The four classes of organic compounds are carbohydrates, proteins, fats and nucleic acid. Protein and nucleic acid contain nitrogen. Plants get their carbon from the air and their nitrogen from the soil through carbon and nitrogen cycles. Animals get their nitrogen and carbon from the plants and other animals which they consume. In all organisms, the organic compounds are converted into various things such as tissue fats, energy, hereditary information, etc. As the organisms died and decompose they are returned to the soil by the decomposers.
The phosphorus cycle does not include an atmospheric component because phosphorus does not cycle through the atmosphere. In comparison, important processes of the carbon and nitrogen cycle occur in the atmosphere.
The diagram in the picture is a cladogram, a diagram that depicts rellationships between different groups of organisms, reconstructing evolutionary history. The cladogram is composed of nodes, clades and roots (initial ancestor, common to all).