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Water and Nutrient Cycles
Energy and nutrients are constantly flowing through ecosystems unseen. Water is taken up by plants, transpired through their leaves, and given off to the atmosphere. This invisible process called evapotranspiration can influence global weather patterns. Eventually, the water falls back to the earth as precipitation. Similarly, the nutrients that sustain every living creature on Earth are transformed and distributed by a perpetual cycle of decay and renewal. The movement of chemical elements among the planet's living organisms and the physical environment is called the biogeochemical cycle. Nutrient and water cycles are two of the most important biogeochemical processes.
Nutrient Cycling
The nutrient cycle is the basis of life. Plants are the foundation of the nutrient cycle. Plants convert the energy of the sun and atmospheric carbon into biomass through a process called photosynthesis. Plants are called primary producers, because they are the starting point, where energy and nutrients enter the system. Animals that eat plants are called herbivores. They vary from the krill in the sea to the bison on the prairie. Herbivores are the lynchpin of the food chain, which is built upon production and consumption. Herbivorous animals are fed upon by meat-eating predators called carnivores. Wolves eat the buffalo that eat the grasses of the prairie. People are also part of the predator/prey system. People also eat the buffalo or, more likely, the cattle that have replaced them on the plains. Carnivores are at the top of the food chain.
Not all biomass is consumed in a living state. When a plant or animal dies, or when a portion of a plant dies, the nutrients are still recycled. When leaves fall from trees in autumn, for example, the nutrients are released by soil insects, fungi, and microbes, which break down this dead living material, called organic matter, through a process called decay, or decomposition. Decomposition of organic matter forms humus, a stable colloidal material that provides a variety of nutrients to plants. Colloids bind nutrients tightly until they are utilized by plants. Some of the nutrients are lost through movement of water down through the soil in a process called leaching. As any good gardener knows, the more organic matter and humus in the soil, the less leaching of water and nutrients there is. In such an efficiently operating ecosystem, most nutrients can be taken up by plants and become new biomass -- new plant growth -- and thus the cycle begins anew.
Nitrogen, an important nutrient for plant growth, also enters the ecosystem directly from the atmosphere through a process called fixation. Nitrogen fixation is the conversion of atmospheric nitrogen into a form usable by plants. Fixation occurs during thunderstorms, when electricity converts nitrogen into a soluble form that dissolves in rainwater, which then falls to the ground. Fixation also occurs in the roots of some higher plants, made possible by bacteria that live symbiotically in thick swellings on the roots called nodules. These bacteria absorb nitrogen gas and convert it directly to a form the plants can use. The prairies and deserts are filled with legumes like lupines and prairie clover that fix nitrogen.
Water Cycling
The movement of water through the environment is called the water cycle. Precipitation falls to earth, and moves through the landscape in a variety of different ways. Some of the water is intercepted by the canopy of trees and other vegetation, where it evaporates before it hits the ground. Evaporation also occurs from the ground itself. Most of the water infiltrates into the ground, where it is either taken up by the roots of plants or seeps into the ground and enters the water table. Some of the water runs off the surface into wetlands, streams, ponds, and, today, storm sewers.
A portion of the water that is absorbed by the roots of plants is returned to the atmosphere by the process called transpiration. This loss of water through the leaves of plants draws water into the roots and pulls it up the stem or trunk of the plant. Even water that flows to the water table eventually reaches the atmosphere, but it is a longer journey. Groundwater moves with the topography of the land until it reaches the lowest point in the water table - a pond, stream, or wetland. Water from these wetlands and waterbodies is constantly evaporating as it moves downstream through the watershed and ultimately to the ocean. Evaporation from the world's oceans returns massive amounts of water to the atmosphere, where it joins the water lost via transpiration, and returns to the earth as rain. Thus the cycle is renewed.
Modern cities and suburbs, with their impervious pavement, wreak havoc with the natural water cycle. In natural systems, surface runoff is minimal. By contrast, parking lots, sidewalks, and rooftops often produce torrents of runoff that never infiltrate into the ground. There are two serious consequences of reduced infiltration. Flooding is the most dramatic, and destructive. As water runs off roofs and streets, it collects in storm sewers and is rapidly deposited in streams and rivers. This fast-moving, enormous volume of water causes massive erosion of stream and river banks. Widespread and damaging flooding also occurs, in the worst cases washing away houses and crops. An ecologically sensible regional landscape would be designed to reduce runoff and maximize infiltration from the top of the watershed to the bottom. Since water accumulates in volume and speed as it runs through the system, the key to reducing flooding is to catch the water at the top of the watershed. This can be accomplished in many ways. The most obvious is to reduce or eliminate runoff. Reducing paved areas, using porous pavement that allows infiltration, and running downspouts into the ground are a few solutions. Where this is not possible, storm-water retention basins must be used to trap water. Keeping natural wetlands intact also reduces runoff.