. This breakdown limits the amount of carbon that can be stored within an ecosystem, allowing materials to be available for other important ecosystem functions and processes, such as plant production (which is essential for increasing forest carbon storage). Increases in the rate of decomposition can have additive impacts on the global carbon cycle (Davidson and Janssens 2006) by increasing nutrient availability to growing plants and by increasing the release of CO2 to the atmosphere. Thus, it is important to understand decomposition and how its controls impact carbon cycling and other potential ecosystem impacts.
Controls of Decomposition Rates
Temperature — Studies have shown that increases in temperature directly result in short-term increases in chemical reaction rates (Pendall et al. 2004, Davidson and Janssens 2006). Indirectly, higher temperatures also increase evapotranspiration, which can inhibit the ability of substrates to interact with extracellular enzymes, water-dependent biota, and reaction microsites. Most studies agree that the effect of increased temperature is short lived, leading to only small relative changes in decomposition rates. However, these changes, multiplied globally, could translate into large changes in carbon cycling.
Substrate Quality and Quantity — The variety of substrates abundant in soil and the amounts present have major impacts on the duration and rate of decomposition. Easily digestible carbon substrates (often referred to as labile carbon) break down faster than more complex carbon substrates. Labile carbon substrates can increase the rate of decomposition because their simple chemical structures require less energy to break down. More complex forms of carbon take much longer because they require greater amounts of energy. In some cases, some forms of carbon (like humus) are rarely, if ever, broken down, allowing for an accumulation of soil C over time. Thus, when the concentration of complex carbon substrates is high, decomposition rates tend to be slower.
- Additionally, substrates that have high nitrogen (N) content also tend to accelerate decomposition, while materials that are higher in lignin tend to slow decomposition rates (Norby et al. 2001). The rate at which litter decomposes has been linked to the initial concentration of N, the initial concentration of lignin, the overall C:N ratio of soils, and its lignin:N ratio (Melillo et al. 1982). These components represent important controls for decomposition rates.
- Soil Composition — The physical composition of soils can have a major effect on the rate of decomposition. High amounts of clay form aggregates in soils that can protect soil carbon stores from decomposition, allowing for more accumulation of belowground carbon. Additionally, the charges found on some soil ions may make them more likely to attract organic materials, making them unavailable and slowing rates of decomposition.
- Disturbance — Disturbance can accelerate decomposition by helping expose new surfaces, promoting aeration, and temporarily increasing soil moisture. This can increase respiration from soils and lead to higher turnover of organic material. Disturbance of soil can be from naturally occurring phenomena, such as fire or weather events, or from anthropogenic impacts, such as those from tilling or soil cultivation.
- Water Availability — Water serves many important functions for decomposition including accelerating the physical and chemical breakdown of organic materials, allowing soil biota to receive nutrients, and facilitating enzymatic secretions from microorganisms. Water must be held in the right balance as a control for decomposition. Too little water makes it hard for soil biota to break down material and for them to receive nutrients. Too much water robs microorganisms of oxygen, increasing anaerobic respiration and resulting in an increase in methane production from soils.
Davidson E.A. and I.A. Janssens. 2006. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature. 440: 165-173.
Melillo J.M., J.D. Aber, and J.M. Muratore. 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecology. 63: 621-626.
Norby R.J., M.F. Cotrufo, P. Ineson, E.G. O’Neill, and J.G. Canadell. 2001. Elevated CO2, litter chemistry, and decomposition: a synthesis. Oecologia. 127: 153-165.
Pendall E., S. Bridgham, P.J. Hanson, B. Hungate, D.W. Kicklighter, D.W. Johnson, B.E. Law, Y. Luo, J.P. Megonigal, M Olsrud, M.G. Ryan, and S. Wan. 2004. Below-ground process responses to elevated CO2 and temperature: a discussion of observations, measurement methods, and models. New Phytologist. 162: 311-322.
For more on Forest Soils:
- Basic Soil Components
- Dynamics of Nutrient Cycling
- Understanding Decomposition and Its Controls
- Soils and Water Availability