Mineral Cycling: Some Basic Concepts and Their Application in a Tropical Rain Forest

Abstract

While the importance of mineral nutrition for plants and animals has been recognized for centuries, only recently has there been a systematic approach to mineral element cycling in entire ecosystems (47) By a systematic approach to ecosystem cycles we mean studies that yield estimates of the total amount of mineral elements and rate of element cycling within a complete landscape unit during a period of time, or under certain atmospheric or other physical conditions. Recent systematic studies (2, 43, 45, 50) have resulted in the formulation of several basic mineral-cycling concepts. In this paper we discuss several of these concepts in relation to a mineral-cycling study of a Puerto Rican tropical rain forest. Worldwide cycles of mineral elements have recently been discussed by Deevey (6). He focused primarily upon carbon, hydrogen, oxygen, and nitrogen and their reactions in the biosphere, lithosphere, hydrosphere, and atmosphere. It is our purpose to discuss principles of mineral cycles at the terrestrial ecosystem level. Because carbon, oxygen, and nitrogen are in a gaseous form for a large proportion of their cycling time, it is difficult to relate their cycles to the processes of a single ecosystem. Likewise, hydrogen, which moves in water vapor in the air is also difficult to relate to a single ecosystem. In contrast, cycles of elements such as calcium, potassium, and phosphorus that do not ordinarily volatilize can be more easily viewed in the context of one ecosystem. For this reason we have limited our discussion to this type of mineral element. The tropical rain forest mineral-cycling study on which this discussion is based was conducted between 1965 and 1969 and was part of the Atomic Energy Commission sponsored Ecology Project near El Verde, Puerto Rico. Background of the project and the results of a forest irradiation experiment have been compiled in a single volume (41), and methods used in the mineral-cycling study will be published elsewhere (20).