Bacteria and protozoa as integral components of the forest ecosystem--their role in creating a naturally varied soil fertility.

The paper explores interactions between the two first organism groups to appear on earth, the bacteria and protozoa, and their interplay with the rest of the ecosystem focusing upon northern boreal forests. The microbial loop is suggested as a mechanism for local inputs of new N to the ecosystem. The possibility to couple short-term microbial processes with their long-term effects,--as registered in plants, soil and the atmosphere, via the abiotic variables--is explored. The latter are investigated in relation to the environments they create for the micro-organisms, and how this results in varying soil fertility. A chain of events is presented that relate high Ca concentration in the mineral soil and high water availability to increased nitrogen availability for plants via the micro-organisms. An example is given of the influence of these parameters directly upon protozoa along an extreme fertility gradient, and also indirect evidence from a Finnish field study of 30 sites with four fertility levels. Finally, there is a discussion about ways to convert knowledge gained in detailed studies of microbial interactions into forms useful when evaluating the present status of and effects of ameliorative management on ecosystems strongly affected by humans.