Abstract: An increasing interest has emerged with respect to the importance of microbial diversity in soil habitats. The extent of the diversity of microorganisms in soil is seen to be critical to the maintenance of soil health and quality, as a wide range of microorganisms is involved in important soil functions. Since, the first estimate of prokaryotic abundance in soil was published, researchers have attempted to assess the abundance and distribution of species and relate this information on community structure to ecosystem function. Present study has investigated the linkage of specific organisms to ecosystem function and an increasing interest has emerged with respect to the importance of microbial diversity in soil habitats. The two main drivers of soil microbial community structure, i.e., plant type and soil type, are thought to exert their function in a complex manner. This review focuses on recent data relating how plant type, soil type, affects the microbial diversity and abundance of soil. Statistical analyses of the microbial counts indicated a significant correlation for bacteria (p<0.01) and no significant correlation, for fungi and actinomycetes, however, microbial enumeration indicated that bacteria were most numerous followed by actinomycetes and fungi, respectively.
INTRODUCTION
Knowledge of microbial diversity and function in soils is limited because of the taxonomic and methodological limitations associated with studying these organisms (Kirk et al., 2004). Soil microorganisms are vital for the continuing cycling of nutrient and for driving above ground ecosystems. It is important to study microbial diversity not only for basic scientific research, but also to understand the link between diversity and community structure and function. Soil bacteria and fungi play pivotal roles in various biogeochemical cycles (BGC) (Molin and Molin, 1997; Trevors, 1998; Wall and Virginia, 1999). Soil microorganisms also influence above-ground ecosystems by contributing to plant nutrition (George et al., 1995; Timonen et al., 1996), plant health (Srivastava et al., 1996), soil structure (Dodd et al., 2000) and soil fertility (O’Donnell et al., 2001). However, activity and species composition of microbes are generally influenced by many factors including physic-chemical properties of the soil, temperature and vegetation. The dynamics of soil microorganisms have important implications for the response of subsurface soil ecosystems to perturbations. Despite all attempts to measure fluxes and gross microbial pools, the soil and its microbiota still remain a black box. Most soil microorganisms are still unknown (Crecchio et al., 2004). The comparison between direct microscopic counts and plate counts indicates that as less than 0.1% of agricultural soil microorganisms are cultivable (Atlas and Bartha, 1998). Understanding the diversity and dynamic of indigenous microbial populations represents one challenge of modern soil ecology. In this context, we undertook a study to evaluate bacterial community structures and diversities located in different ecosystems and tilts correlation with the soil and plant type.
MATERIALS AND METHODS
Study site and species studied: The study was conducted in three field sites (Table 1) located on the same geographic area of Mascara (Northern-Algerian West, 2°,11' W, 35°,26' N). These fields have not the same soil type and topographical features and have received different soil management practices and cropping systems. For each site a farming profile per species was realized.
Total microbial count: Soil microorganisms were extracted by shaking 10 g of soil in 100 mL of one-quarter strength Ringer solution (Oxoid). Data from triplicate readings were expressed as Colony Forming Units (CFU) g-1 dry soil. The total number of bacteria was determined on meat-peptone agar with beef extract and glucose (MPA), actinomycetes-on glycerine-glycine medium, microscopic filamentous fungi-on Martin’s medium with Bengal rose and streptomycin sulfate.
Soil physical and chemical analysis: Soil samples were taken from different depths (5-10, 10-15 and 15-20 cm) and collected randomly from 27 different sites. The samples were analyzed for pH, soil chemistry and texture.
Statistical analysis: All samples (n = 27) were analysed in triplicate. Pearson’s correlation coefficients using a traditional Euclidean distance (Legendre and Legendre, 1998) were conducted on the soil physical and chemical variables, as well as on the vegetation cover and microbial properties analysed to determine how these variables were interrelated. All data processing the Pearson’s correlation coefficients were performed by STATISTICA 7.
RESULTS AND DISCUSSION
Microbiological results: Marked effects were found to have taken place on the bacterial populations under different ecosystems. This is clearly demonstrated by the total number of bacteria CFU recorded from the plates. Our results showed that microbial population was different in soils under different plant covers, soil types and depths (Fig. 1). The total number of isolated bacteria varied in different samples of studied soils, a maximum of 1,8. The 1011 CFU g-1 was signalled in the Lens sp., rhizosphere (0-5 cm) and a minimum of 2,7 109 CFU g-1 in Vicia sp., rhizosphere (5-10 and 10-20 cm). Ecological characterization of studied soil is shown in Table 1.
Soil types had influence on soil microbial organisms’ activities. The highest density of fungi was observed in Triticum sp., rhizosphere (0-5 cm) (1, 35. 109 CFU g-1) and the lowest one in Vicia sp., rhizosphere (1, 8.109 CFU g-1) (10-20 cm). The actinomycetes showed a maximum of 1, 98. 1010 CFU g-1 in the Triticum sp., rhizosphere (5-10 cm) and a minimum of 1,8. 109 CFU g-1 in Vicia sp., rhizosphere (10-20 cm).
| Table 1: | Ecological characterization of studied soil |
In the forest ecosystem, bacteria were abundant in all the rhizosphere of Pinus sp., Asphodelus sp. and Tamarix sp. with values of 2,7.109 CFU g-1. The highest density of fungi (1, 35. 109 CFU g-1) was signalled in all of the horizons analysed of Pinus sp. The actinomycetes were present with a maximum of 3,5. 109 CFU g-1 in the rhizosphere of Tamarix sp., (0-5 cm) but totally absent in Asphodelus sp. Among the soils prospected, microbial enumeration indicated a higher abundance of bacteria in the humid ecosystem than in agricultural and forest ecosystem, with the predominance of the Pseudomonas and Bacillus species. They are the most diverse and ecologically significant group of bacteria on the rhizosphere (Dommergues and Mangenot, 1970).
Physicochemical results: Soil physicochemical properties are presented in Fig. 2. Soils were fine textured (Sand:22.42; Silt:65.57; Clay:38.44) with neutral pH:7.9, CaCO3:19.72, electrical conductivity (177.97) and water holding capacity (15.08) and organic matter (8.01).
Statistical results: A negative and significant (p<0.005) relationship between bacterial count, pH and clay was observed (r = -0.59, r = -0.50), respectively but a positive relationship for the other soils properties however, no significant (p>0.005) relationship between fungi, actinomycetes and all the soils properties (Fig. 3).
The results of soil physicochemical properties varied depending on the ecosystem type. The dominating textural class is that of clay and silty clay (44%). On the basis of our results it appeared that the presences of bacteria was important in the silty clay soils of the humid station.
Soil texture affects the microbial activities (Hassink, 1994; Sorensen and Jensen, 1995). Based on the results, it appears that microbial biomass is influenced by soil texture. These data are in agreement with previous results.
| Fig. 1(a-c): | Numbers of cultivable bacteria, fungi and actinomycetes as
measured by plate spreading |
| Fig. 2(a-c): | Box plot of Soil (a) Physical and (b,c) Chemical properties |
| Fig. 3(a-f): | Significant correlation between of bacteria abundance and
soil properties |
Fine-textured soils typically contain greater quantities of organic matter and microbial biomass than coarse-textured soils. Bonneau and Souchier (1994), demonstrated that this fine texture promoted the bacterial growth (i.e., clay humic complex). Clay-sized particles are thought to protect organic matter through adsorption and aggregation, shelter soil microorganisms from predation (Elliott et al., 1980). According to Alvarez et al. (2002), the fine-textured soil (<50 microns) have a protective effect on total microbial biomass, due to the higher proportion of micropores compared to sandy soil which limited meso fauna development. Fine-textured soils typically contain greater quantities of organic matter and microbial biomass than coarse-textured soils (Schimel, 1986). Clay-sized particles are thought to protect organic matter through adsorption and aggregation and increase substrate-use efficiency (Martin et al., 1976).
This is consistent with the correlation tests between bacterial abundance and these fractions where correlation was significant (p<0.005) and no significant for fungi and actinomycetes (p>0.05). For fungi and actinomycetes this fine texture had not affects on their abundance. Insam et al. (1989) found a significant but weak, correlation (R2 = 0.02) between soil texture (e.g., clay content) and microbial biomass in temperate agricultural soils. They used two measures of soil texture (silt+clay and clay content), but found no relationship between those variables and microbial C. Although, several studies have demonstrated that soil texture has a significant influence on soil microbial biomass (Van-Veen et al., 1985), this relationship may only be of importance within a particular climatic regime.
Bacterial abundance displayed a positive linear relationship (p = 0.005) with soil pH, Bardgett and Leemans (1995) recognized that a reduction in the pH from 5.4-4.7 occurred following the interruption of any fertilization led to a decrease of 18% of the microbial biomass. The highest values were signaled in agroecosystem and the humid ecosystem soils. However, no correlation (p>0.05) with CaCO3, certain authors recognized that this one had a propriety to replace the hydrogen ions and to exhibit atypical high acidity.
The electrical conductivity occur either naturally or as a result of inappropriate soil use and management and the humid ecosystem station tended to be more saline (1163 μsec cm-1) and the correlation was significant only for bacteria.
Soil organic matter (OM) was higher in the forest ecosystem soils but lowest in the agroecosystem soils. These values can be explained by the fact that any OM fraction incorporated was quickly assimilated by microorganisms present especially by the most pathogens (i.e., presence of competition). It may be pointed out that only amount to a smaller percentage of organic soil C, contributes substantially to plant available nutrient content (Diaz-Ravina et al., 1995). According to diverse authors the ratio of microbial C and extractable C to total organic C are related to substrate quality. As previously observed no significant correlation (p>0.05) between OM and microbial abundance (bacteria: r = -0.08; fungi:r = -0.27; actinomycetes:r = -0.09).
The reduction of OM content observed in the soils could also be a cause of reduced soil enzyme activity (i.e., class of OM not hydrolysable). It has been demonstrated by Namour (1999), that the density of heterotrophic community is slightly correlated with the quantity of the substrate available. These results are in agreement with previous report of Namour (1999), focused on the OM kinetic. As described by Servais (1989), the OM is divided into four classes of decreasing lability:a class directly comparable to effectively control microbial growth, two classes slowly hydrolysable and finally a class not used by microorganisms (not assimilated). Our data indicate that this organic matter in these ecosystems was relatively recalcitrant.
In summary, our results suggest that plant type influences microbial abundance and diversity. Plant type displayed a positive linear relationship (p<0.001) with bacterial abundance (r = 072) but a inverse relationship between fungi and actinomycetes (r = -0, 19, r = -0, 55), respectively, this correlation was significant for fungi (p = 0.001) and not for actinomycetes (p = 0,32). Present study suggests that plant production holds promise in predicting patterns of labile organic matter pools and microbial biomass at much larger spatial scales.
CONCLUSION
The diversity and abundance of soilborne microbes may be strongly influenced by some abiotic and biotic factors; however, few studies have described the diversity and dynamics of soilborne bacteria, fungi and actinomycetes in the region of Mascara (Northern-Algerian West).
To obtain an accurate representation of the function and structure of soils, it is necessary to study the inter-relationship between physical, chemical, biochemical and biological properties. Measurement of only one or some of these properties will give only a partial evaluation of the state of the soil ecosystem. The results showed that field data interpretation of soils properties is difficult, particularly when several factors exerting an influence on microbial communities are involved. Further studies are necessary in order to confirm these preliminary field data.