Biogeochemical Cycle

Photosynthesis actually consumes carbon dioxide, converting it into organic matter using sunlight, so it doesn't add CO2 to the atmosphere. Respiration by plants and animals converts organic matter back into carbon dioxide, releasing it into the atmosphere. The decay of organic matter by decomposers also releases carbon dioxide back into the atmosphere as part of the decomposition process. Volcanic action emits carbon dioxide directly into the atmosphere from the Earth's interior. Therefore, the processes that add carbon dioxide to the carbon cycle are 2, 3, and 4.

**Excretion of urea by animals - Correct:** Urea (CO(NH₂)₂) contains nitrogen. When animals excrete urea, microbes in the soil convert it into ammonia, and later into nitrates and nitrites — forms usable by plants. Thus, urea adds nitrogen to the soil indirectly. **Burning of coal by man - Incorrect:** Burning coal releases nitrogen oxides (NOₓ) into the atmosphere, not into the soil. These contribute to air pollution and acid rain, not direct soil enrichment. **Death of vegetation - Correct:** When plants die, their tissues decompose. Decomposers (bacteria and fungi) break down organic matter, releasing nitrogen compounds back into the soil. Hence, this process adds nitrogen to the soil naturally.

The Phosphorus Cycle is unique among major biogeochemical cycles because it does not involve a gaseous phase. Instead, phosphorus primarily moves through the lithosphere (rocks), hydrosphere (water), and biosphere (organisms). - Weathering of Rocks: The main source of phosphorus in this cycle comes from the weathering of phosphate-rich rocks. Over time, geological processes break down these rocks, releasing phosphate ions (PO₄³⁻) into the soil and water. - Absorption by Plants: Plants absorb phosphorus from the soil, and it moves up the food chain as herbivores and carnivores consume plants and other organisms. - Return to the Soil: When organisms die, decomposition releases phosphorus back into the environment, where it can re-enter the cycle.  - Carbon Cycle: The main source of carbon is atmospheric CO₂, released by respiration, combustion, and volcanic activity.  - Nitrogen Cycle: Nitrogen is primarily cycled through the atmosphere, with nitrogen fixation (by bacteria) being a key process.  - Sulphur Cycle: Sulfur is mainly released from volcanic eruptions, decaying organic matter, and fossil fuel combustion, rather than rock weathering.