Effects of Tree Roots on Soil

Soil Reinforcement

• Roots anchor the soil. Alder, willow and cottonwood trees are commonly used for bank stabilization. Willow roots can stretch rather than break when soil shifts. This allows willows to stay in place and their roots to hold onto more soil. Alders have fibrous roots that are deep, which helps in soil reinforcement. Cottonwoods have brittle roots; this makes them less effective at soil reinforcement because their roots break when the soil shifts.

Nutrient Cycling

• The depth at which tree roots take in nutrients from the soil around them varies by the type of tree. Deep-root species, such as willows, keep nutrients from leaching out of the soil. Leaching is the process by which water percolating down through the soil takes the dissolved nutrients with it. Tree roots hold these nutrients and release them back into the soil, making them available to other organisms when the tree root dies.

Nitrogen Fixation

• Acacia and mimosa are two types of trees whose roots fix nitrogen. Plants need nitrogen to grow but cannot use nitrogen gas. Trees in the bean family (legumes) have bacteria (rhizobium) that live in nodules on their roots. These bacteria convert (fix) the nitrogen gas in the air into ammonium. Ammonium is then converted to nitrate, which is a form of nitrogen that plants can use.

Decreased Carbon Storage

• The balance between the amount of carbon held in organic matter in soil and the amount in the atmosphere is important for keeping climates stable. More carbon in the atmosphere can cause climate change. Although trees store carbon for years, the presence of tree roots encourages bacterial growth in the soil around them. Bacteria break down dead particles in the soil. This releases the carbon that is stored in soil into the atmosphere as carbon dioxide.