BIOGEOCHEMICAL CYCLES 



Some 30 to 40 elements are essential to plant and animal organisms (Smith 

 1966). The 18 essential elements for plants have been divided into 

 macronutrients (those needed in large quantities) and micronutrients (e.g., 

 trace elements and minerals). The macronutrients include carbon, hydrogen, 

 oxygen, nitrogen, phosphorous, potassium, sulfur, magnesium, and calcium. The 

 micronutrients are iron, sodium, chlorine, boron, maganese, zinc, copper, 

 molybdenum, and cobalt (Gauch 1972). 



The movement of nutrients within an ecosystem involves uptake or assimilation 

 of nutrients by plants from the available nutrient pool in the soil, or by 

 absorption from the atmosphere (figure 9-5). These elements may be 

 incorporated into long-term storage in woody tissue or released to the system 

 through leaf-fall, herbivore consumption, plant respiration, or leaching from 

 plant tissue. Subsequent breakdown of litter by decomposers in the soil makes 

 the nutrients available to plants again. Weathered soil and minerals 

 contribute to the nutrient pool; some elements may be redeposited as secondary 

 minerals . 



Nutrients enter and leave a forest system by meteorological, geological, or 

 biological processes. Meteorological inputs or outputs exist in the form of 

 atmospheric gasses (e.g., CO2 and O2), dissolved substances in rain or snow, 

 and wind-blown particulate matter. Geological movements include those 

 elements dissolved or suspended in surface or subsurface drainage and movement 

 of colluvial materials. Biological movement is the transfer of animal 

 material from one ecosystem into another. The nutrient budget for a northern 

 hardwood forest in New Hampshire was studied extensively (Likens et al. 1977) 

 and the conclusions listed below probably apply to most of Maine's forests 

 (table 9-12): 



1. precipitation and gasses are the major source of 

 sulfur, nitrogen, chlorine, and phosphorus; 



2. weathering is the major source of calcium, magnesium, 

 potassium, iron and sodium; 



3. photosynthesis and nitrogen fixation play a major role 

 in carbon and nitrogen inputs; 



4. ecosystems show net gains of nitrogen, sulfur, 

 phosphorus, and chlorine and net losses of silicon, 

 calcium, sodium, aluminum, magnesium, and potassium. 



Most of the loss is in surface and underground runoff, for which weathering 

 somewhat compensates . 



In undisturbed forest systems the nutrients available through meteorologic and 

 geologic sources are usually adequate to support forest vegetation. In some 

 areas deficiencies of certain elements or toxic levels of others may exist but 

 this is unusual. Problems concerning biogeochemical cycles arise when people 

 alter the flow of nutrients through ecosystems by air pollution, lumbering, 

 fire, and increased erosion. These problems will be discussed below. 



9-27 



10-80 



