Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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synthesis of ammonia. Much of this synthetic nitrogen is used in the manufacture of fer-

tilizer, which goes directly into increasing the primary productivity of crops.

Many of the transformations that make up the nitrogen cycle are redox reactions.

Nitrogen is present in its most reduced state as organic nitrogen or as ammonia. (The Kjel-

dahl nitrogen determination measures the sum of these two.) The most oxidized form is

nitrate. Intermediate between these extremes are nitrite, NO, N

O, N

, and N

,in

order of increasing reduction (Figure 13.17). The primary biologically mediated steps in

the nitrogen cycle are discusse d next (Figures 14.7 and 14.8).

Fixation: the conversion of N

to ammonia. The prokaryotic aquatic blue-green

bacteria (cyanobacter) are photosynthetic and get their energy directly from the sun.

Bacteria such as Rhizobium an d Bradyrhizobium beneﬁcially infect the roots of

leguminous plants such as alfalfa, clover, peas or beans, and some lichens and marine

algae. Those plants sacriﬁce some of their growth to share the energy they captured

from the sun with the nitrogen-ﬁxing bacteria, in return for a rich source of available

nitrogen. Azotobacter is a free-living bacterial nitroge n ﬁxer. Fixation requires a

considerable amount of energy. Legume bacteria use about 40 kcal (equivalent to about

10 g of gluc ose) to ﬁx 1 g of nitrogen. Free-living nitrogen ﬁxers require about 10 times

as much.

Ammoniﬁcation: the conversion of organic nitrogen to ammonia. This is performed by

saprotrophic organisms which release nitrogen in excess of their own n eeds as

ammonium.

Atmosphere

Land and water

–

Organic N

Lightning

Industrial

fixation

Fossil fuel

Combustion

from air to: Land Sea Total

Acid rain 17 9 26

Dry deposition 15 4 19

Forest fires

Assimilation

1000

Mineralization

(Ammonification)

Nitrification

Denitrification

Land 107-161

Sea 40-120

Biofixation

Land 139

Sea 10-90

Volcanism 5

Figure 14.7 Fluxes in the global nitrogen cycle. Estimated ﬂuxes in Tg/yr. Ammonia, organic

nitrogen, and other forms also enter the atmosphere and oxidize or fall with rain. Dotted line arrows

represent primarily anthropogenic ﬂuxes. (From Odum, 1987.)

458

ECOLOGY: THE GLOBAL VIEW OF LIFE

Nitriﬁcation: the chemoautotrophic conversion of ammonia ﬁrst to nitrite, and

subsequent conversion of nitrite to nitrate. The ﬁrst step is accomplished by bacterial

genuses such as Nitrosospira and Nitrosomonas. The second step is done by bacteria such

as Nitrobacter, and in marine environments by Nitrococcus. Nitriﬁcation is also

responsible for the production of nitric oxide (NO) and nitrous oxide (N

O).

Denitriﬁcation: the conversion of nitrate and nitrite, through a series of steps,

ultimately to nitrous oxide and N

. This process is performed by numerous facultative

heterotrophic bacteria when oxygen is limiting. Den itriﬁcation can occur even in

nominally aerobic environments when anoxic micro environments are present. This can

occur, for example, in clumps of soil where organic matter is present to deplete the

oxygen locally and provide energy for denitr iﬁcation, or in the interior of ﬂocs in the

activated sludge wastewater treatment process. Nitrate serves as a substitute for oxygen as

an electron acceptor for heterotrophic respiration.

Assimilation: the conversion of inorganic nitrogen to organic forms such as amino

acids. Plants can absorb ammonia or nitrate, but mostly they use nitrate. Nitrite is toxic to

plants. Plant fertilizers usually contain ammonia or urea, which breaks down to ammonia.

The ammoni a is then nitriﬁed. After absorption, the plant converts the nitrate back into

ammonia before converting it into amino acids or other organic nitrogen compounds.

The nitrous oxide produced by denitriﬁcation can be oxidized photochemically in the

stratosphere to nitric oxide, which then reacts with ozone to form NO

. Along with the

species from nitriﬁcation, these ultimately form nitrate again. This is deposited with

precipitation and is part of the problem of acid deposition (Section 15.7). About 25% of

the total NO

-N production is thought to originate in lightning and in biological reactions

in soil. The rest is anthropogenic, including fossil-fuel combustion and forest burning.

Figure 14.8 Another view of the global nitrogen cycle, showing storage reservoirs of nitrogen.

Values are kg/m

of the Earth’s surface. (Based on Whittaker, 1975.)

FLOW OF MATTER IN ECOSYSTEMS 459

Ammonia is also released by microbial activity in the soil. Much is reabsorbed by

plants, which also absorb nitrate deposited on leaves. Ammonia in the air can neutralize

some o f the atmospheric sulfate, the other major acid rain component. Rainfall can also

contain signiﬁcant quantities of dissolved organic nitrogen (D ON). The source of the

DON may be from ocean spray tranported to the land.

Overall, rainfall contributes about 17 Tg N/yr in the form of nitrate to the land, plus

15 Tg N/yr as ammonia and 9 Tg N/yr as DON. Dry deposition contributes another 16 Tg/

yr, mostly nitrate, for a total ﬂux to land of 57 Tg/yr. The oceans receive a total of at least

20 Tg/yr.

The uncertainty in estimates for global nitrogen ﬁxation and denitriﬁcation will need to

be reduced before it can be determined if ﬁxation and denitriﬁcation balance each other

(Figure 14.7 and Table 14.3). The biological ﬁxation on land includes an anthropogenic

ﬂux of 44 Tg/yr due to human cultivation of leguminous crops.

The nitrogen cycle is an intere sting example of the Gaia hypothesis at work. Lovelock

points out that one of the effects of life is the maintenance of chemical disequilibrium in

the environment. If you turned off all biotic reactions, chemical equilibrium would even-

tually be achieved among the various species in the environment. In the case of nitrogen,

the atmospheric O

would combine with its N

to form nitrate, which would be dissolved

in the oceans. It is commonly understood that the oxygen in the atmosphere is maintained

there by photoautotrophs. What is less well known is the fact that most of the nitrogen is

in the atmosphere instead of the ocean, because of denitriﬁers. Without life, Earth’s atmo-

sphere might resemble that of Mars, which is 95% CO

Industrial ﬁxation of nitrogen by the Haber–Bosch process has altered human ecology

signiﬁcantly. Each year, 175 million tons of nitrogen ﬂow into the world’s croplands, half

of which is assimilated by cultivated plants. Synthetic fertilizers provide about 40% of all

nitrogen taken up by these crops. The crops furnish about 75% of all nitrogen consumed

by humans (the rest comes from ﬁshing and from grazing cattle). Thus, about one-third of

the protein in humanity’s diet depends on synthetic nitrogen fertilizer. This has undoubt-

edly facilitated the tripling of the world’s population that occurred in the twentieth century.

14.2.5 Sulfur Cycle

Roughly 90% of the world’s supply of sulfur (about 1016 metric tons) is held in sediments

and rocks and the remaining fraction is largely found dissolved within our oceans, which

TABLE 14.3 Global Nitrogen Fluxes of Fixation and Denitriﬁcation

(Tg N/yr)

Source Fixation Denitriﬁcation

Land (biological) 139 107–161

Ocean (biological) 10–90 40–120

Atmosphere (lightning) 4

Forest ﬁres 12

Industrial, including fertilizers 40

Fossil-fuel combustion 22

Total 227–307 147–281

Source: Berner and Berner (1987).

460 ECOLOGY: THE GLOBAL VIEW OF LIFE

in turn represents the dominant reservoir for the majority of sulfur being cycled within our

biosphere. The cycle reactions for sulfur are depicted schematically in Figure 13.25, and

the global cycle in Figure 14.9.

Sulfur is essential for life largely because it is a component of two of the amino

acids, cysteine and methionine. The sulfur cycle may be the cycle that is most affected

by human activities. Human emissions of CO

and nitrogen are about 5 to 10% of natural

emissions. However, human activity releases about 160% as much sulfur as nature

does.

Like nitrogen, sulfur cycles through a number of oxidation states. The most oxidized

form, sulfate, serves as an alternative electron acceptor in respiration. It is generally used

only if both oxygen and nitrate are absent, since it is less energetically favorable to organ-

isms. Sulfate is the form of sulfur taken up by the primary producers, and thereby intro-

duced into the food chain.

The most reduced inorganic form is sulﬁde. The toxic volatile compound hydrogen

sulﬁde is commonly associated with anaerobic environments that have large amounts

of organic matter, such as marsh sediments, anaerobi c digesters in wastewater treatment

plants, and petroleum deposits. In these the organic matter serves as a reducing agent, ﬁrst

reducing oxygen to CO

, any nitrate is denitriﬁed, and then all sulfate is reduced to H

Sulﬁde is also found in deep groundwater aquifers, having been reduced by either soil

organic matter or by ferrous iron. If ferrous iron is present, the sulﬁde will form the non-

volatile, low-solubility precipitate iron sulﬁde (FeS). Sulﬁde is also used by photoauto-

trophic bacteria as an electron donor in place of H

O in a process similar to

Mining

149

Volcanism

Dust

Industrial

Biogenic gases

Wet and dry

deposition

Rivers

213

Weathering and erosion

Deposition

258

Sea

salt

144

Biogenic

gases

Volcanism

Pyrite

Hydrothermal

sulfides

Aerial transport to sea

Aerial transport to land

Figure 14.9 Global sulfur cycle. (Based on Krebs, 1994.)

FLOW OF MATTER IN ECOSYSTEMS 461

photosynthesis. Sulfur here plays the role of oxygen, and instead of O

being produced,

elemental sulfur is the product. In aerobic environments, reduced forms of sulfur are

rapidly oxidized to sulfate by chemoaut otrophic bacteria. Thus, the sulfur cycle is closely

linked to the carbon cycle.

The sulfur cycle is also linked to the phospho rus cycle in aquatic systems (Secti on

14.2.6). When iron sulﬁde is oxidized in aquatic sediments, sulfate is released and phos-

phorus precipitates with the iron, becoming unavailable to organisms. Under anoxic con-

ditions, the iron is reduced, releasing the phosphorus to the water, and the iron precipitates

again as a sulﬁde.

Because reduced forms of sulfur are often associated with deposits of fossil fuels and

mineral ores, combustion and smelting of these resources often results in emissions of

sulfur dioxide to the atmosphere. This is rapidly oxidized to sulfate. In the absence of

alkaline species, this is present in the form of sulfuric acid aerosols. These are easily

washed out of the atmosphere by precipitation, which as a result has greatly increased

acidity and reduced pH. The result is called acid rain or acid precipitation. Acid preci-

pitation has several ecological impacts on aquatic and terrestrial ecosystems where it falls.

These are detailed in Section 15.7.

The ocean is also a signiﬁcant source of atmospheric sulfur, with a biological origin.

Marine phyto plankton produces volatile dimethyl sulﬁde [(CH

)

S]. In the air this oxi-

dizes rapidly to sulfate, much of which is washed back into the sea.

14.2.6 Phosphorus Cycle

The phosphorus cycle differs from those of carbon, nitrogen, and sulfur in several ways:

There are fewer steps, there is no change in oxidation state, there is no signiﬁcant atmo-

spheric component, and it tends to cycle locally. For these reasons, it is easier to study.

Phosphorus is present in three main forms: free or orthophosphate; polyphosphate,

which is a polymer of orthophosphate; and organic phosphate. Although present in living

things in much smaller quantities than carbon or nitrogen, its importance is clear from

biochemistry, as phosphate forms the backbone of the DNA molecule and is central to

energy metabolism in cells. Phosphate is the most common limiting nutrient in aquatic

ecosystem. Humans disturb the phosphorus cycle in aquatic systems by discharging was-

tewater containing phosphate. This stimulates excessive cyanobacter, algae, and plant

growth, which later die and deplete the water of oxygen. Aquatic system s with high nutri-

ent loading are called eutrophic (Section 15.2.6).

Figure 14.10 shows one particular phosphorus cycle based on measurements in a salt

marsh using radioactive tracers. Note that the various compartments do not have to be in

steady state, as deﬁned by equation (14.3). Each compartment may be importing or

exporting phosphorus from the ecosystem. An energy ﬂow diagram was also done for

this ecosystem. An interesting conclusion stemmed from the observation that the ﬁlter-

feeding mussels were more important for their role in nutrient cycling than for energy

processing. In other words, they were more import ant for recycling phosphorus than as

a food source for other organisms.

Sediments store and release phosphate to the water, depending on oxygen concentra-

tions in aquatic systems. When oxygen is available, phosphate is absorbed by ferric

hydroxide. When oxygen is limiting, the ferric iron is reduced to ferrous form, and the

phosphate is released. This occurs seasonally in temperate-zone lakes.

462 ECOLOGY: THE GLOBAL VIEW OF LIFE

14.2.7 Cycles of Other Mine rals

Numerous other minerals are required by living things, including potassium, calcium,

and magnesium. Others are considered micronutrients, such as iron, chlorine, manganese,

boron, zinc, copper, and molybdenum. Yet others, such as silicon, sodi um, and cobalt, are

required only by some organisms. Some are toxic, such as mercury and arsenic.

Iron is the sixth most common mineral element in the lithosphere (at approximately

5%), surpassed only by silica, calcium, magnesium, aluminum, and sodium. With an esti-

mated total mass of 2  10

metric tons of iron, there is roughly 10 times mor e iron than

carbon in the lithos phere. However, the presence of iron in the biosphere is several thou-

sand times lower than that of the macroelements, and its atmospheric fraction would be nil

were it not for dust entrained by the wind. Its low solubility under aerobic conditions lim-

its its availability. The two main forms are ferric (Fe

3þ

) and ferrous (Fe

2þ

). The solubility

of ferric iron at pH 6.0 in waters of moderate alka linity is about 0.13 mg/L. As a result, it is

a limiting nutrie nt in some ecosystems, such as the open ocean. Ferrous iron is much more

soluble, at about 20 to 30 mg/L, but this form is converted to ferric in the presence of

oxygen. Ferrous iron may be found in deep groundwaters or wetland sediments where

oxygen is depleted. When oxygen is present at pH levels above 6, ferrous iron is oxidized

abiotically. At lower pH levels this occurs very slowly. But chemolithotrophic bacteria

such as Thiobacilus ferrooxidans and Gallionella ferruginea can take over, harvesting

energy in the process. Others, such as Sphaerotilus and Leptothrix perform the reaction

but seem not to obtain energy from it.

One of the most thoroughly studied ecosystems in the world is the Hubbard Brook

Experimental Forest in New Hampshire. A calcium budget developed for this forest

found that it receives 3 kg/ha yr from rainfall and 5 kg/hayr from weathering of bedrock.

The output from the watershed via streamﬂow carrie s the sum of these, 8 kg/ha yr. The

biota and abiotic reservoirs exchange 50 kg/ha yr between themselves. This shows how

the biological community recycles the mineral many times faster than the overall through-

put of the ecosystem.

Water

Sediments

500,000

Filter feeders

175

Spartina

660

Detritus

10,000

9.8

16.4 ( avg)

9.8

Figure 14.10 Phosphorus cycle from a Georgia salt marsh. Reservoirs are in mg P/m

, ﬂuxes are

in mg P/m

day. Uptake by Spartina and release from detritus vary seasonally as shown. (Based on

Odum, 1987.)

FLOW OF MATTER IN ECOSYSTEMS 463

Human activity has greatly affected the transport of many minerals. It has even created

new ones. Radioactive strontium-90 is created by fallout from nucl ear explosions. It

behaves like calcium. Radioactive cesium-137 behaves like potassium and is rapidly

recycled by organisms. Industrial activities have resulted in the release to the environment

of otherwise scarce minerals, including mercury and chromium. Acid rain has increased

leaching of aluminum into aquatic ecosystems, affecting ﬁsh life. Aluminum in soil water

also decreases absorption of magnesium by plant roots. Recall that magnesium atoms are

part of the chlorophyll molecule. Aluminum is added in the treatment of drinking water in

the form of alum, although most is then removed.

14.2.8 System Models of Cycles

The cycling of nutrients among compartments can be modeled mathematically using sim-

ple rate expressions. Assuming steady-state nonreactive (conservative) systems, the

expressions become even simpler. We illustrate with an example involving nitrogen bal-

ances, ﬁrst as a linear steady-state system and then as a nonlinear system in both steady-

state and dynamic conditions. The models can be used to explain and predict the distribu-

tion of nutrients in ecosy stems.

Consider the system in Figure 14.11 which represents major nitrogen components in

an aquat ic system. The four compartment s are: particulate nitrogen, which represents

phytoplankton; dissolved organic nitrogen (DON), which is excreted by the phyto-

plankton; ammonia, produced by mineralization of the DON; and nitrate, produced

by nitriﬁcation. Notice that the phytoplankton can utilize both nitrate and ammonia

nitrogen.

The concentration of each species is represented by X and the ﬂux between compart-

ments by J. Each ﬂux is assumed to be proportional to the concentration of the source

species (ﬁrst order kinetics). For example, the uptake of nitrate is assumed to be propor-

tional to nitrate concentration: J

¼ k

. This makes the system linear. The constant of

proportionality is k. The system is set up by writing the mass balance equations for each

Particulate N

Ammonia

Nitrate

DON

= k

Figure 14.11 Simpliﬁed nitrogen cycle in the Bay of Quinte. (Based on Ricklefs, 1993.)

464

ECOLOGY: THE GLOBAL VIEW OF LIFE

compartment based on equation (14.3):

Nitrate : J

þ J

¼ 0 k

þ k

¼ 0

Particulate N : J

 J

þ J

¼ 0 k

 k

¼ 0

DON : J

 J

¼ 0 k

 k

¼ 0

Ammonia : J

 J

¼ 0 k

 k

¼ 0

ð14:6Þ

These equations form four equations with four unknowns. However, they are not inde-

pendent equations. That is, one equation could be derived by combining the others. To see

how this happens, write the equations for two compartments with two ﬂuxes. You will see

that the two mass balances are identical. Another way to see that the equations as given do

not provide a useful solution is to recognize that one possible solution is to set all the

concentrations to zero. In fact, an inﬁnite number of solutions are possible, depending

on the total concentration in the system. Thus, to ﬁnd a unique solution we must specify

the total concentration, M; then the individual X’s will represent fractions of the total:

þ X

¼ M ð14:7Þ

If we replace one of the four mass balance expressions, say the nitrate equation, with

this equation, we will have our four independent expressions. To solve we express them in

matrix form:

11 1 1

k

0  k

0 k

k

00 k

k

 k

ð14:8Þ

The coefﬁcients must be obtained by experimental methods or by ﬁtting the model to

ﬁeld measurements of the X’s. The system could then be solved by standard matrix solu-

tion methods, such as Gaussian elimination. Matrix systems such as this can also be

solved easily usin g spreadsheet software (see Table 14.4 and Problem 14.6).

The linear system described above is powerful but limited. Suppose that we tried to

make it more realistic by improving the model for ﬂux J

, nitrate uptake by phytoplankton.

TABLE 14.4 Nitrogen Cycle Mass Balance Calculation

Process Coefﬁcient

Nitrate uptake (k

)16

Excretion (k

) 4.1

Mineralization (k

Ammonia uptake (k

) 1.6

Nitriﬁcation (k

) 3.1

Form of nitrogen Fraction of total mass

Nitrate-N 0.081

Particulate-N 0.234

Dissolved organic N 0.481

Ammonia-N 0.204

FLOW OF MATTER IN ECOSYSTEMS 465

In the linear model we assumed that this was proportional to nitrate concentration. How-

ever, it would be reasonable to assume that it would also be proportional to phytoplankton

concentration. Thus, a better model would be J

¼ k

. This is a second-order expres-

sion. When we insert it in our system, we can no longer create the matrix as we did above.

We might make similar changes to the equat ions for other ﬂuxes, and might even include

Monod-type expressions.

These changes produce a set of nonlinear simultaneous algebraic equations. Such a

system can be solved by Newton’s method for systems of equations. An alternative and

somewhat easier approach is to write the unsteady-state mass balances for each compart-

ment, then simulate a sufﬁcient period of time until steady- state is practically achieved.

For example, in this case the equation for X

would be:

¼ k

þ k

 k

ð14:9Þ

The result is a system of nonlinear ordinary differential equations (ODEs), which must

be solved numerically, such as by Euler’s method or Runge–Kutta methods. Initial values

must be speciﬁed for the X’s. These can be estimates or solutions obtained from the linear

case. Using the ODE model allows simulation of dynamic effects of disturbances. For

example, the mode l could be used to determine the changes in phytoplankton populations

that would result from a sudden addition of DON and ammonia, such as from a sewage

spill.

Models of these kinds have been developed to simulate complete ecological systems.

For example, models have been created for proposed closed ecological life support sys-

tems (CELSSs) to grow food and recycle wastes in support of human crews on long-term

space missions. Global models have been used to make projections on economic

resources such as food and minerals. Based on current scenarios for population growth

and resource consumption, these models predict a catastrophic resource depletion some

decades in the future. However, they do not take into account social responses to deple-

tion, such as the development of new technologies, reso urce switching, or discovery of

new resource s. Nevertheless, the models make clear that our current world economy is

not sustainable.

14.3 FACTORS THAT CONTROL POPULATIONS

We have just seen how energy and materials move through the ecosystem. Recall that our

primary concern in ecology is to under stand the distribution and abundance of organisms.

Ultimately, this will relate to organisms’ ability to gather and utilize resources. This, in

turn, will depend on the availability of the resources in the environment, on the presence

of other conditions favorable to organism growth and activity, and on their interactions

with other species.

14.3.1 Limiting Factors and Interactions

Resources are factors that are consumed by an organism, resulting in their being removed

from the environment. These include factors such as nutrients, but also less obvious

factors, such as habitats. For example, barnacles require rock surfaces to grow on. As

466 ECOLOGY: THE GLOBAL VIEW OF LIFE

rock surfaces become colonized, growth and recruitment of new individuals is reduced

by the crowding. Factors such as temperature are not resources because they are

not depleted.

Historically, the effect of resources on growth has been described by Liebig’s law of

the minimum, which states that a population will increase until a single resource, called

the limiting factor, becomes insufﬁcient to support further growth. Thus, for every popu-

lation in an ecosystem under steady-state conditions, Liebig’s law predicts that each

would be limited by a single factor. If that factor were known, the population could be

increased by supplementing it.

Although it is still a useful concept and often applies to particular situations, Liebig’s

law is not gener ally true. For example, it may be possible to increase the growth of grasses

in a mea dow by supplementing either nitrogen, phosphorus, or water, and supplementing

two or three of these together could increase growth even more. One of the factors could

become limiting only if the others were present in excess.

Factors can also affect growth in a nonadditive way. For example, growth of Impatiens

parviﬂora was increased 33% by adding nitrogen fertilizer and 19% by adding phos-

phorus. However, when both were added, the increase was 100%. If there were no inter-

action, adding both nitrogen and phosphorus would be expected to increase growth

33% þ 19% ¼ 52%. This is an example of interaction, which is not accounted for in Lie-

big’s law. The concept of interaction is important: An interaction is when the sensitivity

of a variable to one factor depends on the level of another factor. Here, sensitivity can be

given a prec ise mathematical meaning. It is relat ed to the rate of change of one variable

with respect to another. One way to express the sensitivity of variable a to variable b

)is

’

a

b

ð14:10Þ

This expresses the sensitivity as the amount that a would change for a unit change in b.

Example 14.1 If increasing phosphorus concentration, P, from 10 mg/L to 20 mg/L

results in an increase in chlorophyll concentration, c, in a water body from 3 mg/L to

5 mg/L, the sensitivity of chlorophyll to phosphorus, s

, can be approximated as follows:

By equation (14.10),

5  3

20  10

¼ 0:20 mg Chl-a=mgP

That is, each 1-mg/L increase in phosphorus produces a 0.20-mg/L increase in chlorophyll a.

Example 14.2 Peak summer chlorophyll contents in lakes, C

, have been empirically

related to phosphorus concentration at spring turnover, P

, as in equation (15.1):

¼ 0:37 P

0:91

(both concentrations in mg/L). According to this relation, what is the sen-

sitivity of chlorophyll to phosphorus at a concentration of 15 mg P/L?

Solution First, using equation (15.1), the expected concentration of chlorophyll a in

the lake is

¼ 0:37 ð15 mg=LÞ

0:91

¼ 4:35 mg=L

FACTORS THAT CONTROL POPULATIONS 467