Man and Biogeochemicalcycles: Impacts,problemsand Research Needs

Nitrogen, phosphorus and sulphur are essential elements in all living matter and are found in varying proportions to carbon, depending on the chemical nature of the compounds in which they occur. A pattern for the circulation of these elements in nature developed when microorganisms, plants and animals appeared on earth. The pattern was set by the physical and chemical environment, by the chemical nature of these elements, and by the geographic distribution of microorganisms, plants and animals. The appearance of Homo sapiens may not have had any great effect on these patterns as long as man was nomadic. That state of affairs changed when man formed settlements, where he transported vegetable matter and meat and where he deposited most of his wastes. The effect must have been particularly noticeable for phosphorus which is hardly leached from soils. Analyses of soil phosphate have, incidently, been used to locate archaeological settlements, (Arrhenius, 1931). Also in Eastern Canada, clumps of shumack trees are used by archaeologists as indicators of ancient Inchan campfires. These trees flourish on phosphorous enriched soils (R.E. Munn, pers. comm.). Hence a steady flow of phosphorus took place from nearby pastoral and agricultural areas, ending up in the soils of the settlements and their immediate surroundings. It is, of course, difficult to assess the impact of such a process on the environment; it is continuing today, with the difference that in some settlements a great deal of the accreted phosphorus is released into lakes and rivers. In some regions agriculture has been practised for a very long time which must have led to a sizeable depletion of phosphorus in soils, one reason for the present use of phosphorus fertilizers. Similar processes must also have occured for nitrogen and sulphur, although their chemistry would favour leaching from soils or return to the atmosphere. In semiarid areas, however, archeaological sites sometime show accumulation of nitrate in and around former