Ecosystem processes: mineral cycles
by Peter Donovan
Carbon makes up about half of the biomass on earth. Carbon is the dominant element in salmon, sheep, Douglas fir trees, and bluebunch wheatgrass.
Carbon cycles between the atmosphere--mainly in the form of carbon dioxide--and the plants and soil. Photosynthesis fixes carbon from atmospheric carbon dioxide in the form of standing plant matter. Respiration and decay at all levels (as well as combustion) release carbon dioxide back into the atmosphere.
In brittle, seasonal-moisture environments, rapid and effective carbon cycling is often blocked. The symptoms are fuel buildup in grass, shrubs, or timber, desertification, lack of humus, organic matter, and water-holding capacity in soils, repeated hot fire, or what Wayne Burleson in Montana calls land idle-itis.
Carbon, and other biologically important elements such as oxygen, nitrogen, hydrogen, potassium, calcium, phosphorus, sulfur, and the trace elements, as well as water, are bank accounts for the diversity and mass of life. These elements and their combinations cycle between living organisms, the soil, and sometimes the atmosphere. These cycles, like the earth's atmosphere and soil, were created by earth's biodiversity out of the earth's elements. Even some iron-ore deposits are thought to have been the result of the oxygen produced by Precambrian cyanobacteria reacting with dissolved iron in seawater. Not only the gasoline you burn, but your pickup truck itself may owe its existence to those pioneers of photosynthesis.
Human activity greatly influences the mineral or element cycles not only on cropland but on range and forest land as well. As with the other ecosystem processes--the water cycle, solar energy flow, and community dynamics--the soil surface is the crucial interface. Where the soil surface is capped, bare, eroding, or where plant and animal residues are not being incorporated back into the soil, mineral cycling is blocked and ineffective (Allan Savory, Holistic Resource Management). This is often interpreted by land managers as a scarcity, requiring inputs or fire.
Conversely, concentrations of elements or their combinations, where they are not being effectively cycled, constitutes pollution. Nitrate leaching, eutrophication of lakes and streams, smoke, and massive concentrations of waste from agriculture and urban living are examples. Elements and compounds that are toxic to the life forms that cycle them--such as some heavy metals--are another type of pollution.
In this century humans have engaged in large-scale systematic contributions to carbon and nitrogen cycling. With carbon, we are burning fossil carbon and adding it to the atmosphere. Worldwide desertification caused by conventional decision making has reduced the ability of the biosphere to fix and balance atmospheric carbon.
Nitrogen is the key component of proteins that constitute much of biological tissue. Crop yields are often limited by the lack of biologically available nitrogen. Ecologist Vaclav Smil concludes that the enormous human population increase in the industrial era was made possible by the development of nitrogen-fixing crop rotations prior to the Industrial Revolution and then by "breaking the nitrogen barrier" via the Haber-Bosch ammonia synthesis pioneered in Germany in 1913 (Scientific American, July 1997).
Now, Smil estimates, we are one-third synthetic. A third of the nitrogen that is such an important component of the proteins in our bodies comes from ammonia manufacturing, where inert atmospheric nitrogen is made into biologically reactive ammonia fertilizer with the help of fossil or electric energy. Often, much of this fertilizer is not cycled. Widespread leaching of fertilizer-derived nitrates down into the water table is a problem in many agricultural areas.
Effective mineral cycling requires plants and soils full of life. Plant roots recycle enormous masses of material that otherwise would leach into the water table. Deep-rooted plants bring elements and combinations to the surface for subsequent use by shallower-rooted plants. Microorganisms--the bacteria, fungi, algae, protozoa, arthropods, and worms that outweigh the earth's mammals and birds by 50 to 1--break down plant and animal residues into reactive compounds (such as ammonia and urea) that growing plants use.
At the soil surface, air and water are required. Large animals help break down plant residue so that it will cover the ground, feed the microorganisms, and become part of the soil again. Animals of all types--salmon, coyotes, elephants, beetles, geese, and earthworms--move and redistribute these elements and combinations, creating a chaos of possibilities.
The problems that conventional management focuses on are leaching, waste pollution and toxicity, nutrient loading, erosion, sedimentation, scarcity and maldistribution of nutrients, chemical dependence, biofuel buildup, and atmospheric change. These are insoluble problems until we can see them as symptoms--of impaired mineral cycles, and of thinking that regards pollution, competition, and scarcity as fact, rather than as possible results of a process that we can guide and for which we are responsible.