УДК 631.51:631.453:631.466.1


A.A. Danilova

The phytotoxicity of soil microflora was investigated in leached chernozem of Priob’e during different methods of soil cultivation and against the different backgrounds of mineral nutrition. The author determines a raise of phytotoxycity during long-term (more 20 years) minimization of main cultivation and also its increase against the intensive chemical background.

Key words: soil fungy, phytotoxity, no-tillage, pesticide stress.


Phytotoxicity of soil microbial complex is one of its main ecologically important properties closely linked with decomposition of plant foliage. It has been established, that at initial stages of this process phytotoxicity properties are expressed mainly by fungi (1-4). According to M.V. Gorlenko, phytotoxicity of saprotrophic fungi ensures their advantage in struggle for nutrient substrate (5). Other authors conclude the presence of ongoing mechanisms providing phytotoxins accumulation, destruction and removal from a medium, which operate both in nature and in agricultural phytocenoses (6). In Priob’e region, it has been found, that the prolonged absence of plowing fields (15-20 years or more) leads to yield decrease in spring wheat on stubble background, amounting 10-15% despite the use of chemicals (7). The analysis of literature data (8, 9) suggests that this fact can be explained by presence of microbiological processes associated with long monotonous mineralization of plant foliage in a limited volume of soil. The purpose of our study was to determine the dynamics of soil fungi phytotoxicity against the background of different tillage techniques. Methods. In 2001-2005, in conditions of cereal-fallow field rotation, soil properties were compared during a normal plowing and "no-tillage" treatment at two variants of pesticides application. The extensive variant included applying herbicides against dicotyledonous,  the intensive - two herbicides (against dicotyledonous and cereal weeds), fungicide and, if necessary, an insecticide. The scheme of protective activities was described in detail previously (7). The observations were performed in final and in the first crop of the rotation field during a long-term experiment carried out by Siberia Research and Development Institute of Farming and Chemization of Agriculture (RAAS) since 1981. The annual precipitation in the region of experiment (the northern forest-steppe of the Novosibirsk Priob’ye region)  amounted to 390-450, the sum amount of air temperatures above 10 °C  - about 1800 °C, soil – moderately-deep medium-loamy leached chernozem (black-colored soil) formed on loess loam, keeping humus content of 5,1-5,5%, total nitrogen - 0,34%, metabolizeable potassium and mobile phosphorus - respectively, 13 and 29 mg/100g. To assess the role of plant residues composition and quantity accumulated in soil without plowing in the formation of phytotoxic fungi properties, the degree of its manifestation was determined during 2003-2005 in soil under the annual tilled crop (potatoes), perennial grasses (Bromopsis inermis), birch stands, forb-cereal meadow (virgin soil) and natural deciduous forest (10). The study was carried out in conditions of a long-term experiment started in 1980 in the territory of botanical nursery-field of the Siberia Research and Development Institute of Plant Breeding and Selection RAAS (Novosibirsk region). Upon the common methods for determination the toxigenicity of soil microflora (11, 12), authors have developed and tested a simplified method of determining the phytotoxicity of soil fungal complex (13), allowing to combine the recording of colonies number, initial evaluation of taxonomic composition, and also the establishment of toxigenicity as a whole complex and its individual colonies. For test-plants, authors used seedlings of maize and wheat. Inhibition of root growth was calculated by the formula proposed by O.A. Berestetsky (11):  Аph (%) = 100 - (De - Di/Dc - Di) х 100 , where De, Dc – seedlings root length after 24 h of the experiment (De) and in control (Dc), mm, Di - initial root length, mm. The acceptable level of phytotoxicity calculated under this formula is 30%. Results. The content of mortmass in soil layer 0-10 cm at tilling was 0.93, at "no-tilling" - 1,15 g C/kg soil. The number of fungi primordia was equal to 39-97 and 50-100 thousand CFU/g soil, respectively.  The trend of increase in this index was observed when raising  the pesticide background.

Fig. 1. Inhibition of root growth in test-plants seedlings under the influence of fungi isolated from soil at tillage (a) and "no-tillage" techniques (b) against the background of extensive (A, B) and intensive (C, D) pesticide load.
A, B - final crop of rotation,
C, D - the first crop after fallow;
I, II and III -  May, July and September, respectively.
Control - the test-culture grown in a sterile Chapek medium. Horizontal line shows the acceptable level of phytotoxicity calculated under the formula of O.A. Berestetsky (11).

а б А Б В Г – a b A B C D
abscissa – Month
ordinate – Relative to control, %

The frequency of phytotoxicity occurrence in the studied soil samples was relatively high. The lower rates were observed in spring and then they were increasing by the second half of growing season, which may indicate a less significant contribution of fungi in the suppression of cereals seedlings growth (Fig. 1). However, there’s still an open question about a possible role in this process of mycotoxins preserved in soil from autumn.
At a stubble ground ("no-tilling") the soil layer 0-10 cm, the phytotoxicity of fungal complex was almost 2 times higher than at tilling. The increase in pesticide load was accompanied with raising fungal toxigenicity. For example, in 2001, all five cases of suppressed seedlings growth were revealed in test-plants against the intense chemical background, and four of them – when "no-tilling". In following years, deviations from control were observed in stubble backgrounds 2-3 times more often than at tilling.
It has been well known a high sensitivity of fungi to pesticides. Examination of more than 300 pesticides in various soil tests has revealed, that the smaller doses inhibit fungi more actively and for longer periods compared with bacteria and actinomycetes (14 15). During this process,  fungal phytotoxicity can raise in response to increased influence of adverse environmental factors (16-18).

The simultaneous observations carried out in two fields of crop rotation (first and final crop following fallow), allows to conclude that fallowing leached chernozem in Priob’ye contributes to the significant loss in soil micotoxicity. The high efficiency of fallow in purification of soil from phytotoxic fungi has been also established in other regions (19).
Dynamics of fungal phytotoxicity and methods for combating it were studied mainly in agricultural phytocenoses with high intensity of growing certain crops (6, 9, 11, 12). In these conditions, the main reason for such effect is considered to be the reduced diversity of soil microorganisms as a result of accumulation in soil of plant residues with uniform composition and, consequently, the increased competition for food resources. However, some authors have suggested a wider distribution of phytotoxicity in nature and highlighted the specific facts of its manifestations, particularly in virgin soils (20, 21).

Fig. 2. Inhibition of  germination in wheat grains under the influence of fungi isolated from soil of different phytocoenoses in May (a) and September (b): I, II, III, IV and V – respectively, arable field, perennial grasses,  birch stands, forb-ceral meadow and natural deciduous forest.
а б – a b
abscissa – Phytocoenosis
ordinate – Proportion of non-germinated grains to a total number, %

According to our data, phytotoxicity of soil fungi under herbs (both sown and naturally growing) was significantly higher than under potato. The content of mort-mass in these variants was, respectively, 2,0-5,0 and 0,7-0,9 g C/kg, the number of fungal primordia - about 90 thousand and 50 thousand CFU/g. The seasonal dynamics of the studied indicator was similar to that obtained at comparison of tilling methods against a pesticide load background: toxigenicity of fungi in spring was low and increased in the second half of growing season, obviously due to natural dynamics of plant residues income. These facts repeatedly indicate the close relationship between the raise of fungal phytotoxicity and accumulation in soil of plant residues (Fig. 2).
Thus, the cessation of plowing the leached chernozem in Priob’ye  region for more than 20 years is accompanied by an increase in phytotoxicity of soil fungal complex as a result of the combined effects of natural and human factors - the accumulation of plant residues in a limited volume of soil and the increase of soil fungi toxigenicity under the influence of pesticide stress. In the Priob’ye region, fallowing fields can limit this process.


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Siberia Research and Development Institute of Farming and Chemization of Agriculture, Russian Academy of Agricultural Sciences,
Novosibirsk reg., Krasnoobsk vil. 630501, Russia,
e-mail: hainar@mail.ru

Received April 15, 2008