ABSTRACT
Nutrients enter ecosystems from the atmosphere and from rock weathering, and can exit the ecosystem into the atmosphere and by drainage water. The role of the element carbon in the environment is currently receiving enormous attention. The behaviour of energy in ecosystems is referred to as ‘energy flow’ because energy transformations are unidirectional, in contrast to the cyclical behaviour of nutrients. Changes in biomass from year to year indicate the amount of energy or carbon fixed by photosynthesis and incorporated into an ecosystem. The carbon and nitrogen cycles have much in common. Both elements are relatively enriched at the Earth's surface due to microbiological activity. In their studies of the calcium cycle in the natural forest of Hubbard Brook, F. H. Bormann and G. Likens discovered that the forest ecosystem is extremely conservative in its nutrient cycling. Soil micro-organisms play a key role in nutrient cycling in terrestrial ecosystems.
