SOIL NUTRIENT SUPPLY IN NATURAL AND MANAGED FORESTS

Soils differ in their ability to supply the nutrients necessary to sus tain forest productivity. Nutrients are added through natural processe s such as weathering of primary and secondary soil minerals, mineraliz ation of soil organic matter including the forest floor layer, fixatio n of nitrogen primarily through symbiotic microorganisms, and natural or induced atmospheric deposition. Nutrients become unavailable for pl ant uptake through immobilization by soil microorganisms and through c hemical and mineralogical reactions including precipitation and adsorp tion reactions and ionic fixation within lattice structures of clay mi nerals. Losses of nutrients can take place through soil leaching and e rosional processes. Nutrients can also be added or lost through human activities such as fertilization and harvesting. Nutrient supply conti nually shifts with the rate and direction dependent on the prevailing processes in the soil system, but subject to overriding human influenc e. Over relatively short periods of time, the soil nutrient supply can be subject to seasonal fluctuations. Factors affecting long-term nutr ient availability are functions of soil mineralogy, the rate of minera lization of the organic matter of the soil and forest floor layer, and plant-soil relationships of the species occupying the site (deciduous vs. coniferous species, deep vs. shallow rooting, symbiotic nitrogen fixation). The long-term stability of the soil nutrient supply is of i ncreasing concern in the face of a diminished forest land base, increa sed demand for forest products, and reluctance to apply nutrients to m any forest areas because of environmental or economic constraints. The re are questions to consider in evaluating the nutrient sustainability of forest areas if we expect to maintain the long-term nutrient stabi lity of natural and managed forest ecosystems.