THE EFFECT OF CONSERVATION TILLAGE ON MICROORGANISM COMMUNITIES IN THE SOIL UNDER ONION CULTIVATION

Danuta Pięta¹, Tadeusz Kęsik^2
1 Department of Phytopathology, Agricultural University of Lublin, Poland
² Department of Soil Cultivation and Fertilization of Horticultural Plants, Agricultural University of Lublin, Poland

ABSTRACT

The object of studies was the soil from under onion cultivation after spring rye and common vetch as aftercrops of cover plants. The quantitative and qualitative composition of particular populations of bacteria and fungi were determined in this soil. The microbiological analysis found out that cover plants such as spring rye and common vetch had a positive effect on the development of microorganisms in the soil. Spring rye stimulated the growth and development of microorganisms, especially antagonistic ones, and spring vetch caused an increase of the number of cfu of pathogens.

Key words: conservation tillage, onion, antagonistic fungi, antagonistic bacteria.

INTRODUCTION

The microbiological activity in the soil environment depends on biotic and abiotic factors. Among the biotic factors, the plant has the greatest influence on the formation of microorganism communities. The main stimulator of microorganism development in the period of vegetation are the compounds exudated by plant roots and the hulling cells of the roots [8,9,25,29,32,33,34,35]. A special attention should be drawn to the stimulating effect of root exudates on the development of antagonistic bacteria and fungi towards soil-borne pathogenic fungi. In the group of bacteria the best abilities of antagonistic effect towards plant pathogens were found out for representatives of genera Bacillus and Pseudomonas [4,11], while in the case of fungi such properties are characteristic of Gliocladium spp. and Trichoderma spp. [10,14,17,21,23]. The enumerated genera of antagonistic bacteria and fungi are distinguished by the ability to inhibit the growth and development of pathogens and to shorten their viability in soil considerably through competition, antybiosis and parasitism [1,5,12,13,21,24].

The purpose of the studies was to establish the qualitative and quantitative composition of the communities of fungi and bacteria formed in the soil under the effect of onion cultivation, considering different variants of conservation tillage with spring rye and common vetch as aftercrop cover plants. Besides, on the basis of laboratory tests the occurrence of microorganisms distinguished by their antagonistic effect towards the facultative plant pathogens living in the soil was established.

MATERIAL AND METHODS

The object of the studies were the soil samples taken from the depth of 5-6 cm of the plough layer of the field where ‘Wolska’ cv. onion was cultivated with such cover plants as spring rye and common vetch. Those plants, sown in summer aftercrops, created before winter an abundant yield of the green mass, forming the natural mulch on the surface, which was managed in different ways: 1) leaving the mulch in the field and direct onion sowing without any pre-sowing cultivation; 2) disc harrowing before winter and in spring; 3) disc harrowing in spring. The purpose of the autumn or spring disc harrowing was to mix the mulch with the soil. The control was the soil from the field without any cover plants in the traditional cultivation, considering pre-winter ploughing and spring pre-sowing cultivation treatments. The manner of sampling the soil and the microbiological analysis were according to the method described by Pięta and Patkowska [26] and Martyniuk et al. [19]. With the aim of obtaining the total numbers of cfu of bacteria in the soil under onion cultivation, PDA medium was used with an addition of yeast extract and dilutions of the soil solution 10^-5, 10^-6, 10^-7. In order to isolate Bacillus spp., Triptic Soy Agar and the dilutions 10^-4, 10^-5, 10^-6 were used. On the other hand, medium Pseudomonas Agar F and water dilution of the soil solution 10^-2, 10^-3, 10^-4 were used for Pseudomonas spp. Martin’s medium [18] and dilutions of the soil solution 10^-2, 10^-3, 10^-4 were used to calculate the total number of cfu of fungi occurring in the soil.

Among the isolated bacteria colonies, 500 isolates of each Bacillus spp. and Pseudomonas spp. were used to study their antagonistic effect towards Altenaria alternata, Botrytis cinerea, Fusarium culmorum, F. oxysporum, F. solani, Pythium irregulare, Phoma exigua, Rhizoctonia solani and Sclerotinia sclerotiorum. All isolates of Gliocladium spp. and Trichoderma spp. isolated from the soil were tested towards the same pathogens. The antagonistic effect of the tested bacteria was established considering the method of Martyniuk et al. [19], while for fungi the biotic tests were used as described by Mańka and Mańka [15].

The results concerning the numbers of bacteria and fungi were statistically analized and the significance of differences was established on the basis of Tukey’s confidence intervals [22].

RESULTS AND DISCUSSION

As a result of the laboratory microbiological analysis it was found out that particular soil samples taken from under the cultivation of onion differed with the qualitative and quantitative composition of microorganisms (tab. 1).In the experimental combination with the use of spring rye as the cover plant, with different ways of managing it, it was observed that the mean total number of cfu of bacteria was similar (from 3.36 mln to 4.09 mln) to the number of bacteria in the combination with common vetch (from 3.4 mln to 3.65 mln in 1 g d.w. of soil) (tab. 1). According to numerous researchers, grain crops, including rye, stimulated the growth and development of bacteria [6,28,29,38]. The mean number of cfu of Bacillus spp. and Pseudomonas spp. in the soil from under onion cultivation, considering both spring rye and common vetch increased as compared to the traditional cultivation. The use of cover plants in the technology of onion cultivation was conducive to the growth of the biological activity of the soil. Leaving a mulch of spring rye on the soil surface had an especially positive effect on the increase of the number of cfu of Pseudomonas spp. (tab. 1). Similar results concerning Pseudomonas spp. were obtained by Elliott and Lynch [6]. According to them, the discussed genus of bacteria was represented in the soil after ploughing in the straw, while being only scarce after burning the straw.

The mean total number of cfu of fungi from three years of studies ranged from 38.77 thousand to 60.83 thousand in 1 g d.w. of the soil (tab. 1). The number of fungi in the soil from under onion cultivation was much smaller in the case when spring rye was used as a cover plant in comparison to common vetch.

The most frequently obtained species among the fungi isolated from particular soil samples was Fusarium oxysporum (tab. 2). Besides, Penicillium verrucosum var. cyclopium was often isolated. On the basis of earlier studies, the enumerated species were considered as pathogenic towards onion in varying degrees [31]. Fusarium culmorum, F. solani and Pythium irregulare should be included within the fungi occurring in the studied soil samples and infecting onion. Out of the analyzed soil samples, and especially those sampled from the plots of the experimental combination with spring rye. Trichoderma spp. was much more often obtained than from the experimental combination with common vetch. A spontaneous growth of the population of those fungi was probably caused by a big amount of cellulose from rye. The studies of Bujak and Targoński [2,3] showed that both grain crops for green fertilizer and their straw stimulated the growth of Trichoderma spp. It should be supposed that common vetch as a papilionaceous plant constitutes a rich source of nutrients also for plant pathogens. According to Pięta and Patkowska [27] and Pięta et al. [30], plants from this family exudate a lot of acidic aminoacids stimulating the growth and development of pathogenic fungi. The familiar cellulytic and chitinolytic properties of Trichoderma spp. and their generally known antagonistic effect towards others, for example towards the pathogenic representatives of Mycobionta, can significantly affect the reduction of the population of plants pathogens to onion [23,26].

At present attempts to induce antagonistic microorganisms such as Bacillus spp., Pseudomonas spp., Gliocladium spp., Trichoderma spp. to the soil are made through seed dressing [7,16,20,32].

Tests determining the antagonistic effect of both bacteria and fungi showed that there were much more antagonists in the soil after the use of spring rye than in the case of common vetch (tab. 3). It can be stated on the basis of the obtained results and information in literature that rye has a positive effect on the growth and development of antagonistic bacteria and fungi. A big number of propagation units of Bacillus spp., Pseudomonas spp., Gliocladium spp. and Trichoderma spp. distinguished by their antagonistic effect towards soil-borne pathogenic fungi become a barrier making the development of plant pathogens and plant infection impossible. Besides, antagonistic microorganisms cause a decrease or complete loss of vitality of survival forms of fungi [36,37].

The composition of the populations of microorganisms obtained from the soil of various experimental combination considering spring rye and common vetch as cover plants in onion cultivation varied. Therefore, it can be stated that rye and common vetch had a modifying effect on the formation of those communities.

CONCLUSIONS

1. The use of spring rye and common vetch as cover plants in onion cultivation influenced the differentiated qualitative and quantitative composition of microorganism populations.

2. Spring rye had a more positive effect on the growth and development of microorganisms than common vetch.

3. After spring rye as cover plant the population of Trichoderma spp. in the soil environment increased considerably.

4. Much more Fusarium spp. was found in the soil after common vetch than after spring rye.

5. The manner of managing cover plants had no significant effect on microorganism communities.

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The studies were financed by the Ministry of Science and Informatization within grant No. 2P06R 060 26.

Accepted for print: 16.01.2007

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