Forest Nutrient Cycles

Besides participating in global biogeochemical cycles, ecosystems cycle nutrients internally. When the ecosystem is harvested, such as in logging, the nutrients must be replaced if the productivity of the ecosystem is to be sustained. For example, a forest ecosystem receives nutrient input from the atmosphere and from weathering of the bedrock. Atmospheric input includes nitrogen that is fixed by nitrification as well as nitrogen oxides from natural and human-made sources, and from dust and aerosols.

Forests export nutrients to the atmosphere and by streamflow. Streams leach mineral nutrients and carry away biomass that has fallen in. Microbes in anaerobic environments such as bog sediments eliminate nitrogen by denitrification and sulfur by reduction to H2S. Trees also emit ammonia, H2S, and organic compounds such as terpenes. The organic emission is famous for causing the haze in the Great Smokey Mountains of the southeastern United States and for giving President Ronald Reagan a reason to discount anthropogenic pollution, saying that "trees pollute.'' (Actually, it is true that plants may emit more hydrocarbons to the environment than comes from human activities. Aspen and oak leaves emit as much as 2% of the carbon produced by photosynthesis as isoprenes. The isoprenes may play a role in protecting photosynthetic membranes. However, photochemical smog found in urban air is caused not only by hydrocarbon emissions, but by nitrogen oxides produced together with them.)

The fluxes of nutrients within the forest are usually much greater than the external flows. These fluxes amount to an internal recycling. Nutrients taken up by trees are returned to the soil by leaf fall. Biodegradation by saprophytes releases the nutrients, whereupon they are again absorbed by the roots. The nutrient cycles operate faster in warmer climates, as biomass in the litter decays more rapidly. Plants develop thick root systems close to the surface to intercept nutrients before they leach away. As a result, more than half of the organic carbon in temperate forests is in the soil and litter; whereas in tropical rain forests the figure is 10 to 25%. Thus, tropical soils tend to be very poor in nutrients. As a result, harvesting the trees to convert a tropical forest to farmland often leaves the soil depleted of nutrients. Thus, ironically, these highly productive lands become unproductive in their new use. Temperate forest lands, on the other hand, make productive farmland.

TABLE 15.2 Annual Mineral Balances for Several Watersheds of the Hubbard Brook

Experimental Forest, 1963-1969

Experimental Forest, 1963-1969

TABLE 15.2 Annual Mineral Balances for Several Watersheds of the Hubbard Brook

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