Chemistry: Environmental Systems - Carbon, Nitrogen, Water, and Nutrient Cycles

The Water Cycle describes the continuous movement of water on, above, and below the surface of the Earth. Key processes include evaporation ($H_2O(l) \rightarrow H_2O(g)$), condensation ($H_2O(g) \rightarrow H_2O(l)$), and precipitation.

The Carbon Cycle involves the exchange of carbon between the atmosphere, biosphere, oceans, and geosphere. Major processes include photosynthesis, which removes $CO_2$, and cellular respiration/combustion, which release $CO_2$.

Photosynthesis is the process by which autotrophs convert light energy into chemical energy: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$.

Cellular Respiration is the chemical process that releases energy from glucose: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP (energy)}$.

The Nitrogen Cycle converts atmospheric nitrogen ($N_2$), which is unusable by most organisms, into ammonia ($NH_3$), nitrites ($NO_2^-$), and nitrates ($NO_3^-$) through processes like nitrogen fixation and nitrification.

Nitrogen Fixation occurs via bacteria in soil or legume root nodules, or through lightning: $N_2 + 8H^+ + 8e^- \rightarrow 2NH_3 + H_2$.

Decomposition is a critical stage in nutrient cycles where decomposers (fungi/bacteria) break down organic matter, returning $C$, $N$, and $P$ to the soil.

The Law of Conservation of Mass states that matter is neither created nor destroyed in these cycles, only transformed into different chemical states.