Although all of the above are part of the nitrogen cycle, the major reservoir for nitrogen is the atmosphere. The atmosphere contains nitrogen gas which cannot be extracted by plants or animals from the atmosphere. From its reservoir in the atmosphere, nitrogen gas is combined with oxygen to  form nitrate and carried to Earth dissolved in rain. Nitrogen-fixing bacteria produce ammonia. This is absorbed by plants and other producers and incorporated into biological molecules that are passed through the trophic levels. Nitrate and ammonia are released by excretion or by decomposer bacteria. Other bacteria convert these molecules back to atmoshperic nitrogen, completing the cycle.

A source is an organism or physical body that releases a certain compound or element. Through energy dynamics or physical manipulations of the environment, it reaches the sink. The sink is the receiver of the element and can act as another source for a different organisms or physical body. This is how matter travels in the environment, from pairs of source to sink dynamics. 

According to the principles of the Nitrogen cycle, plants can only absorb nitrogen in the form of ammonia. This is achieved by nitrogen-fixing bacteria that convert atmospheric nitrogen into a form usable by plants. Plants excrete nitrogen in the form of waste as nitrates which are absorbed by animals. 

The phosphorus cycle does not contain an atmospheric phase, while both the sulfur and nitrogen cycles do.

The reservoir of phosphorus in ecosystems is rock, where it is bound to oxygen in the form of phosphate. As phosphate-rich rocks are exposed and eroded, rainwater dissolves the phosphate. Dissolved phosphate is abosrbed through the roots of plants. Animals eat the plants and after they die decomposers return the phosphorus that remains in the dead bodies back to the soil and water. It may then be reincorporated into rock. 

All of the other cycles have at least one atmospheric component in their systems. The carbon cycle incorporates carbon dioxide in the photosynthesis portion of its cycle. The sulfur cycle incorporates gaseous sulfur dioxide when it is released by volcanic eruptions. The water cycle has the condensation of clouds in the atmosphere and the precipitation of those clouds as well. The nitrogen cycle incorporates atmospheric nitrogen gas before it is fixed by cyanobacteria. Only the phosphorus cycle does not have a phosphorus containing compound in the atmosphere that is essential for life on Earth. 

The water cycle remains in the form of water throughout the cycle. The major reservoir of water is the ocean. The water cycle is driven by solar energy which evaporates water, and by gravity, which draws water back to Earth in the form of precipitation from water vapor in the atmosphere.

Transpiration is the evaporation of water from plants. Both terms can be defined as the conversion of liquid water on Earth into atmospheric water vapor. Condensation is the formation of clouds from water vapor. Precipitation is the release of water from clouds down towards the earth in the form of rain, hail, sleet or snow. Percolation is the seeping of water through the ground into groundwater sources like aquifers. Runoff is excess water on the superficial soil layers that cannot percolate into the ground. 

K-strategist populations are more commonly regulated by density-dependent limiting factors. Their population sizes hover around a carrying capacity that is dependent on factors that increase in severity with the density of the population. On the other hand, r-strategist populations are regulated by density-independent limiting factors. They reproduce rapidly until a density-independent factor causes many of them to die.

Cultural carrying capacity suggests that humans have secondary needs, such as entertainment, palatable food, and mental/spiritual development. These non-vital resources can only be spread equally among a population of a limited size. Cultural carrying capacity describes the limitation on population size after taking into account these factors. 

A frequent questions asked by sustainability scientists is: Should the optimum sustainable population be based on cultural carrying capacity? Many would agree yes, for the reason that when cultural capacity is transgressed, living conditions spiral downwards and life is reduced to the struggle for mere survival. In such conditions, human inventiveness and creativity become obsolete.