Chapter 8

Soil Nutrient Cycling

(book excerpts)

Although mother Earth gains small amounts of material through meteorite impacts, these extraterrestrial inputs are insignificant in comparison with the mass of the planet. Essentially, at the global level, Earth is an isolated system in terms of matter. As a consequence, nutrients and other materials “cycle” within and between ecosystems. Soils play a critical role in a range of ecosystem services including nutrient cycling—carbon, nitrogen, phosphorus, potassium, sulfur, among other elements. Nutrient cycling by a simple definition describes how nutrients in their organic form move into an inorganic form and back into the organic form. Cycling of nutrients relies on the quality of agricultural soils, either directly through their capacity to receive nutrients and to convert them into or keep them in forms that are available to crops, or indirectly by governing the productivity and harvestability of crops and thereby the effective capture of nutrients from soils. Different types of nutrient cycles are vital to maintain a sustainable life for an ecosystem. In the terrestrial ecosystem, carbon and nitrogen are circulated through the atmosphere, pedosphere, and biosphere system. Although sulfur may be found in the atmosphere as sulfur dioxide and hydrogen sulfide gases and particulates which fall to the ground as acid rain, phosphorus, potassium and other elements do not have an atmospheric component. Phosphorus, potassium, sulfur, among other nutrients are added to soils as organic residues or from parent material released by weathering. These nutrient elements are chemically and biologically transformed in soil. Decomposition of organic residues by soil organisms is at the center of the transformation and cycling of nutrients. A large proportion of nutrients are adsorbed by plants and again added to soils after their death and decay. A large proportion of these elements are also exported with crop harvest and imported as fertilizers. In this way, these elements form ecological cycles. A considerable part of them is transferred to other ecosystems, such as lakes, streams, and oceans where they become a part of the greater global cycles.

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