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VII. Nitrogen Transformations in Grassland Soils

VII. Nitrogen Transformations in Grassland Soils

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THE MICROFLORA O F GRASSLAND



417



1964; Whitman and Stevens, 1952). The latter workers found 27 species

of legumes in western North Dakota grassland. Half of the total legume

productivity, which ranged from 25 to 90 kg/ha, was accounted for by

four species of legumes. Legume productivity usually accounted for less

than 10% of the total range productivity and at times, for no more than

I %. In quadrat counts, legumes averaged about 30 stalks/m2,whereas the

grasses showed from one thousand to several thousand stems/m2. Although of minor importance as structural components in grassland, the

legumes present therein are generally well nodulated, have deep, extensive root systems, and show an active period of growth during the

moist season. Although the amount of nitrogen fixed annually in the leguminous nodules may be quite low, nevertheless their cumulative role in

nitrogen accretion over a number of years can be substantial. Hopefully,

the recent advances in methodology will shortly make available more informative data on the amount of nitrogen fixed in leguminous symbioses

in various types of grassland.

Plants other than legumes may also serve as hosts for nodulating organisms (Bond, 1967; W. D. P. Stewart, 1966, 1967). Although the microbial endophytes involved in nonleguminous symbioses are not as yet

adequately defined, a wide variety of associations is postulated (Table

XVIII). For plants such as Alnus and Ceanothus, the nodulating bacteria

are actinomycetes rather than rhizobia. Eleagnus and Shepherdia are rootnodulating shrubs of this type and both are widely distributed throughout

many areas of grassland habitat. Recently, Farnsworth and Hammond

( 1 968) have reported that sagebrush (Arternisia lodviciunu) and prickly

pear cactus (Opuntiu frugilis) bear root nodules that appear capable of

fixing atmospheric nitrogen. If their observation is substantiated, it will

extend the range of nonleguminous symbioses and should encourage the

search for other associations. Species of Artemisia and Opuntia are widely distributed throughout the western Great Plains. Mayland et al. (1 966)

noted an increase of 15N in Artemisia during studies of fixation by algae

in Arizona desert but attributed the enrichment of isotopic nitrogen to

utilization of nitrogen which has been fixed and excreted by the algal

crust.

The leaf nodules and wet leaf surfaces of vegetation in humid tropical

regions provide suitable sites for the development of nitrogen-fixing

bacteria such as Azotobacter, Beijerinckia, Clostridium, Klebsiella, and

Xanthomonas (Ruinen, 196 1 , 1965; Silver el ul., 1963: Vasantharajan

and Bhatt, 1968). It is doubtful that phyllosphere floras contribute significantly to nitrogen accretion in semiarid grassland.

Mycorrhizal nitrogen fixation has been postulated from field evidence



FRANCIS E. CLARK A N D ELDOR A. PAUL



418



concerning the ability of Pinus species that bear mycorrhizae to grow

vigorously in nitrogen-poor soil (G. Stevenson, 1959). In his reviews on

TABLE XVlIl

Known or Postulated Nitrogen-Fixing Symbioses or Associations

Involving Nonleguminous Plants



M icrobiont



Host genera

involved

Symbioses or associations involving

Alnus, Casuarina, Ceanothus,

Compronia, Discaria,

Eleagnus, Hippophae,

Myrica, Shepherdia; possibly

Arremisia and Opuntia

Dryas, A rctosraphylos,

Cercocarpus, Purschia

Podocarpus



Cycas, Encephalartos,

Siangeria, Ceratozamia,

Macrozamia, Zamia



involved

spermatophytes

Actinomycetes



Unknown

Fungus? (Saxton, 1930);

bacterium? (McLuckie,

1922)

Blue-green algae



Gunnera



Blue-green algae



Numerous and diverse genera

of both graminous and nongraminous plants



Fungi



N-fixing organisms in

the phyIlosphere*

Associations involving lower plants

Pteridophytes

Azolla

Blue-green algae

Bryophytes

Blue-green algae

Blasia, Curvicularia,

A nthoceros

Lichens

Collema, Leptogium,

Blue-green algae

Pelrigera



Site of microbiont and

host interaction

Root nodules



Root nodules are

present'

Root nodules



Coralloid root hypertrophies

Wartlike leafbase

hypertrophies

Mycorrhizal roots'



Leaf surfaces



Leaf pores

Thallus cavities



The lichen thallus,

which is a fungusalga symbiosis



'Nitrogen fixation is assumed on the basis of ecological observations, but definitive proof

is lacking.

*The microbionts involved (Azotobacrer, Beuerinckia, Closiridium, blue-green algae) are

known to fix nitrogen even if not in association with higher plants, but association with

higher plants is believed to augment their ecological role as fixers of nitrogen.



THE MICROFLORA O F GRASSLAND



419



nitrogen fixation by higher plants other than legumes, Bond (1967, 1968)

has cited evidence that podocarp nodules, containing a phycomycetous

nonseptate endophyte, fix nitrogen by means of a mycorrhizal association.

Mycorrhizae are known to occur widely among the grasses (Nicolson,

1959: Dorokhova, 1967), but thus far there is no good evidence that they

are capable of fixing atmospheric nitrogen.

Lawrence et al. ( 1 967) have considered the role of Dryas in vegetation

development on newly exposed mineral soil following glacier ice recession. Nitrogen turnover measurements in marine and brackish environments (W. D. P. Stewart, 1967) and studies of fixation by Hippophae in

dune sand (W. D. P. Stewart and Pearson, 1967) have shown fixation

ranging from a few to 100 kg of nitrogen per hectare to occur in these

nonleguminous communities. Nevertheless, at the present state of knowledge, leguminous symbioses must be considered as more important for

nitrogen accretion in grassland than are the various nonleguminous symbioses involving higher plants.



2 . Asymbiotic Nitrogen Fixation

Recognition of the paucity of legumes in many grassland areas has led

to a continued interest in the role of asymbiotic bacteria and other simple

plants in the nitrogen economy of grassland soils. Moore (1966) has summarized extensive data concerning asymbiotic nitrogen fixation in a wide

variety of soils. Many of the earlier experiments were based on Kjeldahl

measurements of long-term field plots. They tend to show a nitrogen imput averaging about 22 kg/ha annually. In this total is included the nitrogen accruing from rainfall, an imput which in itself can be considerable

(Eriksson, 1952; Junge, 1958; Mackenthun, 1965). Allison (1953, although not doubting that asymbiotic fixation does occur, expressed skepticism on the validity of many of the older measurements showing gains

of nitrogen ranging up to 100 kg/ha per year. It is now generally agreed

that the use of isotopic nitrogen or acetylene methodology is necessary

in order to obtain meaningful measurements of nitrogen fixation.

Delwiche and Wijler (1956) conducted 15Nz fixation experiments on

grass sod and associated cultivated soils and found no nitrogen fixation

to occur asymbiotically unless a source of energy, such as glucose, was

added. More recently, W. A. Rice et al. (1967), also using isotopic nitrogen, measured 19 pg of N per gram of soil fixed in 28 days in laboratory

samples of a prairie soil containing straw residues of the most recent crop.

The addition of higher concentrations of straw or waterlogging greatly

increased the rate of fixation (Table XIX). R. J. Myers et al. (1970)

measured nitrogen fixation in undisturbed soil cores 15 cm in diameter



TABLE XIX

Measurements of Asymbiotic Nitrogen Fixation Using I5N or C2H, Methodology



P

h)



Reference

W. A. Rice et al. ( I 967)



R. J. Myers et al. ( 1 970)



Mayland et al. ( 1966)

Shtina et al. (1968)



Methodology



Experimental conditions



N



Soil aliquots given laboratory incubation

(a) At field capacity, plus 1% straw

(b) At field capacity, plus 5% straw

(c) Waterlogged, plus 1% straw

(d) Waterlogged, plus 5% straw

15-cm diameter undisturbed soil cores

(a) Nostoc surface crust only

(b) 0-2.5 cm deep

(c) 2.5 cm deep

(d) Total 7.5 cm core, and using C Z H Z

15-cm diameter core of cultivated soil plus

2% wheat straw

(a) 0-2.5 cm deep

(b) 2.5-7.5 cm deep

(c) 7 5 cm deep

Algal crusts of Arizona desert

Algal crusts on chestnut soil, USSR

Algal crusts on rendzina soil, USSR

Mountain meadow (Colorado, U.S.A.) soil

block containing algae in moss habitat

Savanna (Nigeria) soil incubated moist and

with 35% oxygen

Sand dune (U.K.) marine environment

dominated by Nostoc, 0-1 cm depth

Jordan Fertility Plots (Pennsylvania, U.S.A.)

(a) No fertilizer added, 0-15 cm depth

(b) 27 kg/ha of N added, plus PK

(c) 81 kglha of N added, plus PK



I5



l5



l5



N



N

N



Porter and Grable ( I 969)

Odu and Vine ( 197 I )



W. D. P. Stewart (1967)

Hardy et al. (1968)



l5



N

N



C,H,



N fixed

(pglgram of soill28 days)



0



19

150



67

240



I”

239

4.3

0

7.3



0.6

0.9

0

43

46.5

62.2

5.0



I .o



47.6



33.9

22.4

11.8



?



3



s



THE MICROFLORA OF GRASSLAND



42 1



and found (Table XIX) lower nitrogen gains than were found by W. A.

Rice et al. According to Myers and co-workers, the top 7.5 cm of cultivated soil amended with straw and incubated in a normal, controlled atmosphere fixed nitrogen equivalent to 0.6 kg/ha during 28 days of incubation. The same clay soil contained Nostoc crusts which fixed 239 pg of

N per gram of Nostoc crust. In the natural field soil, no fixation occurred

below 2.5 cm. The accumulated value of nitrogen fixation, as measured

by 15N, in the Nostoc crust and in the top 2.5 cm of soil, was 1.52 kg/ha.

Measurement of fixation in the same soil by the acetylene technique indicated 1.8 kg of N per hectare was fixed during equivalent time.

techniques

Data from other recent experiments utilizing I5N and C2H2

to measure fixation are also summarized in Table XIX. To present the

information in a comparable form, data from the various authors have

been recalculated on a micrograms per gram basis. Also further to facilitate comparison, data for incubation intervals longer or shorter than 28

days have been recalculated to 28 days.

The microflora involved in asymbiotic nitrogen fixation is diverse. At

least 15 genera of bacteria and many blue-green algae are now known to

fix nitrogen. The classical nitrogen-fixing organism Azotobacter and the

more recently isolated Beijerinckia occur sporadically and in low numbers

in grassland soils (Ross, 1960; Di Menna, 1966; W. D. P. Stewart, 1969)

and in soils generally. Exceptions are soils of the Nile Delta and sugarcane soils of Brazil in which high populations of Azotobacter and/or Beijerinckia occur. In semiarid grassland, neither genus is believed to be of

importance in nitrogen fixation.

Members of the genus Clostridium are widely distributed in grassland

soil (Di Menna, 1966; Campbell et al., 1967; Ross, 1958, 1960). They

can utilize the breakdown products of natural substrates, such as straw

(W. A. Rice et al., 1967), to fix large concentrations of nitrogen. The

clostridia are doubtless the most prolific nitrogen-fixers of the asymbiotic

bacteria. Members of the genera Pseudomonas, Achromobacter, Klebsiella, and Bacillus can readily be isolated on nitrogen-free agar (MeiMejohn, 1968; Paul and Newton, 1961; Moore, 1966; Anderson, 1955), but

they appear of doubtful effectiveness as nitrogen-fixers in field soil. The

significance of such organisms as Methanobacterium and Desulfovibrio

probably is not large. The photoautotrophic bacteria are widely distributed in aquatic environments but their functional occurrence in semiarid grasslands has not been demonstrated. Metcalfe and Brown (1957)

described two Nocardia species from grassland that were able to fix

atmospheric nitrogen. Fedorov and Ilina ( 1 960) also noted evidence of

nitrogen fixation by actinomycetes, but Jensen ( 1965) concluded that



422



FRANCIS E. CLARK A N D ELDOR A. PAUL



evidence for nitrogen fixation by nonsymbiotic actinomycetes is mostly

negative or unconvincing.

Various species of blue-green algae are known to fix atmospheric

nitrogen (Burris, 1956; Moore, 1966; Shields and Durrell, 1964; Shields

et al., 1957; Russell, 1950; Hernandez, 1956). Cameron, Mayland and

associates have calculated that algal surface crusts collected from Arizona desert rangelands could fix up to 10 kg/ha of nitrogen annually if the

soil surface was fully covered with such crusts (Cameron and Fuller,

1960; Fuller et al., 1961; Mayland and McIntosh, 1966; Mayland et al.,

1966). Since native rangeland is by no means fully covered by Nostoc or

other crusts, their calculated value must be scaled down considerably for

field conditions. Porter and Grable (1969) found algae contributed during

10 days approximately 4 kg of nitrogen per hectare in a wet mountain

meadow in which conditions were very favorable for algal growth. Liverworts and mosses may variously contribute to nitrogen fixation. Bond

and Scott (1955) found the liverwort Blasia pusilla capable of fixing

lSNNz.

Moore ( I 966) and W. D. P. Stewart ( I 966) have discussed the contribution that the photosynthetic lower plants may make to nitrogen

fixation in various natural environments.

Heterotrophic nitrogen-fixers appear to play their major role by being

widely distributed and fixing small or very small quantities of nitrogen

over prolonged periods, particularly in microhabitats containing available substrates and restricted oxygen (W. D. P. Stewart, 1969; W. A.

Rice et al., 1967), possibly in the rhizosphere (Mishustin, 1967), and in

the case of the photosynthetic lower plants, singly or in mixed associations, in microhabitats of favorable light intensity and with high moisture,

at least intermittently.



B. NITRIFICATION

Many workers have reported that there is an inhibition of nitrification

in grassland soil (Lawes, 1889; Miller, 1906; Lyon and Bizzell, 1918;

Richardson, 1938; Parberry, 1945; Theron, 195 1 : Theron and Haylett,

1953: Soulides and Clark, 1958; Woldendorp, 1962; Boughey et al.,

1964; E. L. Rice, 1964, 1965; Munro, 1966; Neal, 1969). In such soil,

the nitrate content is commonly observed to be negligible or absent while

the ammonium content, although small, is usually of measurable quantity.

This is in contrast to most cultivated soils, in which more of the mineral

nitrogen is to be found in the nitrate than in the ammonium form.

Various explanations have been offered for the relatively poor nitrification that occurs in grassland soil. T h e most commonly supported hypothesis is that there is direct suppression of the nitrifying bacteria by the



423



THE MICROFLORA O F GRASSLAND



living plant root. This was first suggested by Lyon and Bizzell(l9 18) and

Lyon et al. (1923) and was later endorsed by Theron (195 1 ) and Theron

and Haylett (1953). In recent years other workers have presented data

showing that exposure to either intact plant roots or to aqueous extracts

prepared therefrom influences the activity of the nitrifying bacteria

(Novogrudskii, 1963: Molina and Rovira, 1964; E. L. Rice, 1964, 1965;

Boughey et al., 1964; Munro, 1966; Neal, 1969).

Varying degrees of inhibition of nitrification by grass roots have been

reported. Munro (1966), working in Africa, found that all of the seven

grass species tested inhibited nitrification. In short-term experiments

involving exposure of nitrifiers to aqueous extracts of grass roots, oxidation of both ammonium and nitrite nitrogen was strongly to very strongly inhibited (Table XX). In contrast, Neal (1969) found that while the

native dominant grasses in western Canada failed to depress nitrification,

grasses and forbs that increase on or invade overgrazed land commonly

produced nitrification inhibitors.

TABLE XX

Influence of Aqueous Extracts of Grass Roots on

the Activity of Nitrifying Bacteria"

Nitrite N produced

Source of

root extract



PPm



Percent

inhibition



None, control

Hyparrhenia jilipendula

Cynadon dactylon

Rhyncheletrum repens

Sporobolus pyramidalis

Eragrostis curvula

Themeda triandra

Pennisetum purpureum



7.9

0.8

2.7

2.1

1 .o

1.6

1 .o

3.4



90

66

66

88

80

88

57



-



Nitrite N oxidized



PPm



Percent

inhibition



9.0

0.2

1.9

1.9

2.4

1.6

0.5

5.0



98

79

79

74

82

95

45



Data of Munro ( 1 966).



Other workers have expressed doubt that inhibitions of nitrification

are caused by toxins of root origin and indeed some even doubt the occurrence of any such inhibition in grassland. Brar and Giddens (1968)

undertook to extract possible inhibitors from grassland soil but following

extraction, noted no improvement in the nitrifying capacity of the soil.

They concluded that the low nitrifying capacity in their soil was caused

by soil acidity and a low population of nitrifiers. Ross (1958) also believed



424



FRANCIS E. CLARK A N D ELDOR A. PAUL



that the low nitrifying potential of some grassland soils was due simply

to their acidity. Parberry (1945) and Michniewicz (195 1 ) believed there

might be insufficient aeration for good nitrification in grassland soils

covered by a layer of decaying plant residues. Harmsen and van Schreven

(1955) doubted the validity of any such explanation, since well-drained

grasslands generally have good aeration, often far superior to that of

arable land.

O’Connor et al. (1962) and Robinson (1963) suggested that the low

production of nitrate in grassland was linked to a low population of nitrifying bacteria caused by the scarcity of ammonium ions in grassland and

the direct competition of grass roots for such ions. Chase et al. (1967)

found that grasses grown in Ontario, Canada, did not compete for ammonium to the detriment of the nitrifiers nor did the grass roots inhibit

nitrifiers by other means. Populations of nitrifiers in urea-treated grass

and fallow plots were fully comparable. It would appear that the mechanisms by which and the extent to which different grassland soils inhibit

nitrification are in need of further study. Presently the bulk of the evidence is that such inhibition does exist and, as E. L. Rice ( 1 965) and Neal

(1969) have pointed out, possibly the inhibitory effect may be of importance in conserving the low amount of available nitrogen present in grassland soils or in determining the success of individual species to invade

or to establish dominance in plant communities.

C. DENITRIFICATION

AND VOLATILIZATION

The three general pathways for gaseous losses of nitrogen from soil

are those of enzymatic denitrification, chemodenitrification, and volatilization of ammonia. Recent reviews concerning these pathways are

available (Broadbent and Clark, 1965 : Gardner, 1965).

1 . Enzymatic Denitrifcation



In order for enzymatic denitrification, a process in which microorganisms turn to a nitrate respiration in lieu of an oxygen respiration, to occur,

it is necessary that nitrate be present, that oxygen be absent or greatly

limited in its availability, and that sufficient available substrate or energyyielding material be present to permit microbial activity. Additionally of

course, such environmental factors as moisture, temperature, reaction,

etc. must be favorable for microbial activity. Grassland soils, particularly

those of semiarid rangelands, are characteristically low in nitrate, and

they are usually well aerated. Accordingly, enzymatic denitrification in

them can be expected to be negligible. However, should nitrate be present

and should there be either regions or microsites of poor aeration, the de-



THE MICROFLORA OF GRASSLAND



425



nitrification process should not be greatly limited because of any shortage of microbial substrate (Greenland, 1958, 1965). Grassland soils

usually contain considerable amounts of organic matter in the upper few

centimeters of the profile.

2. Chemodenitrification



Chemodenitrification involves loss of gaseous nitrogen to the atmosphere due to nitrite instability (Clark, 1962). Inasmuch as the nitrite

dismutation requires the presence of nitrite and is promoted by microbial

metabolism (Clark and Beard, I96 I ) , chemodenitrification occurs primarily in soils in which the nitrification process is sufficiently retarded

to permit the accumulation of nitrite as an intermediate product in the

transformation of ammonium to nitrate and in which the organic matter

content is sufficiently high to support an active microflora. The inhibition

of nitrification that occurs in grassland soil, discussed above, is often

characterized by nitrite accumulation (Clark et al., 1960). In the work of

Soulides and Clark ( I 958), the gaseous nitrogen loss via chemodenitrification in some laboratory lots of grassland soils treated with urea nitrogen

was more than twice that measured for nearby intertilled soil.

Even though chemodenitrification losses of considerable magnitude

can be measured in the laboratory (Reuss and Smith, 1965: Soulides and

Clark, 1958; Wullstein and Gilmour, 1964), at present there appears to be

no justification for attempting to extrapolate such laboratory data to

finite values in the field. Most grassland soils do not receive ammoniacal

fertilizers and therefore such soils seldom if ever contain appreciable

nitrite. Only in grassland receiving ammonium nitrogen and in addition

characterized by a poor nitrifying capacity would any serious nitrogen

loss via chemodenitrification be expected. Even then, should warm temperature and drying conditions prevail, losses via ammonia volatilization

doubtless would be of much greater magnitude.

3. Volatilization of Ammonia



The loss of gaseous ammonia from soil is basically a physicochemical

process controlled by meteorological conditions and soil characteristics

(Gardner, 1965; Mortland, 1958). Microorganisms are involved to the

extent that they contribute to the supply of ammonium ions in the soil

by producing enzymes that hydrolyze urea or by decomposing soil organic matter. Microbial release of ammonium nitrogen from soil organic

matter occurs at a rate that is usually less than one or more of the following: the concurrent rate of nitrogen demand by growing plants; the

ammonium retention capacity of the soil: and/or the ammonium-oxidizing



426



FRANCIS E. CLARK A N D ELDOR A. PAUL



potential of the chemotrophic nitrifying bacteria in the soil. Because of

these demands, volatilization of ammonia microbially released from soil

organic matter can rarely be expected to occur.

Volatilization of ammonia is known to occur from grassland soils given

surface applications of urea or aqua-ammonia (Allison, 1955, 1966; Volk,

1959, 1961). There is usually a high urease content in grassland litter:

consequently, ammonia volatilization is usually higher from grassland soil

than from bare soil following equivalent applications of urea fertilizer to

the two soils. On unfertilized grassland in semiarid regions, some loss of

ammonium nitrogen from the system is known to occur from urea nitrogen

voided by grazing livestock. B. A. Stewart (1 970) found that following

addition of steer urine to bare soil with surface drying conditions, volatilization loss could be as much as 90% of the applied nitrogen. However,

urine-voided nitrogen that becomes volatilized is not necessarily lost from

the soil-plant system. It may be returned to the soil in precipitation or it

may be reabsorbed by moist surfaces (plant, soil) or free water surfaces

within the ecosystem.

ACKNOWLEDGMENT

This review stems from participation by the authors in the Grasslands Biome segment

of the International Biological Program. Its compilation was supported in part by National

Science Foundation Grant No. GB7824 and in part by the Canadian National Research

Council, being issued thereunder as CCIBP Report No. 41. The authors gratefully acknowledge helpful suggestions and discussion received from many colleagues at the Pawnee

and Matador grassland sites in the United States and Canada.

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