Читать книгу Exploring the Solar System - Peter Bond - Страница 93

Earth is unique in possessing a biosphere – an environment dominated by living organisms (Box 3.4). These organisms, which range from microscopic bacteria to plants, animals, and humans, have adapted so that at least some of them can survive in every type of environment on or near Earth's surface. Some microorganisms even exist within the crust, several kilometers underground.

The presence of these organisms greatly influences the planet's surface and atmosphere. Studies of newly formed, barren, volcanic landscapes show how plants and animals are able to colonize even the harshest environments. First, mechanical and chemical weathering processes break down the rock into fragments of inorganic material. Later on, plant roots also intrude into every crack and pore in a rock, helping to break it apart.

Figure 3.33 The global biosphere. Different plant ecosystems on the continents are shown in shades of green. Chlorophyll measurements (blue – purple) indicate the amount of phytoplankton in the ocean. Both vegetation and phytoplankton consume atmospheric carbon dioxide and release oxygen during photosynthesis. Note the large area of barren land (tan color), where there is little, if any, vegetation.

(NASA‐GSFC)

Once a small amount of inorganic debris develops, plants, then animals, begin to invade the area. Over time, their waste and dead remains decompose, providing water, carbon dioxide, organic acids, and salts such as nitrates, which are then incorporated into the soil by fungi and bacteria. Earthworms and insects mix the soil as they consume and spread the dead organic material.

Certain bacteria, such as those which occur on the roots of leguminous plants like peas and beans, capture nitrogen from the air then incorporate it into proteins. Other bacteria then break these down into ammonia and nitrates, which can be absorbed by plants. This nitrogen cycle is only one method of circulating soluble nutrients around the biosphere. Micro‐organisms also play key roles in the cycling of elements such as sulfur and phosphorus.

Vegetation also plays an important part in the planet's water cycle, since plants remove large quantities of moisture from the soil and release large amounts through their leaves into the atmosphere during the growing season. An increase in vegetation cover is generally associated with an increase in regional rainfall, and vice versa.

Plants also modify the local climate, for example by slowing the wind, thereby reducing erosion, and reducing the likelihood of frosts. High latitude forests reflect less solar radiation than grasslands or tundra blanketed by ice and snow.

Perhaps the most important process of all is the ability of many organisms – plants, algae, and some bacteria – to take up carbon dioxide from the atmosphere and produce oxygen through photosynthesis. Using sunlight as an energy source, they are able to fix (convert) carbon dioxide into carbohydrates, producing oxygen as a byproduct. It is this process which largely explains why Earth is the only object in the Solar System to exhibit an abundance of atmospheric oxygen and an ozone layer in the stratosphere.

Figure 3.34 The lower reaches of the Betsiboka, the largest river in Madagascar, transport a huge amount of red sediment to the sea. The amount of sediment is increased by large‐scale deforestation inland. The removal of forest cover accelerates soil erosion. Once the soil has been removed it cannot be replaced. Very little of the original forest now remains.

(NASA)