PLANT-PRODUCTIVITY
THE EFFECT OF SOIL RICROORGANISMS ON
J
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.
Y.R. Domergues, H.D. Diem
and F. Crmry*
ABSTRACT
I:oilmicroorganismsaffect p l a n t p r o d u c t i v i t y favourably or unfavourably
e i t h e r i n d i r e c t l y , by a c t i n g upon s o i l physical o r chemical p r o p e r t i e s ,
o r d i r e c t l y by i n t e r a c t i o n with p l a n t r o o t s . B e n e f i c i a l or d e t r i m e n t a l
e f f e c t s on s o i l p r o p e r t i e s concern s t r u c t u r e s , c o a t i n g of p a r t i c l e s with
water-repellent compounds, redox p o t e n t i a l , s o i l n i t r o g e n s t a t u s ( e . g .
g a i n s by N2 - f i x a t i o n and l o s s e s through d e n i t r i f i c a t i o n ) , a v a i l a b i l i t y
of n u t r i e n t s ( e s p e c i a l l y N and P ) and accumulation o r elimination of
phyiotoxic inorganic and organic compounds. S o i l microorganisms d i r e c t l y
a f f e c t p l a n t growth by improving or reducing n u t r i e n t o r wzter uptake (some
a r e well-known, e . g . ecto- o r endo-mycorrhizae; o t h e r s a r e not even
c h a r a c t e r i z e d , such as microorganisms iwlucing proteoid roocs). They may
a l s o produce growth-regulating substances o r p r o t e c t t h e @ a n t a g a i n s t
c e r t a i n 7athogens. Manipulation of t h e s o i l microflora a-wears t o be
highly d e s i r a b l e , but it i s d i f f i c u l t t o accomplish. Some m c c e s s has
a l r e a e y been zchieved with d i r e c t inoculation, e s p e c i a l l y i n t h e c a s e of
N2 - f i x e r s a d mycorrhizae. I n d i r e c t c o n t r o l of s o i l microflora by
methods invol-:ing c l a s s i c a l means, s t e r i l i z a t i o n o r t h e q F i i c a t i o n of
s p e c i f i c ccnp’inds, Ls possible provided some requiremenu e r e f u l f i l l e d .
Altering t h e s o i l microflora by a c t i n g through t h e p l a n t 1s another
promising p o s s i b i l i t y . The processes a r e discussed with s;r?cial
reference t o :heir importance and occurrence i n t r o p i c a l soils.
. .
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’
agronomists today would readily ayree that soil microorganisms affect plant productivity, zspecially in the tropics.
\
Yet this idea took a long time to penetrate, except in the
case of Rhizobium, because microbiologists-were mainly concerned
with the yhysiology of microorganisms that had been isolated
and skudied in %est-tubes or Petri dishes and were therefore
out Òf their natural environment. Another reason is that
i
the study
of the very complex soil-plant-microorganism systems
is mich more difficult that the study of pure culture.
pihny
In this paper, we shall consider some of the mechanisms by
which soil microorganisms favorably or unfavorably affect’
*Microbiologists, QRST@4/CNRS, Dakur and CNRA/ISRA,
tiambey, Senegal
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206
plant growth by altering the soil physical or chemical propFrties, or by directly acting upon the plant itself. Since
other contributors have covered the interactions between plants
and mycorrhizae, or N2 - fixing microorganisms, (Kenya, 1979;
Redhead, 1979) we shall only briefly mention the role of those
microorganisms, focusing our attention upon other groups whose
influence is still not always recognized. Two preliminary remarks
relate to the unique conditioqs that prevail in the tropics.
First, when soil water content is not limiting tropical temperatures are generally high enough to allow much more vigorous
microbial activity than in temperate areas. Second, since the
organic materials thar originate from the plant debris are
only to a slight extent stored as humic compounds and are readil)
decomposed, most microbial life.is located on or around the
root system of the plants (rhizosphere).
INFLUENCE OF MICROORGANISMS ON SOIL PROPERTIES
EFFECTS ON SOIL PHYSICAL PROPERTIES
The role of microorganisms in the genesis and maintenance of
soil structure has recently been reviewed (Hepper, 1975). Our
aim here is to emphasize thc inportance of this process in the
rhizosphere. It has been demonstrated that there are more waterstable aggregates in the rhizosphere than in the non-rhizosphere
soil (Harris et aZ.,1964).
Since the number of polysaccharideproducing microorganisms is characteristically higher in the
rhizosphere, it can be assumed that soil stabilization around
the root can, at least to some extent, be due to 'the rhizosphere
microflora. In tropical soils, where most of the microbial
population is Concentrated in the root zone, it would be worthwhile to elucidate ehe relative importance of the root itself
and that of associated microorganisms in soil structure
stabilization. Such investigations should not be restricted
to free-living microorganisms, (such as Asotojacter spp.,
B e i j e r i n e k i a i n d i c a or L i p o m g e e s s t a r k e y i , which are well-
own p o l y s a c c h a r i d e p r o d u c e r s ) , b u t s h o u l d be e x t e n d e d to
c o r r h i z a e which w e r e r e p o r t e d t o be i n v o l v e d i n sand aggrea t i o n and' dune s t a b i l i z â t i o n i n c o l d e r c l i m a t e s (Koske e t aZ.,
975).
I n c o n t r a s t t o t h i s b e n e f i c i a l a c t i v i t y , microorqanisms
can be harmful i n two ways:
by decomposing t h e a g g r e g a t i n g
compounds o r i g i n a t i n g from p l a n t s o r microorganisms; and by
c o a t i n g ' s o i l p a r t i c l e s w i t h w a t e r - r e p e l l e n t f i l m s (Bond, 1 9 6 4 ;
Bond and H a r r i s , 1964). By a l t e r i n g t h e advancing ' c o n t a c t a n g l e
of w a t e r with the p a r t i c l e s such f i l m s d i s t u r b t h e i n f i l t r a t i o n
of. water i n t o t h e s o i l , inducing a patchy d i s t r i b u t i o n of p l a n t s
and a marked loss of p r o d u c t i v i t y . Water r e p e l l e n c y , which was
mostl? a t t i b u t e d t o basidiomycete hyphae, i s t h o u g h t by G r i f f i n
(1969) t o be of p o t e n t i a l l y wide importance, e s p e c i a l l y i n s e m i a r i d conditions.
Microorganisms may a l s o a l t e r t h e s o i l redox p o t e n t i a l .
Thus
the growth of a e r o b i c microorganisms, most of which grow a t t h e
expense of decaying p l a n t d e b r i s , may l e a d t o a r e d u c t i o n i n
t h e p l a n t . A l t e r n a t i v e l y , p h o t o s y n t h e t i c a l g a e can produce
oxygen and r a i s e 'che redox p o t e n t i a l , t h u s a c t i n g d i r e c t l y o r
i n d i r e c t l y upon t i e p l a n t .
NITROGEN G A I N S AND LOSSES THROUGH BIOLOGICAL PROCESSES
The process of symbiotic N 2 f i x a t i o n h a s a l r e a d y been reviewee
(Keya, 1 9 7 9 1 , b u t mention should be made of t h e e f f e c t of
l i m i t i n g f a c t o r s , an a s p e c t o f t e n overlooked. Besides t h e
p o s s i b l e inadequacy of n a t i v e N 2 - f i x i n g micropopulations
and t h e a t t a c k s of pathogens, e s p e c i a l l y nematodes (Germani,
1 9 7 9 ) , f o u r major f a c t o r s can l i m i t s y m b i o t i c N 2 f i x a t i o n i n
t h e t r o p i c s : moisture stress ( e s p e c i a l l y i n s e m i - a r i d
c o n d i t i o n s ) , s o i l a c i d i t y and a s s o c i a t e d t o x i c i t y , m i n e r a l
d e f i c i e n c i e s and, i n some s i t u a t i o n s , a n e x c e s s of combined
n i t r o g e n i n t h e s o i l (Table 1 ) . As long a s one l i m i t i n g f a c t o r
i s o p e r a t i n g N2 f i x a t i o n i s low o r n i l and t h e i n p u t of n i t r o g e n
t o t h e ecosystem n e g l i g i b l e o r n o n - e x i s t e n t .
Two examples w i l l
i l l u s t r a t e t h e unfavourable e f f e c t of l i m i t i n g f a c t o r s .
These
examples a r e r e l a t e d t o peanut and r e s u l t from f i e l d experiments
208
+Table 1.
Methods to control the effects of environmental factors
limiting symbiotic N2 fixation
Limiting factors
1.
Moisture stress
Methods of control
-
2.
Soil acidity and toxicity
3.
Mineral deficiencies,
especially phosphorus deficiency
-
-
-
4.
Soil inorganic nitrogen
-
-
5.
Pathogens
-
Irrigation
Search for droughtresisting cv. of legumes
and drought-resisting
E%izobi wn
Stimulating VA mycorrhizal
infection
Lizing
Addition of organic matter
Addition of phosphorus
Stimulating VA mycorrhizal
infection
Split application of nitroger
fertilizers
Slow-release nitrogen
fertilizers
C a e of compatible
fertilizers
Sesrch for legumes with
a lower capacity for
nitrate assimilation
Chemical, biological or
integrated control
Crop rotations
Senegal during the last 3 years. The first is illustrated
by Fig. 1, which shows that in the arid conditions prevailing
in Central Senegal, N2 fixation (measured by the acetylene
assay) is closely related to soil water content. The second
example concerns the limiting effect of inorganic nitrogen.
Using'the A value method, (Ganry, 19761, found that by
increasing the rate of application of nitrogen fertilizer
from 15 to 60 kg per ha, N2 fixation by peanut decreased
from 52 to 25 kg per ha. In spite of those limitations, some
N2 - fixing systems can remain active. For example, Casuarina
e q u i - s e t i f o Z . i a , a non-leguminous nodule-bearing tree, largely
used for reforesting sandy soils on the coast of West Africa,
was reported to fix as much as 60 kg N2ha-1 year-1 on the CapVert peninsula (Dommergues, 1963).
Microorganisms can bring about losses through nitrification
and denitrification. The activity of nitrifying bacteria varies
considerably according to the soil characteristics and to the
nature of the vegetation. These bacteria are typically neutrophilic but nitrification is not necessarJly restricted to
neutral soils, but to the neutral micro-habitats. Since such
habitats may occur ( e . g . in the vicinity of organic debris)
in soils whose overall pH is acid, nitrification can be very
active in such soils. Thus acid tropical soils grown with banana,
maize, or rain-fed rice exhibit a high nitrifying activity
when ammonium fertilizer is applied. (Dommergues e t aZ., 1978:
Chabalier, 1978). In forest soils, nitrification may be hindered
by antibacterial substances released by the litter: when the
forest is cleared, a flush of nitrification usually occurs
(Dommergues, 1954). There is increasing agreement that nitrification is a detrimental process since it ip respon'sible for
two types of nitrogen loss: through leaching, since nitrate is
of an anionic nature, and through denitrification (Focht and
Verstraete, 1977). Such losses are highly variable, but they
are seldom lcwer than 20-30 per cent of the nitrogen applied
as fertilizer. The increased cost and shortage of fertilizer
nitrogen, especially in the tropics, must prompt soil microbiologists to gather more information on factors that could
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DAYS AFTER SOWING
Fig. 1:
Variations of acetylene reducing a e t i v l t y ASA pez plant)
of field-grown peanut and of s o l water cszcent throughout
the peanut growth cycle as observed in 197: a t the Bambey
Experimental Station, Central Senegal (Durezï, 1978)
5!
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211
I
it nitrification in soils, since this process is presumably
easily controlled than denitrification. Recent advances
the fie'ld o f methodology (especially direct detection of
cteria in the soil by the fluorescent-antibody techniques)
omise to be most helpful (Schmidt, 19781.
I
AVAILABILITY OF NUTRIENTS
In tropical soils ammonification is usually very active,
so that the potential for the release of ammonium from soil
organic nitrogen is high. Unfortunately, the organic nitrogen
inputs (through N2 fixation, root and litter deposition) into
the spi1 are often limited, so that ammonium release is not
high enough to meet the plant's requirements. It is not
clear whether nitrate, which is the end product of nitrification,
is more available to plants than the ammonium ion..
'
P
Microorganisms, especially those thriving in the rhizosphere,
are often thought to be able to increase the phosphate available to plants by dissolving water-insoluble mineral phosphate, or by mineralizing phosphate from soil organic matter. As
far as mycorrhizae are concerned, their role as solubilizing
agents has not yet been demonstrated. Other soil microorganisms'might be involved. I n v i t r o experiments-have
clearly shown that many common microorganisms, including
Pseudomonas, Achromoba-zer, PZavobacterium, S t r e p t o m y c e s ,
and especially A s p e r g i i l k s and A r t h r o b a c t e r can solubilize
soil phosphorus (Hayman, 1975; Barber, 1978)'. However some
authors argue that the increased uptake of phosphate may not
only result from an increase in the availability of phosphate,
but could also be explained by the effect on plant growth of
stimulating substances synthesized by the micro-organisms. With
regard to organic phosphate, it is readily mineralized by
plant phosphates of the root surface. The soil microflora do
not seem to increase this process significantly.
'
Miçrob{ally-inducsd changes of available trace elements
have recently been discussed; (Bawber, 1978). Since a variety
.4
of microorganisms, as well as plants, synthesize some hydrooxamic
acids known to be powerful chelating agents, it is not surprising
that so51 microorganisms play a prominent role in the iron
metabolism of plants (Waid, 1975). A classical example of the
decreased availability of trace elements is that of manganese.
Manganese deficiency of oats was shown to occur when the activity
of manganese-oxidizing microorganisìzs was too high. Soil
fumigation reduced the population of these microorganisms and
eliminated the manganese deficiency symptoms (Timonin, 1946).
SOIL TOXICITY
Phytotoxic compounds that may accumulate in the soils are
of microbial or plant origin. A classical example of phytotoxicity induced by microorganisms is that of hydrogen sulphide
produced by sulphate-reducing bacteria. The growth and activity
of these bacteria is triggered in the rhizosphere when the
following environmental conditions exist concurrently: active
root exudation, soil sulphate contenz of the rhizospheric soil
above a minimum threshold, and stricz anaerobiosis. Accumulation
of hydrogen sulphide can be high enough to lead to the death of
plants (Dommergues e t aZ., 1976; Jacc and Roger 1978). Manganese
toxicity which occurs in acid soils t h a t are relatively rich in
manganese may be reinforced by rhizosphere microorganisms
capable of reducing manganic sourceç. Partial sterilization of
such soils may prevent toxicity (Barber, 1978).
Phytotoxic compounds of plant or:gin
are responsible €or
diminishing plant growth when thev a r e not decomposed. Many
examples of such toxic effects have Seen described bv Rice
(1974) Recently, investigations carried out at the Agronomic
Research Center of Bambey in Central Senegal showed that sorghum
rbots contained phytotoxic compounds vhich, in some circumstances,
could significantly reduce the yield of subsequent crops, especiñily
sorghum. When sorghum is grown once in a two-course rotation
(peanut-sorghum) instead of once in a four-course rotation (green
manure-peanut-sorghum-peanut) yields are severely depressed.
.
213
ffect (known as “soil sickness“) is induced
e accumulation in the soil of a phytotoxic compound after
irst crop. The phytotoxic compound, which is specifically
itory to sorghum, remains in the soil as long as environmental
rtions prevent its biodegradation by soil microorganisms.
Since such unfavorable conditions may prevail in sandy soils
for seven to eight months, the phytotoxic compounds are still
present when sorghum is re-sown too soon after its last cropping.
It should be pointed out that while “soil sickness” does occur
in sandy soils containing kaolinite-type clays and showing a
poor microbial activity, no symptoms are noted in Vertisols,
which contain montmorillonite-type clays and where microorganisms
are significantly more active. In Vertisols, the sorghum
microflora comprising strains that can actively decompose the
phytotoxic compound (Domergues, 1978b).
I
Another example of phytotoxicity of importance in forestsy
is related to the failurs of GreviZZea r o b u s t a regeneration in
Australia. Seedlings of this species were reported to be killed
by some water-transferable factor associated with the roots of
parent trees. The resjlting regulation of population in G .
r o t u s t a is thought to explain the maintenance of floristic
diversity in complex tropical rain forests (Webb e t O Z . , 1967)
.)
c
-
DIRECT EFFECTS ON THE PLANT
As the root grows through soil, it encounters diverse components of the soil microflora and it is directly affected by
the activity of soil microorganisms. Rhizoplane and rhizosphere
populations affect the host plant in many ways, but there is‘now
increasing evidence that the most important effects of microorganisms on plant growth ccncern the modification of plant nutrition
and water uptake, the production of growth-regulating substances
and the protection of roots against pathogens.
!4ClDIFICATION OF PLANT NDTRITION AND WATER UPTAKE BY MYCORRHIZAE
The best example of the role of microorganisms as regulating
agents of plant nutrition is illustrated by mycorrhizal asso-
7..
-
ciations., The plan
main response to mycorrhizal infection is
an increased uptake
nutr2ent$, especially phosphorus, Mineral
nutrition of plants as stimulated by ectomycorrhizae has been,
well treated by Bowen (1973) and the effects of vesiculararbuscular mycorrhizae (VAM) have been reviewed by Tinker (1975)
Redh.ead (1979) and others.
Many theories have already been proposed to explain the
increased uptake of phosphorus by ectomycorrhizal roots (Bowen,
1973). Some of them could apply to VX4 since Gerdemann (1968)
considers that the function of VAM may also be very similar to
that of the ectomycorrhizae.
.
-
It includes the formation of more efficient nutrient-absorbing
structures than non-mycorrhizal'roots, The extensive strands of
extramatrical hyphae in VAM may also explore a much greater volume
of s o i l than non-infected roots, as do hyphae of ectomycorrhizal
fungi. The possibility of a longer active absorbing life for
mycorrhizal as compared with non-mycorrhizal roots, a s stated by
Bowen and Theodorou (Bowen, 1973) for ectotrophic mycorrhizae,
should also apply to VAM (Gerdemann, 1968), although actual
evidence is still lacking. Another interesting facet of the
biology of mycorrhizae is related to the behaviour of infected
roots under low water regimes in the soil. Tropical soils are
quite different from one another in water content because there
is a wide range of soil textures and climates in the tropics.
In sandy soils, especially in semi-arid regions, plants are
often subjected to a relatively long period of water stress. A
most interesting question is whether soil water supplies could
be improved by mycorrhizae. The physiology of water absorption
by mycorrhizae has hardly been studied but some investigations
have indicated a greater drought resistance in a number of
mycorrhizal seedlings (Bowen, 1973).
In 1971, Safir et aZ. indicated that 'WU4 could probably
decrease the resistance to water transport in soybean. But
later (Safir e t a Z . , 1972) they concluded thatbincreased plant
growth in water-stressed conditions was due to the improvement
of phosphorus nutrition. Recently, however, Menge et aZ. (1978)
have reported that mycorrhizal infection enabled avocado plants
to resist transplant shock, suggesting that mycorrhizae could
rove water uptake by the*hostTplant. Drought resistance
corrh.iza1 plants may be related to the greatex exploitation
soil by extensive hyphal growth, but also to large
ferences between infected and non-infected roots in their
biology. As stated Gy Cromer (in Bowen, 1973), mycorrhizal
oots of Pinus r a d i a t a seemed to renew growth more quickly
han non-infected roots when they are subjected to severe
water stress. Another interesting hypothesis on the relationships between soil-water regime and mycorrhizal infection is
iven by Sieverding (in Moawad, 1978) who found that the amount
f water used to produce lg of dry matter was much lower in
mycor,rhizal than in non-mycorrhizal plants growing in dry
soil fertilized with Ca5 (PO4)30H (Table 2)
According to
Moawad, Sieverding's findings may simply be due to the better
utilization of water by plants growing in phosphorus-deficient
soils. If we wish to explain the greater drought resistance
of plants, the theory of water consumption economy as stated
above seems to be more plausible and more attractive than the
-principle of increased uptake or transport of water in plants
(Safir e t a l . , 1971).
I
.
MYCORRHIZAE UNDER TROPICAL CONDITIONS
r
The impact of mycorrhizal symbiosis in the growth of
tropical plants has been recently discussed by Bogen (1978)
and Black (1978). Black noticed that the number of tropical
plants associated with ectomycorrhizae appears to be very
limited as compared to the wide range of ectomycorrhizal
plants in the temperate region. The only crop recordedwith ectomycorrhizae is P i n u s (Redhead, 1978). Inventories and other
information concerning ectomycorrhizal forest trees are given
in Alwis and Abeynayake (1978).
As for endomycorrhizae, although same families such as
C a s u a r i n a c e a e , C h e n o p o d i a c e a e , U r t i c a c e a e are devoid of VAM
(Khan, 1974), most tropical plant species of economic
importance are infected: cocoa, tobacco, cotton, corn, sweet
potato, peanut, sugar cane, sorghum, rubber, tea, citrus and
many species of timber trees (Redhead, 1971). Spores of VAM
levels of soil water content (80 and 20% available
water) and with two forms of p (after PlOawad, 1978).
Plant
My cor rhi z al.
BPUCiÇ?l?
t rc a t m e n t
Ca ( I I 2 P O 4 ) 2 H 2 0
Ca5 (Po413 OH
80%
80%
- -
E.
odo79atum
'7'.
P>', ,*1c1
20%
NM
M
1208
1237
1207
1177
2860
1574
4112
1436
NM
1073
923
1.005
2563
118 O
3397
1424
M
NM:
20%
1060
Not inoculated with VA mycorrhiza
'
217
\
widely distributed in Niger
d forest to the
77).
?
few
i
from the moiPt lowregions (Redhead,
oliv
e significance of mycorrhizal symbiosis in the cultivation
olives in Pakistan has been discussed by Khan and Saif (1973).
In different soils olc..the arid and semi-arid regions, it is
robable that mycorrhizal associations play an important part
n the growth and drought-resistance of a number of plants
ecause of their ability to regulate uptake of nutrients and
oil water. Unfortunately, little is known about the'mycorrhizal
of mycorrhizal effects in these regions of the world would be
of great practical interest, particularly in the case of
afforestation with plant species that usually are transplanted.
In our laboratory, observations of the roots of Azadirachta indica,
a tree whose growth is wide-spread in dry sandy soils in Senegal,
indicate that most roots, if not all, are infected with VAM
(Fig. 2 ) . It is significant to note that Azadirachta indica
is able to grow vigorously in non-fertilized soils and in arid
conditions.
EFFECT OF VAM INFECTION ON LEGUME-RHIZOBIUM SYMBIOSIS
-
According to a number of papers VAM also occur in many
tropical legumes of ecanomic importance e.g. peanuts, cow-pea,
MacroptiZium atropurpurzum, StyZasanthes spp. (Possingham
et al., 1971;.Sanni, 1976: Graw and Rehm, 1977). As legumes.
have been shown to require high levels of phosphate for nodulation,
it is likely that mycorrhizal infection may affect the
s (Crush, 1974: Islam et aZ.,1976; Mosse et al., 1976;
and Daft, 1977). Recently, in an excellent essay on the
f mycorrhizae in legume nutrition on marginal soils, Mosse
pply of rock phosphate stimulated growth and nodulation of
ny legumes. Although the principal cause of this is undoubtly
Fig. 2:
A z a d i r a c h t a i n d i c a roots infected with VA mycorrhizae
INDUCED PROTEOID ROOTS
,
stimulates the phosphorus-nutrition of host plants.
'
-
Despite
absorb soil phosphate has been attributed to-the formation
of clusters of rootletk in localized parts of the lupin
root system. These clusters of rootlets resemble the den
clusters known as proteoid roots which have been described in
the family of Proteaceae by Purnell (in Trinick, 1977). Other
proteoid roots have also been recorded by Lamont on V i m i n a r i a
jzincea and by Malajczuk on Kennedia (Trinick, 1977). It has
now been shown that proteoid roots play an important role
in thephosphorusnutrition of plants due to their increased
absorbing ability as compared with normal roots (Jeffrey, 1967;
Malajczuk and Bowen, 1974).
Aicording to published literature, very few plant species
form 'proteoid roots. In Senegal, one of the authors (H.G.D.)
observed that rootlet clusters similar to proteoid roots can
be found in Casuarina eqki3ezifoZia usually growing in sandy
and deficient soils. In the cluster, lateral rootlets are
so numerous that they resemble fingers (Fig. 3 ) . Proteoid
roots could therefore provide C. equisetifoZia with an alternative
*
,
system-to mycorrhizae for increasinq P uptake from deficient
ns are now in progress in our laboratory to
cts of these root formations on the physiology
lthough the mechanisms o f the initiation
re not clear, some
tion experiments
oid roots may be in
e root surface (MaLajczuk
I
;il
220
r
i
Fig. 3 :
C l u s t e r of rootlets ( p r o t e o i d roots! of C a s x i 1
*orfa growing i n a sandy soll ( S e n e g a l )
1
. As m i c r o b i a l
r o o t .growth:
d a c t i v i t y are
m
nse i n the
for example r o o t s of tomato, s u b t e r r a n e a n c l o v e r
o v i s a and McDouqall, 1 9 6 7 ) .
However, p a r t i c u l a r a t t e n t i o n has
een paid t o t h e b e n e f i c i a l e f f e c t e x e r t e d by r h i z o s p h e r e ina b i t a n t s . T y p i c a l r h i z o s p h e r e b a c t e r i a such as A r t h r o b a e t e r ,
seudoponas and Azrobacterium w e r e found long ago t o be a b l e t o
roduce s u b s t a n c e s promoting p l a n t growth ( K r a s i l n i k o v , 1 9 5 8 ) .
Ectomycorrhizal f u n g i also p r o v i d e t h e h o s t p l a n t w i t h
phytohormones and growth-regulating B v i t a m i n s ( S l a n k i s , 1 9 7 3 ) .
D e t a i l e d d i s c u s s i o n about t h e d i r e c t e f f e c t s of b a c t e r i a on r o o t
rowth through the production of p l a n t growth-regulating f a c t o r s
an be found i n many reviews ( X r a s i l n i k o v , 1958; Katnelson, 1965;
~
Brown, 1 9 7 5 ) .
The i n f l u e n c e of e c t o n y c o r r h i z a l hormones on t h e
development of r o o t s of t h e h o s t p l a n t has a l s o been amply
demonstrated i n S l a n k i s , ( l 9 7 3 ) . However, i n s t a n c e s of i n c r e a s e d .
p l a n t growth r e s u l t i n g from i n t e r a c t i o n s between s o i l microorganisms and p l a n t s show t h a t when p l a n t s a r e a r t i f i c i a l l y
i n o c u l a t e d with a p a r t i c u l a r microorganism known f o r a determined
b i o l o a i c a l a c t i v i t y ( e . g . N 2 f i x a t i o n ; phosphorus s o l u b i l i z a t i o n ) ,
s t i k i l l a t i o n of p l a n t growth o f t e n was p u t a t i v e l y a t t r i b u t e d t o
t h e e f f e c t of t h i s s p e c i f i c a c t i v i t y , a l t h o u g h i t may simply be
due t o t h e production of phytohormones by t h e same microorganism.
Three examples found i n d i f f e r e n t f i e l d s r e i n f o r c e t h i s p o i n t of
view:
(1) T h i r t y y e a r s ago, G e r r e t s e n (1948) thought t h a t t h e
i n c r e a s e d growth of p l a n t s i n s t e r i l i z e d sand c o n t a i n i n g i n s o l u b l e
,
7
.
.
,.
y i e l d s have o f t e n been recorded after i n o c u l
s m a l l m o u n t s of h i g h l y a c t i v e growth-promoti
the b a c t e r i a (Brown, 1975). SimLlarly, i n o c u l a t i o n w i t h
~
,
Azospim'Zlum b r a s i Z i e n s e , a f
ving NZ-fixing bacterium, can
a l s o induce i n c r e a s e d p l a n t g
t h e a e r i a l p a r t s of rice were
Table 3 s h w s t h a t growth of
i v e l y s t i m u l a t e d by i n o c u l a t i o n
w i t h a n o n - N ~ - f i x i n g bacteri
th A . b r a s f l i e n s e , and that
r o o t growth was even more ac
t i m u l & e d . Moreover, s i n c e
r i g e n e r a l l y does n o t s i g n i f i c a n t l y
i n o c u l a t i o n w i t h ~ z ~ s p iZZum
improve Na f i x a t i o n , Gaskins and Hube11 (1978) and Tien e t a l .
( 1 9 7 9 ) suggested t h a t the e f f e c t of A z o s p i r i Z t u m i n o c u l a t i o n
on p l a n t growth could be due t o g r m t h - s t i m u l a t i n g s u b s t a n c e s
produced by t h i s b a c t e r i u m , as i n t h e case of A z o t o b a c t e r .
(3)
I n some experiments of b i o l o g i c a l c o n t r o l , r o o t disease of wheat
a s s o c i a t e d w i t h R h i z o c t o n i a s o l u n i was reduced and g r a i n y i e l d
i n c r e a s e d by seed i n o c u l a t i o n w i t h b a c t e r i a and actinomycetes.
Merriman e t al.. ( 1 9 7 4 ) s u g g e s t e d t h a t t h e y i e l d i n c r e a s e s are
p r i m a r i l y due t o p l a r t growth-stimulating f a c t o r s r a t h e r th?
t h e b i o l o g i c a l c o n t r o l of r o o t d i s e a s e .
to
IMPROVEMENT O F PLANT RESISTANCE TO INFECTION
Discussion w i l l be r e s t r i c t e d t o t h e c o n t r o l of pathogens
through the improvement of p l a n t r e s i s t a n c e by s y m b i o t i c microorganisms o r microorganisms more o r less l o o s e l y a s s o c i a t e d
w i t h t h e r o o t s l which i s only one a s p e c t of t h e v a s t problem
of b i o l o g i c a l c o n t r o l .
Two t y p e s of mcchaqisms may be i n v o l v e d
i n t h e type of c o n t r o l s t u d i e d here.
I n h i s review, Marx (1975) i n d i c a t e d t h a t if p i n e r o o t s
w e r e a s s o c i a t e d w i t h L a u c o p a x i 1 Z m c e r e a l i s var. p i c e i n a t o
f c r m ectomycorrhizae, they became r e s i s t a n t to i n f e c t i o n s caused
by such pathogenic f u n g i as P h y t o p k t h o r a cinnamomi. Many
mchanisms could be i n v o l v e d t o e x p l a i n t h e p r o t e c t i v e r o l e o f
e c t o m y c o r r h i z a l p i n e r o o t s . Apart from the e x p l a n a t i o n t h a t
a n t i b i o t i c production i n h i b i t s f u n g a l pathog-rns (Marx, 1975) ,
t h e f u n g a l mantle of ectomycorrhizae a l s o c r e a t e s e f f e c t i v e
7
'
224
m c h a n i c a l b a r r i e r s a g a i n g t p e n e t r a t i o n by P. c<nnamomi. There
w a s f u r t h e r evidence t h a t f u n g a l m a n t l e s formed b y n o n - a n t i b i o t i c producing e c t o m y c o r r h i z a l f u n g i a l s o p r o t e c t e d r o o t s f r a
p a t h o g e n i c r o o t i n f e c t i o n s . I t i s a l s o suggested t h a t e n d o p h y t i c
-
In
mycorrhizae may p r o v i d e p l a n t p r o t e c t i o n (Milhelm, 1973)
t h i s c a s e , t h e r e i s no p h y s i c a l b a r r i e r , b u t e a r l y t e r r i t o r i a l
occupation of Living r o o t t i s s u e s by t h e endophyte may promote
b i o l o g i c a l control.
I n t h e presence of s a p r o p h y t i c m i c r o f l o r a many p l a n t s
produce a m u l t i t u d e of compounds, e s p e c i a l l y the s o - c a l l e d
p h y t o a l e x i n s , which can p l a y a r o l e i n r o o t disease r e s i s t a n c e .
Most have been i d e n t i f i e d i n a e r i a l p l a n t p a r t s , b u t it i s
l i k e l y t h a t t h e same compounds can also be f o m d in t h e r o o t
system (Paxton, 1 9 7 5 ) . F o r i n s t a n c e , p i s a t i n , the well-known
p h y t o a l e x i n of t h e pea p l a n t , o c c u r s i n t h e r o o t s as w e l l a s
i n most o t h e r p a r t s of t h e p l a n t and h a s a wide spectrum of
a n t i b i o t i c a c t i v i t y . S t r a w b e r r y r o o t s a l s o produce phytoa l e x i n s i n response t o P h y t o p h t h o r a f r a g a r i a e i n f e c t i o n s
(Mussell and S t a p l e s , 1 9 7 1 ) .
MANIPULATING THE S O I L MICROFLORA
Since t h e major p a r t o f t h e s o i l p o p u l a t i a i i n t r o p i c a l
c o n d i t i o n s i s made up of t h e r h i z o s p h e r e m i c r o f l o r a , and
s i n c e t h e r h i z o s p h e r e m i c r o f l o r a must, be viewed as a component
of t h e whole s o i l - p l a n t - a t m o s p h e r e system [ D m e r g u e s , 1978a) ,
t h e soil m i c r o f l o r a c o u l d p r e d i c t a b l y b e manipulated,' n o t
only d i r e c t l y by a c t i n g upon t h e microorganisms, b u t a l s o
i n d i r e c t l y by a c t i n g upon t h e s o i l and t h e p l a n t . D i r e c t
manipulation of t h e s o i l m i c r o f l o r a can be achieved by inocul a t i o n p r a c t i c e s , s t e r i l i z a t i o n and t h e a p p l i c a t i o n of
s p e c i f i c i n h i b i t o r s o r s p e c i f i c substrates.
l a t i o n of t h e soil-plant-atmosphere
I n d i r e c t manipu-
system can be achieved
by c l a s s i c a l or non-conventional s o i l management p r a c t i c e s ,
o r by a c t i n g upon t h e p l a n t component i t s e l f .
, .
In spite of the fact that root colonization by non-path
icroorganisms is still poorly unaerstood , soil microbiologis
d agronomists have been trying for many years to alter the
osphere microflora by introducing selected microbial strains,
er by coating seeds with an inoculum, or by placing the
inoculum into the soil close to the seed or the seedling.
The value of legume inoculation is well recognized, provided
that the strain used is highly effective and efficient in its
symbiosis with the selected legume cultivar, that it is a good
colonizer of the roots and is able to compete with any ,iative
root microorqanism, and that the proper environmental prerequisites
are fulfilled. However, legume inoculation by classical methods
is not always fully satisfactory.
I
l
-
The value of ectomycorrhizal inoculation is also generally
acknowledged as long as the proper environmental conditions are
met (e.g. Hacskaylo, 1 9 7 2 ; fiarx and Krupa, 1 9 7 8 ) . Inoculation
by endomycorrhizae is currer.cly at the experimental stage excepr.
in special situations. Preliminary reports suggest that larger
responses are more likely in Lropical regions than in temperate
regions, because of higher temperatures and the naturally lowphosphcrrus level of soils (Iiayman, 1 9 7 8 ) .
Recent experiments carried out in the northern coastal area
of Senegal have shown that inoculating .'asuar-ina s ; v i s e r l ' ? Z i ,
with crüshed nodules improved that plant's growth markedly (Dubrekrl
and Andeque, personal communication). Further investigation OT.
the endophyte of i7u T Y . . ~ ~ : is needed in order to improve the
currenr method of inoculation, which is obviously hazardous s x z s
crushed nodules used as inoculum may carry pathogens.
Although techniques
noculation with typically symbioz;z
microorganisms (e.g. R h i c c 1 are already Ln use in the field,
or could be used in the near future (e.g. endomycorrhizaef, techniques of inoculation with loosely symbiotic or non-symbiotic
microorganisms (e.g. rhizosphere N 2 fixers oz phosphate-solubilizing bacteria) cannot yet be safely recommended.
I
'
,'
-.
c
The first attempts at using Npfixing rhizosphere bacteria
to inoculate grasses or cereals were made
(Rubenchick, 1963). Since tha't'date many
performed, at first with A z o t o b a c b e r or B e i j e r i n c k i a ahd later
with AzospiriZZum (e.g. Smith e t al., 1976: Dobereiner, 1977).
Yield increases have sometimes been reported but up to now results
have generally been inconsistent.
Fïeld experiments with phosphate-solubilizing bacteria
(especially BaciZZus m e g a t h e r i u m ) have not shown any consistent
effect on plant yield. According to Barber (19?8), * this
lack of response is not really surprising for two reasons. Firstly,
since a considerable proportion o f soil phosphorus is present in
organic compounds and up to 90%' of the rhizosphere microflora axe
capable of producing phosphatases, the introduction of other
organisms, which would have to compete for available carbon sources,
is unlikely to cause any increase in the supply of phosphate to
plants. secondly, the inoculum used, BaciZZus m e g a t h e r i u m v a r
phosphaticum is a spore-forming bacterium and such organisms grow
far less readily in the rhizosphere than do other types of bacteria".
When stimulation of plant growth consecutive to inoculation
by N 2 fixers or phosphate-solubilizing bacteria has been observed,
it could not be explained by N 2 fixation, nor by an increase of
phosphate solubilization. The stimulation of plant growth probably
has resulted, at least in part, from the effecc of growth substances produced by the microorganisms added w i t h inoculum, as
already mentioned above. In spite of some recent improvements
in the preparation of the inoculum itself (Doumel'gues e t al., 1979)
or in the introduction of mixed cultures (Domergues et a Z . , 1978),
there would seem to be no easy solution to the difficulties that
arise when attempting to inoculate non-sterile soils.
Soil sickness can result from the presence of plant residues
in the soil, especially root litter contáining phytotoxic substances. Inoculating such soils with microorganisms that
actively decompose the root litter appears to be a promising
approach to curing these soils. Thus inoculating a Eerrallitic
sandy soil that contained phytotoxic root debris with E n t e r o b a c t e r
I
227
aeae restored soil fertility (l'able 4 ) . Phytotoxic subnces, pxe-existing in plant residues or formed during decomposition, can possess a broad spectrum of effects that are
jurious to the roots and stems of plants (Toussoun &-ad Patrick,
6 3 ) . Such a deleterious effect could probably be seduced by
il inoculation with proper microbial strains.
SOIL STERILIZATION AND APPLICATTON OF SPECIFIC COMPOUXDS
In sterilization by heating, irradiation and drying is
sed in certain circumstances, sterilization is often achieved
y fumigation with such chemicals as chloroform, carbon-sulfide
methylbromide or chloro-picrin. Such treatments often improve
plant growth even in the absence of pathogens (Wilhelm, 1966;
Rovira, 1976). This beneficial effect can be attribured to
different causes: chemical modifications, especially increase
of NH4 zontent, flush of organic matter decQmposition, including
d microorganisms (Anderson and Domsch, 1978), elimination of
nitrifying bacteria, which are particularly vulnerablt to fumigation (Jenkinson and Powlson, 1976), and re-colonizeiron of soil
by rion-pathogenic microorganisms, especially pseudomcnads, which
are thought to stimulate plant growth (Ridge, 1976).
u
Soil sterilization prior to inoculation wich rr.y-orrhizae
appears to be most helptul in special sltuations ( L a i i b and
Richards, 1978). Among these are fumigated nursery s o l l s where
severe stunting of citrus was reported: inoculation with
vesicular-arbuscular-mycorrhizae appeared to be the kest method
to overcome this stunting (Lamb and Richards, 1978; TFmmer and
Leyden, 1978; Hayman, 1978).
Among the different specific inhibitors that have been studied
[e.g. Anderson and Domsch, 1975), nitrification inhibitors have
received much attention because of their possible use in the field.
Besides the agronomic practices mentioned above, inhibitors such
as 2-chloro-6- (trichloromethy1)-pyridine have been successfully
used to inhibit nitrification, thus increasing the efficiency of
nitrogen fertilizers by reducing de-nitrification ana leaching
f the nitrate ion. Unfortunately, especially in tropical conditions,
he inhibitor is readily decomposed by the soil microflora so
NO
inoculatirn
W i t h inoculatia
(ccntrol)
65
60
3.9
2.7
48.7
14.7
8.2
1.3
Bots
.
stitutes have been proposed, such as neem cake (made of the
eds of A z a d i r a c h t a indica), but this material is not as effective
2-chloro-6 (trichloromethy1)-pyridine (Prasad and de Datta,
The stimulation of a given component of the microflora can
e achieved by adding a specific substrate to the soil. A
lassical example is that of the selective multiplication of
decomposing microflora (Alexander, 1961).
Another example is that of the solubilization of rockphosphate by T h i o b a c i Z Z i . These chemoautotrophic bacteria are
introduced into the soil together with sulphur which is oxidized
to sulphuric acid, thus dissolving the phosphate (Swaby, 1975).
FERTILIZATION AND SOIL MANAGEMENT
..
n
Inoculation even with specific microorganisms, especially
R h i z o b i u m , is unsuccessful when one of the limiting environmental
factors listed in Table 1 is still operating. Therefore, improvement of environmental conditions is a pre-requisite that can be
achieved by different soil management practices, such as irrigation,
liming, application of organic amendments or slow-release
fertilizers. The beneficial effect of liming is illustrated by
Table 5 (Expt. 1) which reports on a study of soybean nodulation
in a ferrallitic acid soil from Casamance, Senegal. The increased
nodulation was attributed to the elimination of Mn and Al toxicity
of liming. Table 5 (Expt. 2), shows that the application of
organic matter even at low rates (400 kg of peat per ha) favourably
affected the growth and nodulation of soybean. This last result
confirms those obtained by Dart e t a l . , 1973 with V i g n a mungo and
V . r a d i a t a . Neither species grew well in a nitrogen-free sandrit mixture. But adding 10% of Kettering loam by volume improved
rowth and nodulation. When added loam had been previously ignited
t 45OoC for 4 h to remove soil organic matter, plant growth was
Eqt. 1
Caltrol
4.0 a
14 a
42a
3.38 a
co3
(2500 kg per ha)
7.0
b
39 b
U2b
3178 a
4.0 a
28 a
44 a
4.0 a
41 b
88 b
Expt. 2
Ccntrol
'
2.16 a
Fe a t
(400 kg p r ha)
Qle p l a n t
per pot cmtaining 5 kg of s o i l fran Sefa &sear&
3.03 b
Statim,
Senegal. A l 1 plants we= i n o c u l a t e d w i t h 1ml of a 3-day old culture
of f i i z o b i m . j a p " m m asp (10a bacteria per ml)
Observatims were
made wfen plants VÆE 6 weeks old. I n e¿& experirrent, n u n h r s in
colunns not hrming the smre letter are statistically difkrent
(P = .05).
.
and the plants eventually died.
A combination of liming, ploughing and the application of
farm-yard manure was reportea significantly to increase peanut
yields in Cent91 Senegal, probably through increasing N2 fixation
(Wey and Obaton, 1978).
Since N2 fixation is not always active enough to meet the
gume's requirements, it is necessary to use nitrogen fertilizers.
t it is known that such applications inhibit N2 fixation. To
prevent this inhibition in legumes, Hardy e t al. (1973) suggested
the use of other form of nitrogen fertilizers that do not inhibit
N2 fixation, while providing the plants with the complementary
nitrogen required for their growth. Such new forms of chemical
fertilizers, which they designated as compatible fertilizers, could
also be recommended for use. The possibility, though promising,
has not yet been seriously explored.
'
+
Nitrification can be controlled by such classical methods as
split application of ammonium fertilizers, localization in mud
balls (International Rice Research Institute, 1978), or banding,
which inhibits nitrification due to the effect of the high
concentration of fertilizer on nitrifying bacteria (Wetselaar e t
a l . , 1972; Myers, 1978). The use of slow-release fertilizers is
also recommended to avoid the harmful effects of nitrification
(Fochts and Verstraete , 1977)
.
MANIPULATION OF THE PLANT COMPONENT OF THE SOIL-PLANT-MICROORGANISMS SYSTEM
Introducing a specific crop in the rotation system has
been used successfully as a basis for the biological control of
some pests. Thus in Florida, soils infested by nematodes
pathogenic to tomato, are cured by growing a grass, D i g i t a r i a
decumbens, after the tomato crop (Salette, personal communication).
Crop rotation is often the best method of controlling soilborne phytopathogenic fungi in cereals (see Baker and Cook, 1974).
he possibility of increasing populations of microorganisms
eneficial to plants through proper crop rotation was suggested
y Krasilnikov (19.58) but the method has not yet been exploited.
w i l l probably be d i f f i c u l t t o i n i t i a t e and develop b e c a u s e of
t h e l a r g e v a r i a b i l i t y o f climate and s o i l c o n d i t i o n s .
Genetic v a r i a b i l i t y i n p l a n t s responding t o lhhizobhium
i n f e c t i o n i s w e l l known. This v a r i a b i l i t y could b e used as a
b a s i s f o r t h e b r e e d i n g programmes of legumes.
The f u t u r e of
t h i s approach w a s envisi0ne.d a s follows by Hol1 and La Rue (1974).
" P l a n t genes c o n t r o l l i n g f i x a t i o n do occur, and e x p e r i e n c e shows
t h a t w e can o b t a i n i n f o r m a t i v e and u s e f u l v a r i a n t s .
There i s no
obvious, r e a s o n why s y m b i o t i c f i x a t i o n cannot be i n c r e a s e d by
g e n e t i c means. W e can e n v i s a g e c u l t i v a r s which n o d u l a t e e a r l y
i n h a r s h s o i l c o n d i t i o n s , f i x d i n i t r o g e n , even i n the p r e s e n c e
of high s o i l n i t r a t e levels, and continue f i e n g throughout t h e i r
l i f e . I t appears t h a t f i x a t i o n may be l i m i t e d by t h e s u p p l y of
photosynthate t o t h e r o o t s . I n c r e a s e d f i x a t i o n may t h e n r e q u i r e
g r e a t e r p h o t o s y n t h e s i s , decreased p h o t o r e s p i r a t i o n , delayed
lodging, o r less pod-nodule competition f o r carbon".
Two examples may s e r v e a s an i l l u s t r a t i o n f o r such a
promising approach, which has n o t y e t been s e r i o u s l y e x p l o i t e d .
The f i r s t concerns t h e n o d u l a t i o n of peanut. Comparing t h e t i m e
course of nodule d r y w e i g h t of three peanut c u l t i v a r s grown i n
1977 a t t h e same t i m e i n i d e n t i c a l c o n d i t i o n s (Dior s o i l , C e n t r a l
Senegal) , Germani ( 1 9 7 9 ) found t h a t t h e maxiinum nodule weight
of two of them was much h i g h e r th^ t h a t of t h e t h i r d ( F i g . 4 ) .
However, such r e s u l t s s h o u l d be i n t e r p r e t e d w i t h c a u t i o n s i n c e
d i f f e r e n c e s i n nodule weight a r e also observed from one y e a r t o
another.
Thus t h e m a x i m u m nodule weight of cv. 55-437,
which
was only 70 mg i n 1977, could reach 100 mg i n t h e same s o i l
d u r i n g more humid y e a r s (1973 and 1975) and even more t h a n 2 0 0 mg
during an even more humid y e a r ( 1 9 7 4 ) (Wey and Obaton, 1 9 7 8 ) .
The o t h e r example i s r e l a t e d t o soybeaq. I n West A f r i c a n
s o i l s , c e r t a i n soybean c u l t i v a r s , such as Malayan, are r e a d i l y
n o d u l a t e d by n a t i v e BaR2izobium of the cow-pea group, whereas
other c u l t i v a r s , such as B o s s i e r , a high y i e l a i n g cv. from
t h e USA, r e q u i r e i n o c u l a t i o n with t h e s p e c i f i c lhR2izobium
japonieum ' % t r a i n s . S e l e c t i o n of high-yielding soybean t h a t
c o u l d n o d u l a t e w i t h n a t i v e %izobitri.: of the cow-pea group would
..
n
F i
I
&
5
a
-
300
c3
z
c
200
I
c3
I
w
3
w
100
O
20
40
60
100
80
AGE OF
Fig. 4 .
120
THE PLANT (DAYS)
T i m e c o u r s e of n o d u l e d r y w e i g h t of p e a n u t e x p r e s s e d
a s mg p e r p l a n t .
B
:
cv. 55-437.
A : cv. 28-206
and GH 1 1 9 - 2 0 ;
A l l d a t a a r e mean v a l u e s f o r c o l l e c t i o n s
i n 1 9 7 7 a t P a t a r , C e n t r a l S e n e g a l (Germani, 1 9 7 9 )
-
CONCLUS IONS
This paper has summarized the numerous ways in which soil
microorganisms can affect the fertility of soil and it has noted,
with examples, how in some cases they can be manipulated in order
to benefit the growth of plants. Up to now practically all work
done has been with agricultural, horticultural or forestry landuse systems. There is clearly a very urgent need now to relate
specific areas of soil microbiological research to agroforestry
systems in which woody and herbaceous plants will be grown either
mixed together or in some sequential manner.
The many possible ways in which the activities of soil microorganisms in the soil-plant association of one of these groups
of plants can affect the other is an almost untouched field of
research. In particular, the effects on microorganisms of soil
management, innoculation and nitrogen fixation and transformation,
and the consequent influence on soil fertility in agroforestry
systems might be given early attention.
D I SCUSSI ON
Keya:
Nitrification inhibitors are produced in the rmts of many grasses.
The neem (Azadirachta indica) plant also produces such an inhibitor,
which might have some prospects in agroforestq.
Scinchez: In North Queensland, Australia, they have observed a competitive
relationship between EucaZ@us and grass pastilre for li, but not in
legume pasture.
Pereira:
The reason for all crops doing poorly after sorghum in dry conditions
is that the stubble continues to utilize water from the 2-m deep subsoil
for many weeks.
bmergues:
A reduction in soil water content may also reduce the microbiological activity responsible for decomposing phytotoxic compounds
added by sorghum roots.
Ahn:
Some grasses are known to inhibit nitrification in West African savanna
soils. Thus, yams (Dioscorea spp.) which demand less U, are grown in
the first season followed by grain crops.
235
es:
Pwbably N immobilized ir! t h e grass r o o t system i s n o t
e r a l i z e d and it i s o n l y p r o g r e s s i v e l y decomposecl. I n c o n t r
p r e s i d u e s a r e e a s i l y mineralized.
Does t h e phytotoxic e f f e c t of sorghum on a succeeding c r o p apply t o
a succeeding crop of sorghum a l s o ?
rgues: Y e s . However, t h e phytotoxic e f f e c t i s o n l y on s o i l s w i t h low
biological a c t i v i t y and w a t e r reserve.
Zsen:
Does g r a s s exude n i t r i f i c a t i o n i n h i b i t o r s , t h e r e b y reducing growth
of Eucalyptus?
ergU0s: Y e s . But t h e r e i s no published r e f e r e n c e f o r t h e i n h i b i t i o n
of Eußalyptus growth.
t:
C i t r u s and peach produce t o x i c m a t e r i a l s i n t h e i r r o o t s which i n h i b i t
the development of new trees. Some seeds of d e s e r t annuals have growth
i n h i b i t o r s t h a t p r e v e n t germination u n t i l t h e s e water-soluble i n h i b i t o r s
sre leached away o r changed chemically.
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