How does deforestation lead to reduction of soil fertility?

Answer 1

Deforestation reduces the fertility of soil because when trees are cut down all the nutrients are taken with them.Normally a tree loses its leaves or the tree dies. The leaves or tree falls to the forest floor and decomposes. The leaves are called the Litter. The decomposed litter has nutrients in it. The nutrients are passed on to the soil and then on to a surrounding or new tree or plant. This cycle keeps on happening and is called the nutrient cycle. Deforestation destroys the cycle. People take the tree with the nutrients in it and go back to their normal lives without knowing what they have done.

Answer 2

The question made is a little misleading. The thought is that what happens after the land is cleared of trees is a decrease in soil fertility. This isn't completely true.

What is true about the premise of soil fertility decreasing upon deforestation is that, on the surface, residue from leaves is no longer present to add any sort of fertility to the soil. Temporarily, soil fertility appears to have gone down after trees are removed. However, one needs to consider this: Soils have most likely been low in fertility from the beginning when afforestation occurred; like when woody species invaded grasslands due to fire suppression and low grazing impact. (For this I am not talking about rainforests.)

Soil fertility when trees are cleared is low because trees get their nutrients not from the surface, but rather from an interaction with soil biota that can turn organic elements into inorganic elements that the tree can take up. In the case of rainforests, there are billions of soil biota that live in the soil. Also, the roots of trees go pretty deep into the sub-layers of soil, deep into bedrock and where trees can access ground water. But they do not have the same type of roots like grasses do, so they are incapable of forming a highly fertile and deep organic layer of topsoil. Their leaves take longer to break down, and actually acidify the soil. Spruce forests and those trees in the aspen belt create soils with thin topsoils and acidic because of the leaf material. Rainforest soils are also acidic. While leaves are one reason for acidity, high rainfall creates leaching of hydrogen ions and soil minerals, which leads to more acidic soils.

After deforestation and a bit of time, soil that was once considered infertile and acidic eventually become more fertile and neutral in soil pH with care of soil amendments with cropping that adds litter to the soil, and allowing for herbaceous species like grasses and forbs to quickly come in to replace those trees. You can increase soil fertility and organic matter with a-grassland-ification, so to speak, it just takes time and patience.

No doubt forests and trees belong to this Earth as well, but the truth is that there is a lot of trees and shrubs invading a lot of land, and a bit of deforestation in these areas that once were large tracts of grassland many years before European settlers came along and enforced laws to outlaw regular burning of woody species, is certainly welcome.

Deforestation leads to the appearance of reduced soil fertility simply because trees are removed and soil biota associated with them are also removed. But, soils do heal by themselves when given the chance. Deforestation doesn't automatically mean desertification will occur. That in itself is simply not true. Plants and animals are the key to heal soils. Desertification happens because of lack of water in the soil. Compaction, lack of vegetative cover, lack of animal impact in a "brittle" landscape, and yes overgrazing (animals on a piece of ground for too long) causes desertification.

Deforestation by mechanical means, or fire, removes an upper canopy that denies seeds of plants the sunlight and heat needed to germinate and grow. When that canopy is removed, forbs and grasses move in. Disturbances that are detrimental to woody shrubs but beneficial to grasses and forbs will keep the plant community as a grassland or savannah. As these plants die and regrow and get trampled by grazers and burned by regular fire (either disturbance may be more frequent than the other, and we'd most likely prefer the former over the latter for safety reasons), they add organic matter to the soil, and also soil fertility.

This is why the historic vast grasslands of the Midwest were once seen, and still are, as incredibly valuable ground for growing crops. Forest lands were not suited for crops, and this has not changed today.