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| 4680396885 | Hydrological cycle | The natural cycle by which water is circulated between various reservoirs on or beneath the Earth's surface and the atmosphere. The main source of energy that drives the cycle is heat from the Sun. Evaporation, transpiration, runoff, condensation, precipitation and infiltration. |  | 0 |
| 4680402661 | Nitrogen fixation | The process by which atmospheric nitrogen (N2) is converted from unusable diatomic form into usable ammonia (NH3) (by bacteria such as rhizobium or cyanobacteria) that can be absorbed by plants. |  | 1 |
| 4680406581 | Ammonification | Nitrogen is converted into ammonifying bacteria; may occur when nitrogen in organic wastes in the soil are converted to ammonia or when atmospheric nitrogen (N2) is converted to (NH3) |  | 2 |
| 4680408352 | Nitrification | The biological oxidation of ammonia or ammonium (in dead organic material) into nitrates and nitrites by soil bacteria. The transformation of ammonia to nitrite is usually the rate limiting step of nitrification. | 3 | |
| 4680412766 | Denitrification | Bacteria convert nitrate (NO3-) and nitrite (NO2-) back into atmospheric nitrogen. Bacteria convert ammonia (NH3) back to nitrogen (N2) or N2O-typically done by anaerobic bacteria |  | 4 |
| 4680420013 | Assimilation | Inorganic nitrogen (N2) is converted into organic molecules such as DNA/amino acids and proteins. Plants assimilate N as ammonium (NH4+) or nitrate (NO3-) through their roots Animals assimilate organic N compounds by eating plants |  | 5 |
| 4680425868 | Distinguishing characteristic of Phosphate Cycle | Phosphorus does not circulate as easily as nitrogen because it does not exist as a gas, but is released by slow weathering of phosphate (PO4 3-) rocks. This is an exclusively sedimentary cycle...it is never found as a gas | 6 | |
| 4680429465 | Excess phosphate in aquatic ecosystems | Added through runoff of animal wastes, excess fertilizer, and discharge of sewage. A limiting factor in freshwater ecosystems. Excess P leads to eutrophication. | 7 | |
| 4680432172 | Carbon sinks | Places such as forests, ocean sediments, and rocks and soil, where accumulated carbon does not readily reenter the carbon cycle. | 8 | |
| 4680438917 | Biome | A large region characterized by a specific type of climate, soil, and certain types of plant and animal communities. Terrestrial biome determining factors are temperature and precipitation. Aquatic biome determining factors are salinity and sunlight. |  | 9 |
