INTRODUCTION
Serious environmental problem worldwide is the trend towards reducing the quality of soils and their natural fertility. In the modern agricultural systems, subject to the principles of sustainable agriculture paying increased attention to the quality of soil as a natural resource and a major production area (2, 5). Indicative of soil fertility are organic matter content and stocks of nutrients for plants. The fertility depends on many factors, but the main determinant of its creation, preservation and enhancement are soil microorganisms. Soil microcenoses are important structural element of each ecosystem. The quantity of soil microorganisms of different ecologo – throphic groups is an important biological characteristic of soils and constitutes an integral part of modern environmental studies. Various basic biological processes related to the transformation of nutrients and important for soil fertility are depending on the activity and the normal functioning of micro-cenoses (1, 6, 7). In the specific physicochemical properties of a soil are formed a specific microbial communities and the ongoing processes are depending on the quantities of numerous trophic groups of microorganisms involve with similar physiological functions and performing the same biochemical processes in the soil. The contemporary understanding is that the microbiological indicators are more informative about changes in the quality of soil from the agrochemical indicators, which are more stable and remain longer, unchanged. When building systems for diagnosis and prognosis of changes in the quality of the soil, for the essential criterion has been recomended microbiological component (2, 8, 6, 9)
The objective of this study was to characterize and assessed by quantitative microbiological parameters the present biological condition of some of major soil types occurring in Bulgaria
MATERIALS AND METHODS
The present work covered the results of the analyses of 70 samples collected in the period 2001 — 2012 from 30 sites all over the country. The soils presented belonged to eutric vertisols (Northern and Southern Bulgaria); chromic luvisols in various stage of erosion (Sredna Gora, Rhodopes and Middle Struma – Ossogovo Provinces); eroded humic cambisols (Central and Western
Rhodopes, Belassitsa Mountain,); gleyic colluviosols and rendzinas (the Rhodopes Yaka in the Northern Chernatitsa Ridge); eutric fluvisols and phaeozems (Plovdiv Region); eroded and gleyic haplic chernozems. The determination of the types of soils have been made according to Koinov at al. (4). Samples were collected in spring and summer from the humus horizon at depth of 0 — 20cm, from arable and non-arable areas, under different plant cover (mainly tobacco). Samples were processed according to the standard techniques and subjected to conventional chemical and microbiological analyses (viable counts in various specific media). The set of microbiological parameters was designed to comprehensively characterize the biological condition of the soil through microbial communities structure, trophic groups diversity, and their potential to perform the major transformations of carbon and nitrogen compounds in the soil. The microbiological analyses were executed according to Koh’s method – culture of diluted soil suspensions on specific for every trophic group microorganisms nutrient media, in three replications (3). The most probable number technique is used to estimate microbial population sizes per gram absolute dry soil (MPN/g a.d.s), with confidence level 0.05. The microorganisms of the following trophic groups were counted: autochtons — on soil extract agar and oligotrophs — on diluted soil extract agar (connected to the humus complex); carbophils — on meat–peptone stock agar; aerobic nitrogen fixing from genus Azotobacter — on Ashby agar; mineral nitrogen consuming organotrophs — on starch–ammonium agar, Actinomycets — on starch–ammonium agar and microscopic fungi — on Chapek agar were also estimated. Two structural indices were calculated – oligotrophy index and nitrogen immobilization-mineralization index, which enlighten the accessibility of nutrients and the direction of nitrogen transformations in soil.
Agrochemical analysis of soil samples have been made – total organic carbon % (by Turin); total nitrogen content % (by Kjeldahl) and soil reaction (by potentiometry).
The statistical analysis of the data has been carried out with SPSS Statistics, using multivariate Spearman’s rank correlation and cluster-analyses, using weighted pair-group average approach with Euclidian distances.
RESULTS AND DISCUSSION
Clustering of the objects studied by similarity based on abiotic parameters (altitude, content of total organic carbon, total nitrogen; soil reaction) was not revealed, including of soil type (fig.1)
Fig. 1. Grouping the soils of abiotic parameters
The soil samples were collected from cultivated and uncultivated areas. Crops of arable land were mainly tobacco, tomato and sunflower. The vegetation cover on uncultivated land was mainly natural grass, and forest at soil type — humic cambisols.
The average data for the content of total organic carbon and total nitrogen have been showed trends to decrease at all surveyed soil types in arable land.
Not observed trend in changes in the values of soil reaction (Table 1). The impact of anthropogenic factor is with strength 25.34% on the content of total organic carbon and with 20.22% on the content of total nitrogen, but not statistically significant (Fexp.= 4.41 and F exp. = 3.30 ˂ F tab. 95% = 4.67).
Table 1.
Average data of the agrochemical indicators
|
Soil type |
vegetation |
total organic carbon, content (%) | total nitrogen content (%) | pH
(H20) |
| Eutric vertisols- arable land | tobacco | 3.40 | 0.188 | 7.26 |
| Eutric vertisols – non-arable | natural grassy | 3.94 | 0.162 | 6.50 |
| Chromic luviols — arable land | tobacco | 1.49 | 0.101 | 6.85 |
| Chromic luviols — non-arable | natural grassy | 5.55 | 0.2795 | 7.81 |
| Humic cambisols — arable land | potatoes | 4.06 | 0.210 | 6.76 |
| Humic cambisols — non-arable | forest /natural grassy | 5.52 | 0.353 | 6.72 |
| Gleyic colluviosols — arable land | tobacco | 2.01 | 0.123 | 7.52 |
| Gleyic colluviosols non-arable | natural grassy | 2.62 | 0.154 | 7.72 |
| Rendzinas- arable land | tobacco | 1.51 | 0.1845 | 7.7 |
| Rendzinas — non-arable | natural grassy | 3.41 | 0.213 | 7.92 |
| Eutric fluvisols — arable land | tobacco | 1.75 | 0.101 | 7.22 |
| Eutric fluvisols — non-arable | natural grassy | 2.48 | 0.143 | 7.81 |
| Gleyic phaeozems — arable land | sunflower | 4.6 | 0.250 | 7.87 |
| Gleyic phaeozems — non-arable | natural grassy | 5.32 | 0.432 | 8.18 |
The average values of the microbiological parameters separating investigated soil types in three groups: Gleyic colluviosols and Rendzinas were densely populated; Humic cambisols and Eutric fluvisols were moderately populated, followed by Chromic luvisols; Gleyic phaeozems and Eutric vertisols were sparsely populated (fig. 2).
Fig. 2. Average population density in the microbial communities in the different soil types –MPN/g a.d.s.
In thus grouped soils were observed some differences between the distribution of the different trophic groups microorganisms into the microbial communities (table 2). At all soil types with the highest proportion are authochtones and oligotrophic microorganisms, which is normal for soil micro-cenoses. Autochtones there are preponderance in soils with the highest population density (Gleyic colluviosols and Rendzinas). There was an increase in the proportion of assimilating mineral nitrogen microorganisms and disbalance in the biological equilibrium between them and carbophils microorganisms, i.e. in those soils the processes to immobilization of nitrogen are intensified. Numeric expression violations in nitrogen balance are high levels of mineralization-immobilization index. At the soil type with the lowest microbial population density — Gleyic phaeozems are disturbed the equilibrium ratios between autochtones/ oligotrophic microorganisms and between carbophils/ assimilating mineral nitrogen also. This is an indication for reducing the content of easily digestible nutrients increased level of destruction and accelerated immobilization processes. With this type of soil is registered the highest relative gave to microscopic fungi. The share of actinomycetes is relatively high, which is also an indicator of depletion of easily degradable and rapid mineralization of organic compounds hard degradable. In microbial communities at another soil type with low population density — Eutric vertisols the shares of groups of microorganisms show prevailing processes of nitrogen immobilization. Very high percentage of oligotrophic microorganisms was reported in microbial communities formed under soil type — Eutric fluvisols and also increase to some extent at soil type Chromic luviols which is an indication for the reduction of organic matter and destructive changes in these soils types. The shares of the other trophic groups of microorganisms at the different soil types vary. Actinomycetes were with highest levels in soils types — Gleyic colluviosols, Chromic luviols and Eutric vertisols, and with lowest in soil type — Humic cambisols. Bacteria aerobic nitrogen-fixing of the genus Azotobacter (Az. chroococcum) were with highest share in soils types — Chromic luviols, Humic cambisols and Gleyic colluviosols. They were with lowest in soil type — Eutric vertisols.
Table 2.
Relative shares of trophic groups of microorganisms (%) in microbial communities and values of microbiological indices (Au –autochtons; Ol – oligotrophs; Mf – microscopic fungi; Ac – Actinomycetes; Car- carbophils; AmN – assimilating mineral nitrogen; Az – Azotobacter)
| soil type | Relative share of trophic groups of microorganisms (%) | Microbiological indices | |||||||
| Au | Ol | Mf | Ac | Car | AmN | Az | OI | MII | |
| Chromic luviols | 24.54 | 23.39 | 0.015 | 4.24 | 16.79 | 11.29 | 6.22 | 1.50 | 0.67 |
| Humic cambisols | 44.49 | 29.37 | 0.013 | 0.88 | 15.33 | 5.46 | 4.46 | 0.66 | 0.36 |
| Gleyic colluviosols | 50.34 | 20.03 | 0.009 | 4.94 | 4.40 | 15.91 | 4.36 | 0.40 | 3.62 |
| Rendzinas | 51.06 | 18.18 | 0.005 | 1.70 | 19.33 | 7.39 | 2.33 | 0.36 | 0.38 |
| Eutric fluvisols | 16.79 | 48.69 | 0.007 | 1.53 | 20.86 | 9.39 | 2.72 | 2.90 | 0.45 |
| Gleyic phaeozems | 35.36 | 40.58 | 0.093 | 3.47 | 7.29 | 11.76 | 1.45 | 1.15 | 1.61 |
| Eutric vertisols | 26.06 | 14.30 | 0.006 | 1.87 | 13.15 | 43.71 | 0.90 | 0.55 | 3.32 |
Table 3.
Quantities of of investigated trophic groups of microorganisms (MPN/g a.d.s) depending on the condition of the soils areas — arable or non-arable ((Au –autochtons; Ol – oligotrophs; Mf – microscopic fungi; Ac – Actinomycetes; Car- carbophils; AmN – assimilating mineral nitrogen; Az – Azotobacter)
Variations in the quantities of the different trophic groups of microorganisms depending on the condition of the soils areas — arable or not arable have been observed without statistically proven (table 3). Generally, in all soil types, except Chromic luvisols, the average population density in the microbial communities is higher in not arable areas. At soil type Chromic luvisols the trend in the studied trophic groups of microorganisms was to increase their number, with the exception of microscopic fungi in arable areas. At Humic cambisols only the numbers of Az. chroococcum were higher in arable areas, . The quantities of groups involved with the transformations of nitrogen and especially assimilating mineral nitrogen, and the microscopic fungi were higher in cultivated areas at soil type Gleyic colluviosols. At Rendzinas number of actinomycetes and microscopic fungi were more in arable, also.
Detected is a statistically significant relationship between abiotic and microbiological parameters (Rho 0.370 at a level of significance 0.1%) which confirms the potential of the test-microbiological parameters in determining the properties of the soils and the direction of flowing of the processes there (fig.3).
Fig. 3. Multivariante Spearman’s rank correlation between abiotic and quantities of throphic groups of soil microorganisms
CONCLUSIONS
- The results divided studied soil types, depending on the population density in microbial communities into three groups: Gleyic colluviosols and Rendzinas were densely populated; Humic cambisols and Eutric fluvisols were moderately populated, followed by Chromic luviols; Gleyic phaeozems and Eutric vertisols were sparsely populated.
- Differences between arable and non-arable areas have been established. In cultivated areas in Gleyic colluviosols, Rendzinas, Chromic luviols and Eutric fluvisols have been observed reduction of organic matter and strengthened immobilization processes
- Detected is a statistically significant relationship between abiotic and microbiological parameters in soil. The results obtained can be used for diagnosis of the soil quality.
REFERENCES
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- Doran J. W., Sarrantonio M., Janke R. Strategies to Promote soil quality and health, In: Mangement in Sustainable Farming Systems (ed. Pankhurst, Double, Gupta, Grace), Soil biota, CSIRO, East Melbourn, 1994: 230-247
- Koleshko O. I. Ecology of soil microorganisms Handbook, University Press., Minskр 1991: 19-136
- Koinov V., Kabaktchiev I., Boneva K. Atlas of soils in Bulgaria. press: Institute of Soil Science “N. Pushkarov” (Agricultural Academy), 1998, Sofia, ISBN 954-05-0116-41
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- Santos VB, Araujo SF, Leite LF, Nunes LA, Melo JW. Soil microbial biomass and organic matter fractions during transition from conventional to organic farming systems. Geoderma. 2012 ,170:227–231.
- Sessitsch A., B. Mitter. 21st century agriculture: integration of plant microbiomes for improved crop production and food security. Microbial Biotechnology. V.8, issue 1, 2015: 32-33
- Singh JS, Pandey VC, Singh DP. Efficient soil microorganisms: a new dimension for sustainable agriculture andenvironmental development. Agric Ecosyst Environ. 2011;140:339–353
- Yakovchenko V., Lawrence J. S., Kaufman D. D. A biologically based indicator of soil quality. Biology and Fertility of Soils, vol. 21, 4, 1996: 245-251[schema type=»book» name=»MICROBIOLOGICAL CHARACTERISTICS OF CERTAIN MAJOR TYPES OF SOIL IN BULGARIA» description=»The objective of this study was to characterize and assessed by quantitative microbiological parameters the present biological condition of some of major soil types occurring in Bulgaria. The work summarizes the results of the analyses of 70 samples collected in the period 2001-2012 from 30 sites all over the country. The soils presented belonged to: chromic luvisols in various stage of erosion; eroded humic cambisols; gleyic colluviosols; rendzinas; eutric fluvisols; gleyic phaeozems and eutric vertisols. Samples were collected in spring and summer from the humus horizon at depth of 0 -20 cm, from arable and non-arable areas, under different plant cover. Samples were processed according to the standard techniques and subjected to conventional chemical and microbiological analyses. The set of microbiological parameters was designed to comprehensively characterize the biological condition of the soil through microbial communities structure, trophic groups diversity, and their potential to perform the major transformations of carbon and nitrogen compounds in the soil. The results divided the studied soils into three groups: Gleyic colluviosols and Rendzinas were densely populated; Humic cambisols and Eutric fluvisols were moderately populated, followed by Chromic luviols; Gleyic phaeozems and Eutric vertisols were sparsely populated. Differences between arable and non-arable areas have been established. The results obtained can be used for diagnosis of the soil quality.» author=»Tsveta Hristeva, Svetlana Naumova» publisher=»Басаранович Екатерина» pubdate=»2016-12-06″ edition=»euroasia-science_30_22.09.2016″ ebook=»yes» ]