Types of agrobiological development in the permafrost regions

Geocryological research in the course of development of agriculture is becoming more and more important. It is associated not only with the permafrost regions but also with those with deep seasonal freezing: the non-fertile zone of European Russia, Western Siberia, the Chitinskaya Oblast' the BAM zone, etc. For the present the problem of using only small areas for agriculture is considered here. One of the reasons for poor harvests as well as for their unreliability lies in inadequate warming of the ground.

According to A.M. Shul'gin the main critical thermal conditions of soils are the following: 1) the sum of active temperatures; 2) minimum temperatures of the soil; 3) depths of seasonal freezing and thawing; 4) duration of seasonal freezing and of thawing. The sum of active temperatures of a soil at the depth of 10-20 cm is the main factor responsible for development of vegetation. By this term is meant the sum of mean daily temperatures over the period when the soil temperatures are above 10°C. Knowing the sum of active temperatures of the soil one can establish the possibility of development of one or another kind of agriculture. As seen from the diagram (Fig.

17.14) the sum of active temperatures at the depth of 20 cm is 1200-2000°C over the considerable area constituting the zone of risk for agriculture. According to the values of minimum soil temperatures, of seasonal freezing depths and duration, the greater part of agricultural lands in the former

Fig. 17.12. Formation of thaw subsidence cone as a result of thawing of permafrost around an operating gas well (according to data from field observations).

USSR lie in the zone of risk agriculture where it is necessary to solve the problems of improving the climate of the soil and carrying out thermal reclamation.

The main problems of soil thermal reclamation are the following: a) control of deep winter freezing of a soil in the cold period; b) inducing rapid heating of the ground in the warm period. In addition it is necessary to provide the most favourable temperature regime for plants in winter time in the region of winter crop development. At a temperature higher than -3 °C at 5 cm depth, rotting and damage of winter plants take place, and at temperatures below — 15°C freezing of these plants occurs.

Prevention of deep soil freezing is carried out by snow retention and snow accumulation, with arrangement of shelter belts (protection forests) which can lead to increase of snow thickness on fields of up to 45-50 cm compared to 20 cm within open areas, or with the help of shelter belt farming (measures based on snow retention using belts of plants that do not fall in winter such as sunflower, corn and buckwheat). Snow ploughing is often used for this purpose. When snow ploughing is carried out the snow is pushed up with ploughs into banks of 0.5-0.6m height, spaced at 10-15m intervals from each other and oriented at right angles to the wind. Autumn soil ripping contributes to increase of evaporation surface, causes soil drainage, decrease of thermal conductivity, creation of supplementary thermal-insulational cover and decrease of thickness of the freezing layer.

Fig. 17.13. Structure of an operating gas well with thermal insulation of well bore: 1 - guide pipe; 2-3 - jigs (2 - outside; 3 - inside); 4 - thermally insulating air space; 5 - circulation hole; 6 - cement stone; 7 - operating casing string; 8 - casing string; 9 - perforation interval.

Within the zones of excess moisture the snow-removal method, i.e. removal of snow from fields with snow ploughs and cultivators, is widely used in the spring period. When the snow is removed the mean daily air temperature should be higher than the temperature of the upper horizon of a soil. Adequate removal of the snow, in European Russia, advances the beginning of thawing by 10-12 days and reduces the duration of the seasonal freezing of the ground by 7-10 days.

Rolling of soils in the spring period causes consolidation of the upper layer of the soil and increase of thermal conductivity, contributing to better warming and more intensive thawing. Observations show that rolling of soil can raise its temperature by 4.5 °C at the depth of 20 cm during the first

Fig. 17.14. Sum of active soil temperatures (°C) at depth of 20cm (after V.N.

Dimo): 1 - <400°C; 2 - 400 to 1200°C; 3 - 1200 to 2000°C; 4 - 2000 to 2800°C;

5 - 2800 to 4400°C; 6 - 4400 to 5200°C; 7 - 5200 to 6800°C.

Fig. 17.14. Sum of active soil temperatures (°C) at depth of 20cm (after V.N.

Dimo): 1 - <400°C; 2 - 400 to 1200°C; 3 - 1200 to 2000°C; 4 - 2000 to 2800°C;

5 - 2800 to 4400°C; 6 - 4400 to 5200°C; 7 - 5200 to 6800°C.

summer months. This method is most efficient with dry soils and less efficient for moist ones. The good point of this method is the fact that the rolling has an anti-erosion effect and causes moisture retention as well. Mulching of surface (both complete and interrow covering of the soil with sawdust, straw, peat, manure, soot as well as with paper, emulsified asphalt, etc.) is performed in order to raise or to lower the soil temperature.

In the Nechernozyomnaya (non-fertile) zone, ridge and row sowings are used for fast soil heating. Ridges and rows increase the surface of heating by 20-25%, raising summer temperature on rows by 2-5 °C compared to a horizontal surface.

In northern regions where the common methods for increasing the provision of heat do not perform very well, covering of the soils with translucent polyethylene and nylon films is used. These films allow the passage of shortwave solar radiation and hold back up to 90% of longwave radiation from the soil surface, retarding heat and moisture exchange, which results in the warming-up of the surface under these films by 7-10°C (compared with open areas) and in thawing depth increasing. In the north the methods of thermal hydroreclamation, in particular, basin irrigation, are also used. In this case late in May to early in June, the fields are flooded with a water layer 25-40 cm thick for 6-10 days. After water removal the active temperatures (higher than 10°C) penetrate into the soil 20-25 days earlier and 15-20 cm deeper than under natural conditions.

A group of Magadan scientists under the direction of S.V. Tomirdiaro has developed recommendations for the creation of meadows on bottoms of drained thermokarst lakes. Investigations have shown that bottom deposits of thermokarst lakes are of high productivity. The main point of the method consists of artificial drying of thermokarst lakes, the creation of a drainage system for some dewatering of bottom deposits and in sowing crop grasses. With the observance of proper rules for the usage of such meadows one can provide heavy grass productivity as a feeding base for cattle breeding for 5-6 years, and more seldom for 10 years, and then grass vegetation is replaced by moss-lichen and shrub.

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