TROPICS Vol . 4 (213): 173-186 Issued February 1995

Effect of Burninll on Soil Nutrient Status andAbundance of \,,A-mycorrhizal Fungi in aSavannah Type Grassland Ecosystem in SouthIndia

K. SENTHILKUMAR*, S. MlNtlX, K. UDAIYAN AND V. SUCAVAXdVT

Department of Botany, Bharathiar University, Coimbatore-641 046, India

ABSTRACT The influence of fre on the frequency of occurrence and species richness of VAM fungiin a tropical grassland in southem India was examined for a period of one year from July l99l to lune1992. Buming in ropical grassland increased the VAM spore density in soils. Glomus mosseae and G.geosporurn were the most frequently occurring species in the study site. There was no considerablevaniation in species richness of VAM fungi between burned and unbumed control plots. Buming wasfound to increase the soil P content" however, subsequent watering was shown to deplete the soil P statussuggesting the uptake ofP by plants after watering treatrnent. This was further supported by a gradualincrease in the P content of plants over a period of one year in burned and watered plots. However, thereexhibited no considerable variation in the VAM infection intensity in major crops of the plots viz.,Cymbopogon caesius, Heteropogon contartu.s, Rhynchosi.a cana and Azadirachta indica. T\e influenceof various ecological factors and climatic conditions on the dynamics of VAM fungal populations afterfire is discussed.

Key Words: ropical grassland / savannah / fire i VAM fungi / buming

Fire is a natural disturbance component of the Westem Ghats grassland ecosystem in southernIndia. Frequent fire results in the changes in soil temperature (microclimate), water potential,plant species composition, nutrient and microbial status (Knapp & Seastedt, 1986; Gibson &Hulber, 1987; Gibson & Hetrick, 1988; collins & Gibson, 1990). The effect of buming and

its influence on nutrient cycling has been examined in several ecosystems, but such studieshave generally been shrub or tree cover developing during the fallow period (Algren &Algren, 1960; Jorgensen & Hodges, 1970; Amaranthus & Trappe, 1993). Through thebuming process nutrients accumulated in the plant biomass become released to soil and largeamounts of C and N are released into the afinosphere. Despite a number of studies on the

influence of fire on soil physical and chemical properties, very little is known on its impacton soil biological properties. A few studies have shown that soil biologicat properties can be

directly affected by fire (Perry et al. ,1987).In natural grassland communities the vesicular-arbuscular mycorrhizal (VAM) symbiosis

is widespread and play a major role in nutrient cycling in such soils. The influence of fire onforest myconhizal populations has been studied (Pickett & White, 1985; Amaranthus &Trappe, 1993) and it was suggested that the mycorrhizal fungi play an important role in post

fire nutrient availability and thereby the establishment of vegetation. There are accounts offire effects on ectomycorrhizal density (Buchholz & Gallagher, 1982; Reddell & Matajczuk,

* Corresponding author and address: K. SsNtIilLxuMAn, Research scholar, Department of Botany, BharathiarUniversity, Coimbatore - &l M6, Tamil Nadu (state), INDIA

t74 K. SSNTHILKUMAR, S. MeNnN, K. UpRIYAN &V. SUCNVNNAM

1984) and soil fungi (Wicklow, 1973). The effects of burning on mycorrhizal or othermicrobial populations are varied in different ecosystems and depends on duration and

intensity of fire as well as on soil and site conditions (Perry & Rose, 1983). There are few

accounts of the effects of fire on VAM species composition. Studies of the impact of various

disturbance processes on beneficial microbial activities in soils is essential (Kannan &Oblisami, 1990; Kannan et al ,1990) to develop efficient management strategies for natural

ecosystems. In this regard, our earlier investigations dealt with the litter degadation process

in a natural grassland ecosystem and the role of various Fungi in the degradation process

(Senthilkumar et al., t992, 1993). Although such studies including those of fire effects on

grasslands are predominant for sub-topical and temperate regions, not much is known about

the influence of natural disturbance processes in a nopical grassland ecosystem.

The present study was conducted with the objectives of examining the effects of fire on

the frequency of occurrence and richness (number of species) of VAM fungal species and to

know the changes in nufiient status of such soils in a tropical grassland plots.

MATERIALS AND METHODS

Study siteThe study was conducted from June 1991 to June 1992in a Savannah type natural grassland

located in the foothills of Maruthamalai (11o04'N latitude,77"93'E at 426 m MSL),Coimbatore, southern India (Fig. 1). The total area of this grassland is about 3, 500 acre. The

soil was a red sandy loam (sand 44 Vo, silt 33 Vo, clay 23 Vo) of pH 7. 0 and EC O. 2-0. 4

mmhos/cm (at 25'C). The nutrient status of the soil at the beginning of this experiment was

as follows: total N lM k{ac, P20s 6 kglac,Kr0 150 kg/ac, Zn 0. 56 kg/ac, Cu 0. 8l kg/ac,

Mn 5. 0 kg/ac and Fe 7. 5 kg/ac. The climatic condition of the study area during the period ofinvestigation is shown in Fig. 2.

Bharathiar Universityo

COIMBATORE DISTRICT

TAI\'TII NADU

N

tFig. 1. Map showing

the study area.

Effect of burning on soil nutrient status and abundance of VA-mycorrhizal fungi 175

N!! Rainfall inmm+ Temp.<nax tfl Temp.-min t+ RHatTary.

O nHatl4pm

Fig. 2. Climatic cnnditions of the study area during the investigation period betweenJuly 1991 to June 1992.

The study was undertaken in a good vegetation of grasses and the site was ungrazed. Thedominant crop species at the study site were, Cymbopogon caesius, Heteropogon contartus,Rhynchosia cana and Azadirachta indica. Four plots, each measuring 100m x 100m, lyingadjacent to each other were selected based on vegetational homogenity and topography.Following teatrnents were made under each plot Plot I - Unburned and not watered; Plot II -Unbumed and watered; Plot III - Burned and not watemed; Plot IV - Bumed and watered.

Plos III and tV were burned on l3th June 1991 while plots I and II remained as unburnedcontrols. Plots II and IV were watered at weekly intervals for l5cm depth each time whereas,plots I and III were covered with polyethylene bags so as to avoid effects of watering duringrainy days. Although we did not replicate each treatment, soil and root samples werecollected at several locations in each plot at bimonthly interval from July l99l to May 1992.

Collection and analysis of soil samplesComposite soil samples were collected from all the four plots and examined for pH and majorplant nutrients such as, N, P and K contents.(i) Nitrogen estimationTen gram soil (0.5 in the case of plant material) was taken in a clean dry kjeldatrl flask anddigested with concentrated sulphuric acid and catalyst (copper sulphate, potassium sulphateand selenium powder). The extract was distilled in Markham stream distillation unit and theammonia liberted was collected in boric acid which was tifiated against dilute hydrochloricacid (Jackson, 1958: Misra, 1968).

ii) Phosphorus estimationTen gram soil sample (or 0.5 g plant material) was digested with ternary acid mixture (10 mlconc. HNOr, I ml conc. H2SO o and 4 ml 60Vo HCIOJ until it became colourless. Afterdigestion, it was extracted with distilled water and then filtered. The volume of the extractwas made to 100 ml. Ten ml of the solution was taken in 50 ml volumetric flask and 2 dropsof dinitrophenol and 2 ml of sulphomolybdic acid were added and the volume was made to

,---o---l)-.-o--,' N

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ttASONDIFMAMJ

176 K. SgNTHTLKUMAR, S. MeNnN, K. UpaIyAN &V. SucevaNAM

50 ml. The solution was transferred to 100 ml conical flask and added with 3 drops ofchlorostannous acid with intermittent shaking. After 5 minutes, optical density of the

developed blue color was determined in a photoelectric colorimeter at 660 nm. The

concentration of phosphorus in the test solution was calculated from the standard graph

developed from potassium dihydrogen phosphate (KH2PO.) solution of differentconcentrations. The result is expressed on per cent dry weight basis ( Jackson, 1958 ) .

(iii) Potassium estimation

It was analysed by flame photometry (Peach and Tracey, 1956). Five gram air dried soil

sample (0.5 g in the case of plant material ) was added with 25 ml of Morgan ' s reagent

(4850 ml lN sodium acetate + 150 ml acetic acid), shaken for about 5 minutes and filtered.

The filtrate was directly used in the flame photometer and the readings noted. Potassium

concentrations werie calculated from a standard graph where the potassium concentrations

were plotted against galvanometer readings. If the concentration of potassium exceeds 100

pglml, the samples may be suitably diluted.

Estimation of VAM spore density in soils

Soil samples were collected from rhizosphere at a depth of ca. 0-15 cm from soil surface. Atleast five samples were collected from different sites in each plot (i.e., treatment), and from

each plant species, composited and quantity of 500 g was packed in clean polyethylene bags

and stored at 4"C for wet sieve analysis to determine the VAM spore density.

VAM spore density in each soil sample was estimated by a modified wet sieving and

decanting technique as described by Gerdemann and Nicholson (1963). A quantity of 100 g

soil was suspended in 500 ml water and the soil particles were allowed to settle down for l0min. The suspension was passed through 780 m and 38 pm sieves. The residues in 38/1 m

7.t

7.6

1.4

1.2

AI{. rro5\r' lmaa-lr:e0ct'

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aSrI!r7-E€6oesaE4(r,

3

JSNJMM-'-Q'-' PlOt I "-f--' PlOt tI

JSNJMM

---t1- Plot III -{- Plot IV

Fig 3. Seasonal variations in mil pH, N, P and K status of different treatment (burning and

watering treatmenb) plots in a tropical grassland during July 1991 to June 1992..

Effect of burning on soil nunient status and abundance of VA-mycorrhizal fungi 177

sieve were resuspended in water, transferred to a burette and left undisturbed for 5-10 min.The soil particles settled at the bottom were removed by opening the stopper. The spores insoil suspension were collected over a filter paper by passing the suspension over it. The innersurface of the burette was washed two to three times to collect the adhering spores. The filterpaper was spread over a glass plate and the spores were count€d under a magnification of100x. The population ofspores was expressed as number ofindividuals per g dry soil. Spores

were identified to species based on microscopic observation (Trappe, 1982). Frequency of the

most abundant species and total richness (species number) were assessed in each of the fourplots.

Collection and analysis of plant root samples

Feeder root segments of Cymbopogon caesius, Azadirachta indica, Heteropogon contartusand Rhynchosia cana occurring up to a depth of ca. 15 cm in different treatrnent plots werecollected, washed to remove adhering soil particles, cut inlo small pieces, fixed in FAA (13

ml formalin 200 ml SOVo ethyl alcohol + 5 ml glacial acetic acid) and brought to laboratoryand observed for VAM colonization.

Examination of VAM colonization in plant rootsThe procedure described by Phillips and Hayman (1970) was used for the rapid assay ofmyconhizal colonization in plant roots. The root segments preserved in FAA were cut into Icm pieces and washed thoroughly in distilled water. The tissues were softened by boiling inSVo KOH at 90oC for 15 min for an hour depending upon the hardness of the material, thenthe root bits were washed three !o four times in distilled water, acidified with 5 N HCt for l0min and stained with0.O1Vo ffypan blue (in lactophenol) for 15-30 min. The excess stain, ifany, w:ls removed by washing with lactophenol. Stained root bits were mounted on glass

slides in lactophenol and examined under a low power compound microscope for thepresence of VAM hyphae and spores. For each treatrnent, at least one hundred root bits wereexamined for VAM infection/colonization. This technique has been widely used for theexamination of VAM infection in plant roots (Giovannetti & Mosse, 1980). VAM infection inroots was calculated as follows:

Per cent root infection = t tOONumber of root bis examined

The method described by Piper (1950) was used for the determination of N, P and K contentsof plant materials collected from different plots at rimonthly interval.

RESULTS AND DISCUSSION

Soil nutrient statusSeasonal variations in pH and N, P and K contents of soil samples from four differenttreatrnent plots from July 1991 to June 1992 are shown in Fig. 3. The pH of soils in unburnedplots (I and II) did not vary during the study period. However, in burned plots (III and IV) thepH increased slightly over time as watering was continued periodically. It seems thatwatering has no significant effect on soil pH as was observed by the unalteration in soil pHbetween watered and unwatered plots. The increased pH in burned and watered plot (IV)perhaps reflects the influence of ash formed by the fire. Amaranthus and Trappe (1993)

r78 K. SeNTHTLKUMAR, S. MeNmN, K. UperyAN &V. SucaveNAM

Table 1. Frequency* of occurrence of various VAM fungal species in a grasslandsoil following firing and watering.

Species Frequency (%)

Plot I Plot II Plot III Plot IV Mean

Ac aul o sp or a b ir e t i c ul at a

A. elegans

A. gerdemaniiA. nicolsoniiA. sporocarpia

Sclerocystis rubifurmis

Glomus albidum

G, ambisporumG, constrictumG. deserticola

G. etunicatumG. formosanumG, geosporum

G. microcarpumG, mosseae

57

67

33

50

67

50

67

67

57

33

33

100

83

50

100

83

33

100

50

83

83

83

576750

67100

100

50

100

67

33

83

100

67

50

83

83

83

83

67

67100

33

100

83 75

67 50

67 77

83 7'1,

83 75

100 71,

100 83

67 7r100 79

83 62

83 63

83 88

100 96

83 54

100 100

TotalMeantSD

934 711,6

62.b. 74.40+

21.44 21.68

1,099 1282 1190

73.27! 85.47! 73.93+

27.65 72.31, 13.99

*Frequencies are based on the percentage occurrence in soils following the examination duringsix sampling months.

Table 2. Seasonal variations in spore densities (number of spores/g soil) of VAM fungalspecies in a grassland soil after buming and watering treatrnents from fuly 1991 to

Jwre192-Treatment* Acaulospora spp. TotalGlomus spp.

Iul. Nov. |an. Mar. Ma Iul. Nov. ]an. Mar. Ma

Plot I 2.9 1.6 I.7Plot II 1.8 2.2 2.7

Plot III 3.8 4.7 5.0

Plot IV 3.8 5.4 3.0

2.A 2.8 2.r1.0 2.3 3.6

3.8 2.9 4.0

2.8 4.5 6.9

13 76 72 18

13 19 14 15

25 37 20 12

19 22 28 13

20

22

20

30

23 115

26 1,23

29 16r31, 1,69

'Details of the treatments are mentioned in text.

reported a similar increase in the pH of bumed soils compared to unbumed ones following

forest fire in southwest Oregon, USA. Similarly, Tester (1989) reported an increase in soil pH

with frequency of burning on oak Savannah in east-cental Minnesota.

The average N contents of plots I, II, III and IV wete 92.3, 86, 90 and 85 kg/ac

respectively. The N content in burned, watered plot (IV) tended to decrease whereas, the

control - unburned, non watered plot (I) showed a slight increase in N with time (Fig. 3).

Generally, in watered plots N decreased when compared to nonwatered plots. Probably the

supply of water might have enhanced the uptake and availability of nifiogen for plant growth

ultimately leading to decreased soil N status. In other words, burning might have not

contributed to an increase in soil N status. As a result, the uptake of N from soil by plants

without external input might have contributed for the depletion of soil N content. A similar

pattern was observed in prairie grasslands in which legumes were shown to utilize more N in

Effect of burning on soil nutrient status and abundance of VA-mycorrhizal fungi 179

bumed plots resulting in a decreased N content of soils (Hobbs et al.,l99l). Amaranthus andTrappe (1993) showed a decrease in N content of soils and an increase in P content followingwildfire in coniferous forests. Buming increased soil phosphorus status, although subsequentwatering seemed to decrease it. The burned but unwatered plot (III) showed a maximummean P content of 7. 0 kg/ac (Fig. 3). Watering followed by buming was shown to decreasethe P content (plot IV). This may imply that burning has increased VAM colonization;however, subsequent watering has resulted in an increase in the P uptake by plants. The VAMfungi form an extensive net work of external hyphae, thereby increasing the effectiveabsorptive surface of the root and the ability to absorb a variety of essential poorly mobilenutrients such as P (Harley & Smith, 1983). The effects of fire on VAM spore counts andtheir colonization have been discussed in the later part of this article. In contrast to ourresults, Bentivenga and Hetrick (1991) showed that fire did not change soil p content intallgrass prairie in Kansas. A significant increase in K content was noticed in all the fieabnenrplots (II, III and IV) when compared to confrol plot (I). The increase in K content may be dueto the influence of ash formed from vegetation buming which is rich in potassium content.The variation in the nutrient status of various treafinent plots between different samplingmonths may also be due !o the influence of climatic conditions and vegetation at differentseasons. Therefore, results obtained from different climatic regimes by several authors in theworld are varied, suggesting the need for site specific investigations to elucidate fue effectson a vegetation.

VAM spore density in soilsA total of fifteen VAM fungal species were present in soils from the study area (Table l), themost widespread species being Glomus mosseae and G. geosporum with frequencies of

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Fig 4. Seasonaf changes in the percent root infection by VAM fungi on Azadirachta indica, Cymbopogoncaesius, Heteropogon cont&rtas anil Rhynchosia cana in a tropical grassland following furning anOwatering treatments in different plots (July l99lto June 1992).

Andirachta indica Cymbopogon caesius

Rhynchosia cana

180 K. SeNTHTLKUMAR, S. MRNnN, K. UoRIyAN &V. SuceveNAM

o(t)o0C')q)trg(t)

ocn

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ct)

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oo(n

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Fig. 5. Seasonal variations in the rhizosphere VAM fungal populationsol Aztilitachta indba,Cymbopogon

caesius, Hcteropogon contarlrrs and Rhynchosia cana in a tropical grassland following burning and

watering treatments in different plots (July 191 to June 1992)'

occurence of 100 and 96 70, respectively. The abundance of both these species was not

related to buming or watering, as observed by their maximum occulrence in all the treatment

plots. This also indicates that these two species were not affected by fire or climatic

variations in this area. Usually, these two species are widespread and common in alkaline and

neutral soils (Khan, 1978). Gibson and Hetrick (1988) and Hetrick and Bloom (1986)

reported a similar pattern of the highest occurrence of G. mosseae from six sites surveyed

monthly for 1. 5 yrs on Konza Prairie Research Natural Area, Manhattan, Kansas. Liberta and

Anderson (1986) found out that G. fasciculatum to be the most abundant species on Illinois

prairies; however, this species was not detected in the present study area. Other abundant

species in this investigation were G. formosanum and G. albidum.In bumed plots (plots III

and IV), the frequency of occurrence G. formosanum was lower than in unburned plots. Apart

ftomGlomus spp, other taxa observed include, Acaulospora spp. and Sclerocystis rubifunnis.

However, their density of occurrence is lower than those of Glomus spp. Among Acaulospora

spp, A. nicolsonii was found to occur more frequently in burned sites. Similarly, buming

increased the frequency of occunence of G. constrictum. Although species richness was not

related to either watering or buming, however, mean spore number per plot was positively

related to burning because of the increase in spore numbers per g soil in plots III and IV

(Table 2). Anderson et al. (1984) observed that VAM fungal species richness across a

topographic gradient was positively correlated with percentage soil organic matter and

negatively correlated with Ca, Mg and P content of soil, while total spore number was

positively correlated with nitrogen and organic matter content and negatively with Ca, Mg

and P content of soils. However, in this study, no obvious relationship can be explained

Effect of burning on soil nutrient status and abundance of VA-mycorrhizal fungi

Thble 3. Dominant vAM structures, root colonization (%) and rhizosphere population(spores/g soil) of VAM fwtgi occurring in major plants in the grassland after buming andwatering treatment.

Treat- Plantment species* Iul Sep Nov fan Mar Muy colonization***

181

Dominant VAM structure** Mean root Meanrhizospherepopulation***

Plot I

Plot II

Plot III

Plot IV

AH AVHAH AVHVH AVHAVH AVHAVH AVHAVH AVHAVH AVHAVH AVHHVHAH AVHAVH AVHHVHVH AVHAVH AVHVH AVHVH AVH

53111

45t2249X1,5

58+8

45!2043!21,40!2747-2444_3445t2342!2349-2950!2944t3256!2646133

18r1.5

17!1.11613.0

73+4.6

17!3.820+5.6

1g+3.3'].,6+3.7

20!424t4:J.

2419.8

24e5J,

28+6.2

26!71,

25+5.9

27t7.1

AiAVHHHHCc AVH AVH H HHcAHHHHRc AVH AH AH HAiAHAHHVHCc AVH AV H AHHc AVH VH AH VHRc AVH VH H AHAiAVHHHCc AVH AHHc AH VHRc AH AVH HAi AHAVHH HCc AVH AVH HHc AVH AVH HRc VH AH

*Ai, Cc, Hc and Rc are Azadirachta indica, Cymbopogon caaius, Hetuopogon contartus and Rbynchosia caw, respectively.

l*A,v and H represent arbuscular, vesicular and hyphal shuctures of vAM fungi, respectively.+oMean of six sampling months t SD.

Table 4. N, P and K contents (mg/d of four major plants* in a hopical grassland afterbuming and watering treatments.

Plant species Plot I Plot III Plot IVNPK

Plot IINPK

Azadirachta indica 1.8

Cymbopogon caesius 1,.2

Heteropogon contartus I.4Rhynchosia cana 1.8

0.51 1.5 0.89 0.51

0.43 0.94 0.94 0.520.38 0.71, 0.74 0.47

0.51 1.9 0.75 0.55

1.3 0.93 0.77 1.58 0.81 0.58 1.31.1 7.2 0.47 L.1 1.0 0.49 0.gg0.69 O.gt 0.M 0.77 0.gg 0.45 0.901.1 0.91,0.63 1.9 1.0 0.51 1,.3

*Data indicate the mean of four sampling months.

between nutrient status, VAM species richness and VAM spore density in soils due to theshort-term nature of the study period. A longer investigation period may throw light on thepossible relationship between soil nutrient status and VAM abundance and species richness.

In general, Glomus spp. constituted 80-90 Vo of the total spore counts in soils. Glomusspp. was the most highly invasive mycorrhizal fungi, having the highest percent of rootlenglh colonized in a tallgrass prairie (Dhillion, 1992).In facr, the various Glomales VAMfungi recorded in this study are ubiquitous in arable soils (Harley & Smith, 1983). Eventhough earlier investigations showed both positive and negative effects of fireTburning onVAM fungal community (Reddell & Malajczuk,1984; Pendleron & Smith, 1983; parke era1.,1983, 1984), the present results suggest an increase in soil population of the VAM fungi.The exact reason for the increase in the abundance of VAM fungi in burned soils is not clear.However, several factors such as rapid dispersal of spores upon burning, reduction in

t82 K. SnNTHTLKUMAR, S. MeNnN, K. UpAIyAN &V. SucnveNAM

Azadirachtn indica1.0 1.0

s 0't

gE 0.6(,T

6 0.4a

t 0'8

f 0.6

!04

IbE

f-,

flEI,-EE

t

0.2

0.t

0.1

0.6

0.5

0.4

03

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G Jan Apr

-# ptot I -F pbt II

Fig. 6. Seasonal trends in P content of AztdirachU indica, Cymhopogon caesius, HeEn pogon

contortus anll Rhynchosia cana in a tropical grassland following burning and watering

treatments in different plots (July 1991 to June 1992)'

competitive pathogenic microbial populations, changes in plant composition and soil nutrient

status are atfibutable for the variation in the VAM status in soils. Amaranthus and Trappe

(1993) indicated that the fire did not kill any of the vAM propagules and showed that erosion

of soils following fre had adverse effect on VAM inoculum potential. The magnitude of soil

erosion followed by fue is very high in hilly slopes. The present study was conducted in plain

grasslands and we did not examine soil erosion rates. Further studies are in progress to

estimate the erosion in the tropical grassland following fire.

VAM infection in plant roots

In order !o examine the influence of variation in soil fungal population after buming on plant

root infection, we collected root segments of major plants such as, Cymbopogon caesius,

Azadirachta indica, Heteropogon contartus and Rhynchosia cana in all the four treatrnent

plots and examined for the presence of VAM colonization and dominant VAM structures

(Table 3). Seasonal variation in the percentage VAM colonization in the roots of these plants

over a one year period following the fire is illustrated in Fig. 4 (the results are based on the

examination of a few hundred root bits from each of the four plots during different sampling

months). Furthermore, the population of VAM fungal spores in the root zone (rhizosphere) of

these plant species was estimated for a period of one year and the results are shown in Fig. 5.

The percentage of roots colonized by VAM fungi for all the crop species was almost

uniform in all the treatment plots, indicating that burning has not significantly influenced the

root infection by VAM fungi. However, it can be seen that ttre population of VAM spores in

the rhizosphere ofthese plant species increased markedly in plots III and IV highlighting the

role of burning on VAM abundance. This increase in soil VAM fungal spore population in the

rhizosphere is similar to those observed in non-rhizosphere soils after burning.

Although it is known that vAM infection density varies in space and time in natural

Heteropogon contartus

Effect of burning on soil nutrient status and abundance of VA-mycorrhizal fungi 183

ecosystems (Gay et al.,1982:, Giovannetti, 1985), little is known about the causes of variationin infection and whether infection is related to plant nutrition and growth. Informationregarding temporal patterns of VAM infection together with nutrient uptake and plantperformance is essential to an understanding of their significance in natural ecosystems. Allthe plant species examined showed an initial decline in the infection per cent (colonization)by VAM fungi followed by an increase in plot IV, which was burned and watered (Fig. 4). Incontrast, the rhizosphere VAM population showed a steady and gradual increase during theone year period in burned plots (Fig. 5). The unalteration in VAM infection intensity invarious plant species, in spite of the increase in the rhizosphere VAM spore population afterfire may again be explained by several environmental factors. As an example, it was shownthat the root systems of certain gr:rsses (Bromus tectorum, Stipacomnta and Chrysothatnnusviscidiflorus) rapidly occupy the below ground space and compete for soil resources after fire(Melgoza & Nowak, 1991). Pandey (1938) showed that burning stimulated the growth ofshoot and root components of Dicanthium annulatun but reduced the growth of rhizomes in atropical grassland in India. Apart from these plant variables, soil P status has been shown roaffect VAM infection intensity on plant roots (Arines et al.,1990; Sanders & Fitter, l991a,b).Only during the periods of high P demand(periods such as high respiration andphotosynthesis during flowering and seed formation stages of crops) VAM fungi infect andcontribute to the necessary rate of uptake. If it is the case, the results of the present studyshow that, despite increase in soil P status and VAM spore density in rhizosphere and non-rhizosphere soils, infection on plant roots was not affected because the p demand for cropafter burning may be low Therefore, the VAM fungal infection depends on numerousvariables including plant P requirement, soil P status etc. However, a long-term study periodmay provide information on the possible causes of infection in natural ecosystems.

We also determined the N, P and K contents of Cymbopogon caesius, Azadirachta indica,Heteropogon contartus and Rhynchosia cana and the results are shown in Table 4. N contentof plants in plot I (unburned, unwatered)was higher than other in plots. p and K contents didnot exhibit clear cut variation between various treatments. The P content of plants in varioustreatment plots over a l0 month period is shown in Fig. 6. In bumed plots(plot [V) there wasa gradual increase in P content of all the four plant species. This indicates that VAM fungimight have actively played a role in translocating soluble P after firing and subsequentwatering.

It may be concluded from the present study that burning in a tropical grassland insouthem India increased the VAM spore density in soils. Although no considerable variationin VAM species richness was evident between burned and unburned plots, it is probable thatthe abundance of VAM fungal species depends on the climatic features of an area. In thisstudy, G. mosseae and G. Seosporum were the most frequently occuning ones both in burnedand unburned plots. There exhibited no considerable variation in the nutrient status of the topsoils (0-15 cm depth) after fire, except P and K contents, which showed an increasing trend.However, subsequent watering following the fire resulted in the reduction in soil p status.This was compensated by an increase in the P content of four major plants in the burned plot.There exhibited no considerable variation in the VAM infection density of the plants. Thisimplied that the burning might have activat€d certain VAM structures already existing inroots and frans located ofP from soil to plants.

It is also worth mentioning that the variations in results obtained on the role of fire in

184 K. SENTHlLKUMAR, S. MANIAN. K. UOAIYAN &V. SUGAVANAM

natural ecosystems by several workers may be due to the differences in the climatic features

of the study area concerned. Therefore site specific studies are needed to discuss the role of fire on an ecosystem. Furthermore, such studies should allow considerable length of time to

obtain a long-term and valuable information. To our knowledge, this is one of a very few reports available on the effect of fire on a tropical grassland.

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186

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Ecology 54: 1302-1310. Received July 15, 1994

Accepted Jan. 30, 1995

K. SENTHEILKUMAR, S. MANIAN, K. UOAIYAN & V. SUGAVANAM

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