People are totally dependent on plants for their survival because plants can convert the energy of the sun into sugar through photosynthesis. This process fixes carbon dioxide into organic forms, which is a critical step in the carbon cycle. Plants obtain water and minerals from the soil. One of the most limiting minerals in agriculture is nitrogen which must be in the form of nitrate or nitrite for absorption by plants. Nitrogen is an important component of chlorophyll, DNA, proteins, and many other biological compounds. Both the carbon cycle and nitrogen cycle have been disrupted by human activities, and these cycles determine our ability to provide a sustainable food supply.
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