Biogeochemical Cycles
  Spheres of B. Cycles
  Nitrogen Properties
  Simple Nitrogen Cycle
  Human Influences
  Spheres of the N Cycle
  Choose a Sphere



The biosphere consists of all of the earth's organisms living in the geosphere, hydrosphere and atmosphere. Research in recent decades has shown that the organisms participating in the nitrogen (N) cycle are essentially ubiquitous - living in guts of humans, animals, insects, worms, in plants, in dust and rain, to the bottom of the oceans, and down to many 100s of meters beneath the surface of the earth. Many of these recent research findings have yet to be incorporated into estimates of N budgets and reservoirs.

Nevertheless, it is clear that the biosphere's N reservoir is by far the smallest of the four spheres (Properties of Nitrogen) but its cycling of N is by far the largest (i.e., the turn-over rate of N is much greater in the biosphere than in any other sphere). The relatively large size of biospheric N cycling is fortunate because N is the limiting nutrient for much (if not most) life on earth and cycling increases the availability of N for growth.

The biosphere appears well designed from the viewpoint of energy efficiency in its use of N. Massive amounts of energy are required to reduce oxidized (nitrified) forms of N (e.g., NO2- and NO3-) to the ammoniacal forms of N which make up the proteins, enzymes and other essential biocompounds. Most of the N acquired and cycled is internal to the biosphere, with fixed, reduced forms of N making up the bulk of transfers within the food web of the biosphere's universe of plants, animals, and microorganisms.

An example of above-ground N transfer would be a plant producing a flower whose pollen is eaten by a butterfly. The butterfly is, in turn, eaten by a dragonfly which then gets eaten by a frog. The frog gets eaten by a snake which then gets eaten by a hawk. Each organism of this food chain represents a trophic level - level one being the plant and level six the hawk.

N transfer in the biosphere is more accurately depicted as a web than as a ladder or pyramid. The living tissues of all six trophic levels of the food chain are fed on by parasites. And generally 90 percent of the food eaten by any one tropic level is not passed up to the next highest level but is excreted as waste organic matter. And, as will be discussed, the N in these wastes feed whole communities of their own. Organisms can act on different trophic levels. For example, the frog can eat either the butterfly or the dragonfly. And a highly successful omnivore, such as Homo sapiens, can acquire N from any trophic level. Even the first trophic level (plants) acts on different trophic levels, e.g., plants utilize both inorganic- and organic-N. And organisms in the second trophic level - plant eaters (herbivores) - vary in the number of trophic levels capable of eating them. This is illustrated by the herbivores living on Illinois' most prevalent natural landscape element, the prairie:

"A considerable portion of the above ground biomass of a prairie was consumed each year by the grazing of a wide range of grazing animals, such as bison, elk, deer, rabbits and grasshoppers" (Robertson, 2000).

The number of trophic levels (levels of predators) above the bison and elk are different from the number existing above grasshoppers and rabbits.

Most of the N eaten by the herbivores, predators, and parasites was returned to the soil as organic wastes:

"Grazing increased growth in prairies, recycle[d] nitrogen through urine and feces, and trampling open[ed] up habitat for plant species that prefer some disturbance of the soil" (Robertson, 2000).

These organic wastes, along with dead biomass from above ground, and the roots of plants growing below ground - the roots of prairie plants can grow deeper into the soil that their aboveground parts grow into the air - supply the organic matter which supports the complex world of subterranean organisms.

Living in the soil are vast numbers of microorganisms such as bacteria, fungi, actinomycetes, protozoans, nematodes, and algae. A cup of soil can contain as many bacteria as there are people living on earth. Less numerous microorganisms (such algae and protozoans) typically number only 1 to 10 billion in the 10 cm. surface slice of a m2 of soil. Given their short lifetimes and extremely high potential reproductive rates, living and dying microbial biomass turns over large amounts of N, more than half of the 5.8 billion metric tons/yr estimated to occur in the top 1 m. of world soil. As in the above-ground world, microbial herbivores (and parasites) are fed on by at least three trophic levels of microbial and soil animal predators.

The density of organisms living in the immediate root zone (rhizosphere) ranges from 10 to 100 times that of the bulk soil. Roots excrete readily decomposable organic substances and slough off tissue, thereby stimulating a high degree of microbial activity in the neighboring soil. High numbers of highly active free-living microorganisms attack soil particles to release plant nutrients which ultimately become available to the root. Other organisms infect the root and send strands well out into the soil. Dissolved soil nutrients pass through the microbial strands into the root and organic matter passes from the root into the strand to feed it.

A similar relationship exists between roots and N-fixing microorganisms. Organic matter exterior to the root stimulates free-living, N-fixing microorganisms. Other N-fixers infect roots and live in symbiosis with the plant. The symbiotic N-fixers generally fix more N that their free-living counterparts because the plant provides not only food but also a reducing environment to protect the N-fixing enzyme (nitrogenase) from the deleterious effects of O2 gas.

Clearly, the biosphere is complex. The difficulties this complexity imposes upon our understanding of the biospheric N cycle are compounded by the fact that so many units of the biosphere have virtually complete N cycles unto themselves. The typical plant growing in the soil of the geosphere stimulates N fixation. This typical plant also stimulates N mineralization, and nitrification by providing complex root exudates of carbohydrate and hormone-like substances, which stimulate microbial growth, and the growth of worms and other soil animals. The mucus, wastes, and other excretions of soil animals also promote N fixation, mineralization, and nitrification, and denitrification. Conversely N-fixing bacteria not only supply fixed N, they also supply plant growth stimulating substances and stimulate the growth of N-fixing nodules in legumes. Furthermore, "Root infective arbuscular mycorrhizal fungi (AMF) increase root surface area thereby assisting the roots in exploring a large soil volume. AMF infection is known to increase nutrient uptake, N2 fixation, plant tolerance to soil chemical constraints (acidity, alkalinity, salinity), toxic elements and drought. They also help in biological control of root pathogens and are involved in nutrient cycling (solubilization, mineralization)" (Baligar and Fageria, 1999, p. 199).

Aquatic ecosystems present an even more complex picture than do the plant-based ecosystems of the geosphere. This is because the rooting zone of aquatic vegetation not only presents the same complex N cycle picture as does terrestrial vegetation, but also the same suite of interactions exists to a significant extent on the plant surfaces exposed to water - the aquatic surfaces of plants being mini-ecosystems unto themselves and in free-living (planktonic) algae living in the water.

For example, planktonic algae of an productive (eutrophic) lake can fix an amount of N equal to nearly half that received annually in runoff and atmospheric deposition. It is not unusual for aquatic plants (macrophytes) to have amounts of algae and bacteria living on them equal to the total weight of the plants themselves. The productivity of algae living on aquatic plants (epiphytic) occupying 1/4th of the area of a lake can account for 3/4ths of the total algal productivity of even a productive lake in a limestone watershed. "Nitrogen fixation by epipelic [bottom living], as well as epiphytic, blue-green algae and bacteria is common..." (Wetzel, 1983, p. 567). Blue-green algae living on the surfaces of vascular aquatic plants have been reported as fixing N at rates from 15 kg N/ha-yr to greater than 500 kg N/ha-yr at plant densities of 200 g plant dry weight/m2 (e.g., Goering and Parker, 1972; Head and Carpenter, 1975).

Individual wetlands can have vascular plant densities that range from 0 to 1,000s g plant dry weight/m2. N-fixation may be appreciable across the whole range of values. For example, N-fixing microorganism living in and on the stems of living and dead Spartina (2200 g plant dry weight/m2) fixed an amount of N during the month of July alone that "...was at least 1550 kg N2/ha. The barren zone, during its peak fixation month of July had a potential nitrogen input of 210 kg N2/ha. In comparison, the growing season of an alfalfa crop encompasses at least two months, and during this period approximately 250 kg N2/ha are produced. During the growing season, a crop of soybeans fixes about 150 kg N2/ha (Stewart, 1966)." (Green and Edmisten, 1974, p. 125).

So-called barren zones of wetland areas lack growth of vascular plants but often have appreciable growths of algae. Algae living in the top 0.25 cm. of marsh soil have been shown to fix 64 Kg N2/ha-yr and blue-green algal mats living on top of marsh soil have been shown to fix over 200 Kg N2/ha-yr (Carpenter et al., 1978). N-fixation in the top 8 in. of wetland soil at 15oC has been shown to be 144 kg N/ha in a 180-day period (Tjepkema and Evans, 1976).

Furthermore, aquatic and wetland sediments undergo especially intense N cycling. For example, populations of N-fixing bacteria are especially large in oxygen-free, waterlogged sediments. In such environments N-fixing microorganisms do not have to expend as much energy to protect the N-fixing enzyme (nitrogenase) as they would in oxic sediment. This, plus a usually plentiful supply of organic matter to use as sources of energy, make for relatively intense N-fixation on the order of 10s to 100s of kg N/ha-yr in natural wetlands and aquatic sediments. This has also been also been demonstrated experimentally. For example a review of the literature reported, "In waterlogged soils amended with 1 % straw, or less, nitrogen fixation rates up to 150 kg ha-1 a-1 were achieved; with 5 to 20 % straw and waterlogged conditions 500 to 1000 kg ha-1 a-1 were fixed. The responsible organism was Clostridium butyricum and Meiklejohn (1967) also found that the number of clostridia increased considerably when approximately 678 to 1356 kg/ha of compost was added to the soil." (Stewart, 1969, p. 370).

N fixation has been reported to be especially intense in the rhizosphere of aquatic plants, e.g.,

"For a typical T. testudinum stand, N2 fixation is estimated to be 100 to 500 kg N/hectare per year. Numbers of N2-fixing in the rhizosphere sediments were roughly 50 to 300 times more abundant than those in the non-rhizosphere sediments..." (Patriquin and Knowles, 1972, p. 49).

And for a stand of maximum productivity, N-fixation was estimated to be "...350 to 1700 kg N/hectare per year. These estimates are comparable to estimates of N2 fixation associated with legumes, which are typically in the range 110 to 220 kg N/hectare per year" (Patriquin and Knowles, 1972, p. 56).

These reducing aquatic and wetland sediments also are the site of appreciable denitrification of NO3-N produced in the surface oxidized sediment layer (if present) and/or brought in from overlying oxygenated water (if present) to the reducing environment of the underlying waterlogged sediments. However some of this N gas is recycled by N fixation.

Whereas the biosphere appears to be energy and material inefficient in utilizing only a small part of available N - that portion which undergoes nitrification/denitrification - these apparent inefficiencies are also part of an excellent design. Namely, the leakage of N through nitrification/denitrification allows N to spread out from N-rich environments in solution as NO3- and as gases - much as seeds can spread out and away from trees. In this way the N-rich portions of the biosphere "seed" the entire earth with N.

People are part of the biosphere. Since the harnessing of fire, long before the dawn of recorded history, essentially wherever people have lived they have had a profound influence on the biosphere and its N reservoir and N cycle. Fire generally promoted the growth of N-rich vegetation which, in turn, promoted the accumulation of soil N through the humification of the N-rich organic matter. Fire also stimulated symbiotic and asymbiotic microbial N fixation directly. N fixation was indirectly promoted by fire as it enhanced the biomass to support large herds of grazing animals to live on the N-enriched landscape.

Fire also enhanced the flow of terrestrial N to the atmosphere and the hydrosphere (surface and ground waters). Nevertheless, in spite of the increased transfers, the fire-induced gains in terrestrial N were generally greater than the fire-induced losses to the atmosphere and hydrosphere (Human Influences on the Nitrogen Cycle).

In Illinois, and much of the Midwest, annual burnings by Native Americans kept the N cycle (and other nutrient cycles) at high rates of fixation, transfer and accumulation. Fire also shaped the vegetation. Whereas now Illinois is ecologically mapped as being in the eastern hardwoods ecoregion (if you abandon land, trees grow on it) when Europeans began settling the state, 9 million ha. of Illinois' 14 million ha. of land was prairie due to repeated burning by Native Americans and intense grazing by buffalo and other animals. And the one-third of Illinois that was forested had more park-like forests than the forests of today - the forest floor being rich in grasses and legumes, and the species composition of the forest overstory was different.

Early explorers and settlers noted than once Native Americans were cleared off the land in advance of European settlement, prairies soon stated growing up into trees. And the large herds of buffalo and other animals whose dung and bones littered the landscape also disappeared within 20 years after the departure of the herds. As with the forests, the remaining prairies changed in plant composition and lost most of their N-fixing legumes.

The clearing of land by the European settlers went slowly at first. Populations were low relative to today's standards and modification of the land had to be done by hand and animal power, not machines as are used today. By 1900 only about 4 million ha. of Illinois was plowed and growing row crops. Machine power enabled the drainage of about 4 million additional ha. of land (mostly in the 20th century) and the total area of Illinois in crop land increased to 9.3 million ha. Drainage and leveeing reduced the organic and nutrient loadings that aquatic and wetland vegetation imposed on Illinois' surface waters.

At first, the prairie soils were too rich in N and other nutrients to grow crops well. After some years fertility declined but bumper crops could still be grown for decades without addition of N and other nutrients. For decades there was still so much N in the soils that crusts of nitrate salts were an unremarkable phenomenon in Midwest soils. And so much nitrate could accumulate in animal forage that animals eating it were poisoned.

However, over time soil-N levels and crop yields dropped to low levels. In the second half of the 20th century crop yields increased to record levels due to improved soil and crop management techniques, development of new plant varieties, and the use of chemical fertilizers.

There are major challenges to estimating historical mass balances for the biosphere and human impacts on it. For example, in estimating the impacts of agriculture to Illinois' N mass balance the effect of removing the N-fixing prairie ecosystem needs to be taken into consideration. Such mass balance estimates need to take into account the agriculturally-induced decrease in the soil-N reservoir and concomitant decreases in natural soil-N cycling. Human impacts must consider both anthropogenically-induced reductions as well as additions. So far, only additions are considered in N mass balances.

For example, today's natural N cycle is generally considered only from the viewpoint of its being an imperfect filter against the removal (by denitrification) of anthropogenic N additions. However, today's natural ecosystem still has a N-fixing component which needs to be taken into consideration. Mass balance studies in Illinois have yet to consider how anthropogenic N additions interact with the whole of the remaining natural N cycle: an example being that N additions will reduce asymbiotic and symbiotic N fixation in both terrestrial and aquatic ecosystems.

Conversely, massive pollution of surface waters by phosphorus (P) in detergents and other pollutants greatly increased blooms of N-fixing algae which contributed to the N enrichment and eutrophication of lakes and rivers. Fortunately, timely recognition of the importance of controlling phosphorus pollution in the 1960's and 1970's led to a subsequent reversal in eutrophication and associated N enrichment by blooms of N-fixing algae (e.g., Paerl, 1990). The recognition of the relationship between P-enriched environments and concentration of NO3- in water goes back to the 1920s (McHargue and Peter, 1921). But such interaction of N with P and other elemental cycles in the biosphere has yet to be considered in Illinois mass balance studies (e.g., David and Gentry, 2000a, b) nor have they been used to explain trends in water quality (Krug and Winstanley, 2000).

The challenges to estimating historical mass balances for the biosphere and human impacts on it are formidable; and we have a long way to go in recognizing and overcoming them.

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Overview w B. Cycle w Spheres w N Props w N Cycle w Influences w N Spheres