PROTEIN ---... ~ ,
SYNTHESIS BACTERIA AND "
I-IITROGE:N-f"lXING ' \ . \ \
/BACTE:RIA AND ALGAE , \
" " \ . NI T ROGEN AMINO-AC.IDS AND \ (
fUNGI Of DECAY \
~ IN THE I
""{,]R~\::"",,, "',,:] O~;~~~:'''~~'/
AND \~SH ) PHOTOCHE:MICAL fl)CATION AMMONI~"'''''
SHALLOW MARINE DENlrRlrVING / GAIN fROM
SEDIMENTS BACTERIA VOLCANIC ACTION
/ NITRATE NITRITE \
i/ BACTERIA BACTERIA IGNEOUS
LOSS TO DEEP ... NITRITES~ ROCKS
5rDIMENTS
A
STEPS REQUIRING ,PROTOPLASM
'ENERGY FROM ( ~
:OTHER SOURCES ,AM I N O-AC IDS
ISUNLIGHi O R : : ~
ORGANIC MATTER) : /7 AM M ON I A
I STEPS PROVIDING ICNERG'1 +TO THE DECOMPOSER
ORGANISMS
, J. ENERGY
: / ,'N IT R I T E BARflER
;' '" ~ NITROGEN
'----NITRATE ~ ~ '---=+GAS
B
Figure 7. Two ways of picturing the nitrogen biogeochemical cycle, an example of a relatively perfect, self-regulating cycle in which there is little overall change in available nitrogen in la.rge ecosystems or in the biosphere as a whole, despite rapid circulation of materials. In A the circulation of nitJ:Ogen bctween organisms and environment is depicted along with micro- organisms which are responSible for kcy steps. In D the same basic steps are arranged in an ascending-descending series, with the high energy forms on top to distinguish steps which require energy from those which release energy.
Some quantitative estimates of interest are:
1. The loss of nitrogen from the atmosphere to sediments is apparently balanced by the gain from volcanic action; in fact, the nitrogen content of the air may possibly have increased throughout geological time.
2. According to Hutchinson (1944a), the amount of nitrogen fixed from the air (non-cyclic nitrogen) is estimated to lie between 140 and 700 mg. per square meter, or between 1 and 6 pounds pel" acre pel" year for the biosphere as a whole. Most of this is biological; only a small portion (not more than 35 mg. per square meter per year in temperate regions) is non-biological (electrification or photochemical fixation). Biological fixation in fertile areas may be much greater than the biosphere average, up to 200 pounds per acre, according to Fogg (1955).
(Continued on facing page)
THE ECOSYSTEM AND BIOGEOCHEMICAL CYCLES: §3 33 valve of the system. Nitrogen is continually entering the air by the action of denitrifying bacteria and continually returning to the cycle through the action of nitrogen-fixing bacteria or blue-green algae and through the action of lightning (i.e., electrification).
In Figure 7 (B) the components of the nitrogen cycle are shown in terms of the energy necessary for the operation of the cycle.
The steps from proteins down to nitrates provide energy for organ- isms which accomplish the breakdown, whereas the return steps require energy from other sources, such as organic matter or sun- light. Likewise, nitrogen fixers must use up some of their carbo- hydrate or other energy stores in order to transfolm atmospheric nitrogen into nitrates.
The importance of the nitrogen-fixing bacteria associated with legumes (Fig. 9) is well known, of course, and in modern agri- culture continuous fertility of a field is maintained as much by crop rotation involving legumes as by the application of nitrogen fertilizers. Secretion from the legume root stimulates growth of the nodule bacteria, and bacterial secretions cause root hair de- formation, the first step in nodule fOImation (Nutman, 1956). Neither legume nor bacteria can fix nitrogen alone. Strains of bac- teria have evolved which will grow only on certain species of legumes. The bacteria get carbohydrates from the host and the host gets nitrogen, some of which is excreted into the soil and may be used by other plants. Other nitrogen fixing bacteria live free in the soil and do not require the partnership of a vascular plant. In water and moist soil, blue-green algae often perform the vital operation of nitrogen fixation; these organisms may also be associated with higl1er plants but seem to be much less specialized symbionts. In the Orient it has been found that the blue-green algae which occur naturally in the rice paddies are very important
3. The capacity to fix atmospheric nitrogen was thought, until recently, to be limited to a few, but abundant, organisms, as follows:
Free-living bacteria-Azotobacter and Clostridium
Symbiotic nodule bacteria on legume plants-Rhizobium (see Figure 9);
Blue-green algae (free-living or symbiotic)-Anabaena, Nostoc, and probably others.
In 1949 it was discovered that the purple bacterium Rhodospirillum and many other representatives of the photosynthetic bacteria are nitrogen fixers (see Kamen and Gest, 1949 and Kamen, 1953). Ability to fix nitrogen is proving to be widespread among photosynthetic, chemosynthetic and saprobic microorganisms. However, no higher plant is able to fix nitrogen alone; legumes and a few species of other families of vascular plants do so only with the aid of symbiotic bacteria.
34 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CH. 2
PROTOPLASM
PLAN T S __"'A NIMALS<::: - - - - . . _ l>IBACTERIA'<" -... ...,
El\CRETION' "
"
'\. BONES, "-PHOSPHATE ROCKS \ TEE~H
PROTOPLASM GUANO DEPOSIT} \ I
SYNTHESIS FOSSIL BONE DEPOSITS I I
I
PHOsPHATislNG! /BACTERIA / /
VOLCANIC ! /
APATITE I !
EROSION / /1
1 / /
MARINE BIROS If / / /
AND FISH DISSOLVED _ / / / /
PHOSPHATES""'- / /
./
-/-
II? - --
SHALLOW MARINE k - - SEDIMENTS
~~~~OE~Eg~
Figure 8. The phosphorus cycle. Phosphorus is a rare element compared with nitrogen. Its ratio to nitrogen in natural waLers is about 1 to 23 (Hutch- inson, 1944a). Chemical erosion ill the UniLed States has been estimated at 34 metric tons per s'luare kilometer per year. Fifty-year cultivation of virgin soils of the Middle West reduced the P20r. content by 36 per cellt (Clarke, 1924). As shown in the diagram, the evidence indicates that return of phos- phorus to the land has not been keeping up with the loss to the ocean.
in maintaining fertility under intensive cropping. Seeding the rice fields with extra algae often results in increased yields (Tamiya, 1957). Fogg (1955) has reviewed the cntire subject of nitrogen fixation in a very readable manner, and Shields (1953) has re- viewed the environmental factors which affect the nitrogen cycle as it pertains to the seed plants.
The self-regulating, feedback mechanislTIs, shown in a very sim- pImed way by the arrows in the diagram (Fig. 7), make the nitrogen cycle a relatively perfect one, when large areas or the biosphere as a whole is considered. Thus, increased movement of materials along one path is quickly compensated for by adjust- ments along other paths. Some nitrogen from heavily populated regions of land, fresh water, and shallow seas i.~ lost to the deep ocean sediments and thus gets out of circulation, at least for a while (a few million years perhaps). This loss is compensated for by nitrogen entering the air from volcanic gases. Thus, vol- canic action is not to be entirely deplored but has some use after
THE ECOSYSTEM AND BIOGEOCHEMICAL CYCLES: §3 3S alll If nothing else, ecology teaches us not to make snap judgments as to whether a thing or an organism is "useful" or "harmful." One must consider all the aspects of a problem before arriving at a judgment. There will be many other examples of this in subse- quent chapters.
The phosphorus cycle appears to be somewhat simpler. As shown in Figure 8, phosphorus, an important and necessary con- stituent of protoplasm, tends to "circulate," the organic compounds being broken down eventually to phosphates which are again available to plants. The great reservoir of phosphorus is not the air, however, but the rocks or other deposits which have been
,
Figure 9. Root nodules on a legwne, the location of nitrogen-fixing bac- teria of the symbiotic or mutualistic type (see also Figure 7). The legwne shown is blue lupine, a cultivated variety used in southeastern U. S. (U. S.
Soil Conservation Service Photo.)
36 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CH. 2 formed in past geological ages. These are gradually eroding, re- leasing phosphates to ecosystems, but much phosphate escapes into the sea, where part of it is deposited in the shallow sediments and part of it is lost to the deep sediments. The means of returning phosphorus to the cycle may presently be inadequate to com- pensate for the loss. In some parts of the world there is no exten- sive uplifting of sediments at present, and the action of marine birds and fish (being brought to land by animals and man) is not adequate. Sea birds have apparently played an important role in returning phosphorus to the cycle (witness the fabulous guano deposits on the coast of Peru). This transfer of phosphorus and other materials by birds from the sea to land is continuing, but apparently not at the rate which occurred in some of the past ages.
Man, unfortunately, appears to hasten the rate of loss of phos- phorus and thus to make the phosphorus cycle less perfect.
Although man harvests a lot of marine fish, Hutchinson estimates that only about 60,000 tons of elementary phosphorus per year is returned in this manner, compared with one to two million tons of phosphate rock which is mined and most of which is washed away and lost. Agronomists tell us there is no immediate cause for concern, since the known reserves of phosphate rock are large.
However, man may ultimately have to go about complcting the phosphorus cycle on a large scale if he is to avoid famine. Of course a few geological upheavals raising the "lost sediments"
might accomplish it for us, who knows? At any rate, take a good look at the diagram of the phosphorus cycle; its importance may loom large in the future.
Quantitative study of biogeochemical cycles
Diagrams such as those in Figmes 7 and 8 show only the broad outlines of biogeochemical cycles. Quantitative relations, that is, how much material passes along the routes shown by the arrows, and how fast it moves, are but poorly known. However, an in- creasing number of studies on cycling rates in specific ecosystems are being undertaken. Radioactive isotopes, which have become generally available since 1946, are providing a tremendous stim- ulus for such studies since these isotopes can be used as "tracers"
or "tags" to follow the movement of materials. It should be empha- sized that tracer studies in ecosystems, as in organisms, are designed so that the amount of radioactive element introduced is extremely small in comparison with the amount of non-radioactive
)SYSTEM AND mOGEOCHEMICAL CYCLES: §3 37
t already in the system. Therefore, neither the radioactivity extra ions distmb the system; what happens to the tracer can be detected in extremely small amount by the tell- liations which it emits) simply refleets what is normally ing to the particular material in tIle system.
s and lakes are especially good sites for study since they ltively self-contained. Following the pioneer experiments n, Hayes, Jodrey, and Whiteway (1949), and Hutchinson wen (1948, 1950), numerous papers lHlve appeared report-
results of the use of radiophosphorus (P32) in studies of oms circulation in ju],es. Hutchinson (1957) has summar-
~se studies and reviewed the general knowledge on cycling phorus and other vilal clements in lakes.
. Estimates of the turnover time of phospho1"US in watc1' and
~r7irnenls of "three lnkes as detcnllinecl wit-h the use of 1m
(() fter H ulchinson, 1957)
Ratio
'IIIo!>i[r;
Ama Deptl' '1'lIr1IOtWI' Time in days P /0 total Km .. ~ 1n. water sedim ents Pin. watc'r
0.4 7 5.4 39 6.4
wI 0.3 6 7.6 37 4.7
2.04 3.8 17.0 176 8.7
,s been generally found that phosphorus does not move and smoothly from organism to environment and back to
In as one might lhink from looking at the diagram in 8, even though, as we havc already indicated, a long-term :ium tends to be established. At anyone time, most of the Jrus is tied up either in organisms or in solid organic or ic particles which make up the sediments. In lakes, only
o
per cent is the maximum likely to be in a soluble form one time. Although some back-and-forth movement or~e occurs all of the time, extensive movement between .d dissolved states is often irregular or "jerky," with periods release from the sediments followed by periods of net by organisms or sediments, depending on seasonal tem- e conditions and activities of organisms. Generally, uptake more rapid than release rate. Plants readily take up phos-
in the dark or under other conditions when they cannot )uring periods of rapid growth of producers, which often
1 the spring, all of the available phosphorus may become
36 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CH. 2 tied up in prodllcers and conSllmers. The system must then "slow down" since little new protoplasm can be synlhesized until the bodies, feces, etc., are acted on hy the decomposers. Thus, the concentration of phosphorus in the water may be higher after the
"bloom" than during it. In other words, the concentration of dis- solved phosphate in the water at anyone time is not necessarily a good index to the total amount of phosphorus present. In gen- eral, the greater the surface area (as provillecl hy both living organisms and inert partides) per unit of volume, the more rapidly will phosphorus be removed from the water.
Radiophosphorus has been especially useful in quantitative measurement of E'xchalJge rates in components of the ecosystem, that is, the rates at which phosphorus moves in and out of com- ponents after equilibrium has heen established. To understand this it is necessary to introdnce two concepts: T,n'Hollel' rate is the fraction of the total amount of a suhstance ill a eOlllpollent which is released (or which enters) in a given length of time. Ttl1'l1over
time is the reciprocal of this, that is, the time re(l11irec1 to replace a quantity of sllbslance equal to the amount ill the component (see Rohertson, 1957, for a discussion of these concepts). For example, if ] 000 units are present in the component and 10 go out or enter each hour, the turnover rate is 10lJOOO or 0.01 Or I per ccnt per hour. Turnover time would then be 1000/10 or 100 hours. Data OIl turnover time for two large components, the water and the sediments, in threc lakes are givcn in TaJ)Je 1. The smaller lakes have n shorter turnover time prf'sumedJy because the ratio of bottom "mull" surface to the volume of water is greater. In general, the turnover time for the water of small or shallow lakes is ahout one week; for large lakes it may he two months or longer.
Studies with p;i~ tagged fertilizers in land ecosystems have revealed simiJnl" patterns; 111llell of the phosphorus is "locked up"
and unavailable to plants at allY given time (sce Comar, 1957, for a summary of some of these cxperimen ts). One very practical rcsult of intensive studies of llutrien t cycles has IJeen the rcpeated demonstration that overfertilizatioll can he jllst as "bad" from the standpoint of man's inter '!it as underf crtilization. When more materials are added than call be used by the organisms active at the time, the excess is often quickly tied up in soil or sediments or even lost completely (as by leaching), and is unavailable at the time when increased growth is most desired. In experiments
THE ECOSYSTEM AND 1l10GEOCHEMICAL CYCL1,S: §3 39 with crops, little of tagged phosphate fertilizer added to soil ends up in the crop when the concentration of phosphate in the soil is already high. The "blind dumping" of fertilizers in ecosystems such as £5h ponds is not only wasteful but may even back£l'e insofar as the desired results are concerned. Since different organ- isms are adapted to speci£c levels of materials, continucd excess fertilization may result in a change in the kinds of organisms, perhaps discouraging the one man ·wants and encouraging the kinds he does not want. Among thc algac, for example, Botl'ljo-
coccus lJraun:ii cxhibits optimun1 growth at phosphorus concentra- tion of 89 mg/rvp while Nitzschia palea grows best at 18 mg/M3.
Increasing the amount of P fro)11 18 to 89 wOllld likely result in T3otl'Yococcus replaCing Nitzschia (assuming other conditions favorable for both species), and this could have considerable effect on the kinds of animals that could be supported. The com- plete destruction of an oyster industry as a result of increased fertilization by phosphorus and nitrogen materials is described in Chapter 4 (sce pages 96-97).
The sedimentary cycle
The nitrogen and phosphorus cycles in a very broatl way illus- trate the two general types of biogeochemical cycles. Cycles of oxygen, carbon dioxide and water resemble the cycle of nitrogen in that a gaseous phase is important in the continuous flow be- tween inorganic and organic states. Most elemcnts and compounds are more earthbound, and their cycles follow the pattcrn of phos- phorus in that erosion, sedimentation, mountain building and volcanic activity, as well as biological transport, are the primary agents cHccting circulation. A generalized picture of the sedi- mentary cyclc of earthhound elements is shown in Figure 10.
Some estimates of the amounts of material which pass through the cycle are markcd on the arrows. Of course, vcry little is known about the flow of materials in the deep earth. The movement of solid matter through tIle air as dust is indicated as "fall out." To the natural fall out; atomic age man is adding additional matcrials, small in amount but significant biologically, as will he discussed latcr. The chemical elemcnts which are available to the com- munities of the biosphere are those which by their geochemical nature tend to be enclosed within the types of rocks that come to the surface. Elements which are abundant in the mantle are scarce at the surface. As already indicated, phosphorus is one of the
40 DASIC ECOLOGICAL PruNCIPLES ANI) CONCEPTS: CR. 2
Figure 10. A diagram of the sedimentary cycle involving movement of the more "earthbound" elements. Where estimates are I?ossible, the amounts of material are estimated in geograms per million year (one geogram = 1020 grams). The continenLs are sediment covered blocks of granite floating like corks on a layer of basalL which unuerlies the oceans. Below the black basalt is Lhe Mantle layer which extel1(1s 2900 Km. down to the core of the earth. Granite is tile light colored very resistant rock often used for tombstones;
basalt is the black rock found in volcanoes. (Diagram prepared by H. T.
Odum.)
elements whose scarcity on the earth's surface often limits plant growth.
The general "downhill" tendency of the sedimentary cycle is well shown in Figure 10; the lowlands and the oceans tend to gain soluble or usable mineral nutrients at the expense of the uplands during periods of minimum geological activity. Under such con- ditions, local biological recycling mechanisms are extremely important in keeping the downhill loss from exceeding the re- generation of new materials from underlying rocks. In other words, the longer vital elements can be kept within an area and used over and over again by successive generations of organisms, the less new material will be needed from the outside. Unfortu- nately, as already mentioned in the discussion of phosphorus,