18. The right answer is C) as carbon dioxide.
In the food chain that traditionally represents the food web, decomposers occupy a special and essential place, as they contribute to recycling organic matter to all the "floors" of the pyramid, sustaining the carbon cycle. A disturbance in the life of the decomposer can result in the degradation of the fertility of the soil or the environment (aquatic for example). For this reason, we seek to better integrate microbial ecology into biogeochemical models.
19. The right answer is C) nitrogen.
The nitrogen cycle is very complex; three basic processes are involved in nitrogen recycling: N2 diatomic nitrogen fixation, nitrification and denitrification.
Human activity contributes to the increase of denitrification (one of the stages of the nitrogen cycle), among others, by the use of fertilizers that add ammonia compounds (NH4 +, NH3) and nitrates (NO3) to the soil). The use of fossil fuels in engines or thermal power plants transforms nitrogen into nitrogen oxide NO2-. With N2 and CO2, denitrification emits a small amount of N2O nitrous oxide into the atmosphere. The concentration of this gas is low, 300 ppb (parts per billion). However, be aware that a molecule of N2O is 200 times more effective than a molecule of CO2 to create a greenhouse effect. Today, it is estimated that the concentration of atmospheric N2O increases annually by 0.3% and that this increase is practically related entirely to the emissions due to the denitrification of soils.
20. The right answer is C) carbon dioxide from automobile exhaust.
Plants require CO2 for growth. The solar energy harvested by photosynthesis is used to convert CO2 into glucose, which plants then use to feed their life cycle, including reproduction.
However, plants need more CO2 for growth and reproduction. Importantly, they require relatively stable environmental conditions. Since CO2 is a greenhouse gas, its rapid increase warms our planet and contributes to more frequent and extreme weather events. These events make plants more vulnerable to damage and disease, reducing reproductive potential. Increased temperatures can decrease breeding in crops such as maize, an effect that is not mitigated by higher concentrations of CO2. In fact, the reproduction period can be decreased under high CO2 conditions. The entire environmental network must be taken into account to optimize plant reproduction.
21. The right answer is D) More runoff water is created, and less is returned to the atmosphere.
Trees play an important role in the processes of runoff, infiltration and evapotranspiration and consequently in the rainfall of a region, because their roots retain water in the soil, and their leaves by transpiring produce and maintain some moisture in the air. For example, when too many trees are cut down, rainfall rises, runoff increases, and soil erosion accelerates as the soil is no longer held by the roots.
Any deforestation is therefore locally involved in the disruption of the water cycle, contributing to the desertification of some sparsely watered regions of the world.
In addition, during the erosion of the soil, the torn fragments are dragged to the streams, which they raise the bottom of the beds. The deforestation of pastures, for example, transformed into pastures around the year 1000, raised the level of the Rhône upstream of Lyon by about one to two meters.
Finally, the disappearance of alluvial forests, which act as a filter between the terrestrial environment and the rivers, eliminates a natural means of water purification, particularly with respect to nitrates. A thickness of 30 meters of alluvial forest, for example, is enough to prevent almost all nitrates from reaching the waters of the river.