The document summarizes two important biogeochemical cycles - the nitrogen cycle and the carbon dioxide-oxygen cycle. The nitrogen cycle describes how nitrogen exists in different forms and moves between the atmosphere, living things, and the lithosphere through natural processes like nitrogen fixation, assimilation, decomposition, nitrification, and denitrification. Human activities like using nitrogen fertilizers and burning fossil fuels can impact the nitrogen cycle. The carbon dioxide-oxygen cycle explains how carbon dioxide and oxygen are exchanged between living things and the atmosphere through photosynthesis, respiration, and combustion. Photosynthesis produces oxygen and food, respiration uses oxygen and food to produce energy, and combustion releases carbon dioxide through burning fuels. Human activities such as def

1. Biogeochemical Cycles

6. Let’s Review

7. Let’s Review

15. The End