Griep Research Group

In Nebraska, the landscape seems to go on forever

by Mark Griep
Department of Chemistry
University of Nebraska-Lincoln
for Swimming in Nebraska (2010) by Jon Jost

In Nebraska, the landscape seems to go on forever

The green and brown fields are punctuated by trees and houses, and traversed by poles carrying telephone signals or electricity. There are hills in the distance and the asphalt road disappears on the horizon ahead. The chemistry of the plants, soil, water, and sky form an integrated whole that connects Nebraska with every other part of the Earth’s biosphere.

Soil is made from organic matter, inorganic matter, water, and air. The water and air are complementary in that the spaces between the soil particles are either filled with air when the soil is dry, or water when the soil is wet. The organic matter is any living or dead carbon-based material left lying on the soil. The inorganic matter in soil is the major component, and one of the most common types is silica. For instance, sandy soil is composed of large crystals of silica whereas clay soil is composed of very small crystals. Silica is silicon dioxide, a hard crystal in which each silicon atom is bonded to four oxygen atoms and every oxygen atom is bonded to two silicon atoms. The silicon-oxygen bond is one of the strongest chemicals bonds, which means that silica is a very hard mineral. The surface of silica contains no silicon atoms, only negatively charged oxygen ions. As the crystal becomes larger, its surface becomes less densely negatively-charged. The large silica particles in sand, such as are found in western Nebraska and the rest of the Great Plains, don’t bind cations as strongly as clay soil. This means that both the cation and anion of a fertilizer such as ammonium nitrate wash quickly through the soil and into the underground water system. The cation ammonium is retained longer because its positive charge is attracted to the negative charge of the silica. The anion nitrate is not attracted at all to the silica because both are negatively charged and repel each other.

Aquifers, or groundwater, can form when there is a layer of porous sandy soil on top of a less porous layer such as bedrock or even clay soil.  When rain or melted snow seeps into the soil, it is filtered and purified as it makes its way through the silica particles until it can fall no further because of the impermeable layer.  The resulting groundwater is tapped to make wells and for irrigation. The Ogallala aquifer is the largest in North America and it is beneath the Great Plains, extending from North Dakota to the panhandles of Oklahoma and Texas. The thickest part of this aquifer is located beneath the western half of Nebraska and is used for the irrigation.

If life is a miracle, its name is water. Plants draw their water from the soil and then use the chlorophyll’s electrical energy to split two water molecules into diatomic oxygen, two hydrogen ions and two hydride ions. The oxygen eventually diffuses into the atmosphere where animals breathe it in and use it to break down their food during metabolism. The hydride ions actually reside on a special molecule called NADPH. The hydrogen ions build up on one side of membrane and the concentration differential across the membrane is used to drive the synthesis of a molecule with the acronym ATP. In a series of reactions that don’t require light, CO2 from the atmosphere is fixed to make carbohydrates, starch, and cellulose using the chemical power of the ATP and NADPH that could only be formed with light. Plants also draw nitrogen, phosphorus, and sulfur from the soil to use in making proteins, more chlorophyll, nucleic acids, vitamins, and fats.

When an atom or molecule absorbs light energy, it excites an electron in that atom or molecule from its ground state to an excited state. The excited electron usually returns to the ground state coupled with an increase in the atom or molecule’s vibrational or translational motion, causing it to heat up. When a suitable electron acceptor is nearby, such as is usually the case with chlorophyll, the excited electron jumps from the chlorophyll to that acceptor. At that moment, light energy is converted to chemical energy, the reaction that sustains life on Earth and by which the biosphere harvests the Sun’s energy.

The green in the plants is due to chlorophyll, the molecule that converts light energy into electric energy so the plant can fix CO2 from the atmosphere to make carbohydrates. In fact, the word photosynthesis means to put things together using light. Chlorophyll is a flat, nitrogen-containing molecule that binds one magnesium ion and is itself bound to a protein.  The protein is found in the membrane of chloroplasts, a subcellular organelle found in all photosynthetic plants.

In Nebraska and the rest of the Northern Hemisphere, plants take CO2 from the atmosphere in the spring and summer to make carbohydrates during photosynthesis. Plants are then slowly decomposed by microbes and animals throughout fall and winter to release the CO2 back into the atmosphere. For this reason, they contribute to the annual fluctuations of CO2 that are recorded at the many scientific stations that have discovered CO2 is highest in the early spring and lowest in late fall.

In Nebraska, the landscape seems to go on forever.