BISC 302: Unit 4 - SimU Text-Nutrient Cycling

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(1) Atmospheric nitrogen is not only fixed by microorganisms, but also during thunderstorms. Lightening can provide the energy needed to break the bonds that hold nitrogen atoms together in atmospheric N2 to form nitric oxide (NO) and nitrogen dioxide (NO2), which are collectively known as NOx.

(2)Fires also release fixed nitrogen to the atmosphere, but since this nitrogen comes from the combustion of organic matter the fire is not actually responsible for fixing new nitrogen from atmospheric N2.

(3) Humans are also nitrogen fixers. Burning fossil fuels releases NOx into the atmosphere where it often reacts with other molecules in the atmosphere to form nitrate and nitric acid, a contributor to acid rain. Although some of the fixed nitrogen released from fossil fuel combustion originates in the fuel, much of it comes from N2 in the atmosphere. As the fuel burns it releases enough energy to break the triple bond in the atmospheric N2, producing NOx.1 Regardless of where the nitrogen originates, NOx released during combustion is considered a new source of fixed nitrogen, since it would have remained unavailable to the biota in the absence of human activity. Industrial nitrogen fixation involves the manufacture of ammonium compounds which are used to fertilize agricultural crops and timber plantations. Application of artificial nitrogen fertilizers allows crops and trees to grow in locations where they would otherwise be unable to grow due to nitrogen deficiency. However, anthropogenic nitrogen fixation and artificial fertilization can have serious negative environmental impacts.2
The chemical properties of nutrients affect how they move via water flow. In the case of nitrogen, ammonium ions (NH4+), are positively charged. Soil contains negatively-charged clay particles. Since positives attract negatives, ammonium tends to bind tightly to these clay particles. In contrast, nitrites (NO2-) and nitrates (NO3-) are negatively charged ions that do not bind to clay particles. Nitrites and nitrates are also easily dissolved in water. These forms of nitrogen are therefore more mobile. Water moving through the soil carries free nitrites and nitrates. High rates of streamflow can therefore, lead to losses of nitrogen from the ecosystem.
The process of converting nutrients from a solid to dissolved form that leads to loss from the system is called leaching. Nitrites and nitrates are susceptible to loss via leaching. Leaching can thus lead to decreases in soil fertility and nitrogen enrichment of the water bodies located downstream. In most undisturbed ecosystems, nitrogen is in high enough demand that little is lost via leaching. Disturbances, whether naturally occurring phenomena or human-caused disruptions of the ecosystem, can produce large increases in the amount of nitrogen that water carries out of the ecosystem.

In forestry plantations, a repeated disturbance occurs when timber is harvested. Clearcutting obviously has a major impact on the forest ecosystem. A virtual tree-removal experiment in a simulated forest will allow you to investigate how nitrogen cycling is impacted by such a disturbance,

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