SOILS AS BIOTIC CONSTRUCTS FAVORING NET PRIMARY PRODUCTIVITY

Many, if not most, physical and chemical properties of soils required for plant growth are affected strongly by biotic processes. Feedback p rocesses involving primary producers and decomposers may be involved i n the development of properties that favour net primary productivity i n terrestrial ecosystems. However, both constructive and destructive e ffects of biota on soils can be observed. Apparently, effects favourin g net primary production have accumulated and presently prevail in the various terrestrial ecosystems of the world. In some ecosystems, howe ver, the dominant vegetation gains competitive advantage by making soi ls un''favourable'' for most other plants. Ombrotrophic peat bogs and heathlands are cases in point. On the global scale, biotic processes c an be seen as responsible for the persistence of water on the earth, t hrough control of the earth's surface temperature under the influence of greenhouse gases. As a result, therefore, the large-scale geochemic al and hydrological cycles, which are essential for chemical rejuvenat ion of the earth's surface, also depend on life processes. This is an aspect of Lovelock's Gaia hypothesis, which states that the earth has evolved over geologic time by feedback processes keeping the earth in a state comfortable for life by the action of living organisms. While the development of soils with ''favourable'' properties may be explain ed in evolutionary terms, such an explanation apparently does not suff ice for the development of Gaia as a whole.