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<Forum on the Caspian, Aral and Dead
Seas-Perspective Salt-Affected Soils under Large-Scale Irrigation Agriculture in Kazakhstan Takashi Kosaki, Reiji Suzuki, Norio Ishida
Introduction Salt-affected soils in North and Central Asia occupy more than 200 million ha, which corresponds to about 20% of the total area of such soils in the world (Szabolcs, 1986). The Soviet Union started many large-scale irrigation projects for the establishment of rice- and/or cotton-based agriculture in arid regions in Central Asia in the 1960s. It has recently been reported that the introduction of irrigation water into a farm from major rivers drained off to the Aral Sea and Lake Balkhash, causing a decrease in the surface area and depth of these water bodies (Williams and Aladin, 1991), resulting in soil salinization and adverse effects on human health (Mainguet, 1991). Rozanov (1984) showed 1 million ha of land was lost in Central Asia because moderate to severe salinized soil in irrigated areas reached 60 to 70% and crop yields decreased by 30 to 33% in Kazakhstan. This study aims to investigate the distribution pattern of saline soils in a farm and its surroundings in order to understand the mechanism of salinization due to irrigation practices, and to establish an appropriate technology for sustainable agricultural production in arid climates. Kazakhstan can be considered one of the representative places for conducting such a case study, and the outcome may be transferred to other areas having similar constraints in a temperate zone. Study Site And Soil Sampling We set up a study site near Bereke, 250 km north of Alma Ata in Kazakhstan (figure 1). Bereke is at an elevation of 375 m above sea level and located in the flood plain of R. Ili, originating in the Tien-Shan Mountains. Bereke has a precipitation of about 150 mm per annum, total evapotranspiration of about 1000 mm, and an average air temperature of 9°C, ranging from minus 11°C in January to 27°C in July. The study site belongs to a former state farm established in 1979. Farms are mainly planted with rice, alfalfa and barley, besides being utilized for the grazing of sheep and beef cattle. Rice is irrigated and water-logged for four months of the growing season, while alfalfa and barley are not, although irrigation water does run in canals during the growing season for early May to early September every year. The yield of rice is reported to be about 4 t/ha. Individual farms are cultivated with a 4 year rotation, i.e. barley and alfalfa for the first year, alfalfa for the second and rice for the third and fourth. Soils encountered in the farms are Sierozems in the Russian Classification System, correlated with Gypsiorthids and/or Calciorthids in Soil Taxonomy and Gypsisols and/or Calcisols in the FAO/Unesco systems. The study site included about 2 ha of virgin land, 2 ha of farm planted with barley and alfalfa at the time of sampling, and irrigation canals between them (figure. 2). While the farm was leveled, virgin lands have an undulating topography with a difference of 250 cm in elevation from the level of a valley bottom to the top of a sand dune. Zero point of the relative elevation was set on the level of a bridge across the irrigation canal in the study site. The crest parts of dunes are dominated by shrubs of sakusaul (Haloxylon aphyllum), the valley bottom by reed (Polygonum orientale) and hill slopes of sand dunes by halophytic weed such as Karelinea caspia and Atriplex tatarica.
The surface soils were sampled at an average interval of 25 m along four transects, being 50 m apart from each other, both in the virgin land and the farm (figure. 2). Seven soil profiles were prepared from representative sites with respect to the topography of the study site, and soil samples were collected from soil horizons designated in these profiles. Irrigation water was running in the canals in the study site at the time of sampling in late June. Additionally, we investigated some soil profiles and collected samples under natural sakusaul forest some distance from the study site, where we observed no effect of irrigation on the morphology of soils but similar environmental conditions in terms of parent material, climate, topography etc.
The collected soils were air-dried, sieved and analyzed for selected physico-chemical properties such as pH (1:2.5 in water), electrical conductivity (1:5 in water), water soluble cations and anions (1:5 water extraction), CEC (pH 7.0 in ammonium acetate), organic carbon (Tyurin method) and soil texture. Results And Discussion The distribution pattern of electrical conductivity (EC) of surface soils is shown in figure. 2. The soils near irrigation canals in the farm showed high salinity. The relative elevations of the farm and the surface of irrigation water was -330 cm and -230 cm, respectively. Irrigation water at a higher level than the farm suggests that the high salinity was induced by seepage water from irrigation canals. although the surface soils of virgin land generally sowed a similar pattern, their salt contents were also affected by topography. The highest salinity was observed in the mod-slope of high sand dunes as well as in the crest and mid-slope of low to medium sand dunes, which corresponds to areas at a relative elevation between -250 cm and -150 cm. The virgin land generally showed higher salinity than that of the farm. Figure 3 shows the distribution pattern of water-soluble salts in the soil profiles investigated in the study site. The profiles located in the areas showing high salinity in figure 2, i.e. P103 and P104, accumulated high amounts of salts in the surface layers. In contrast, salt accumulated layers were observed in the subsoil of profiles P108 and P105, which were located in the crest part of the high sand dunes and in the area distant from the irrigation scheme and, therefore not affected by irrigation practices, respectively. The distribution pattern of water soluble salts in soil profiles suggests that salts originally accumulated in the subsoil in natural conditions were dissolved with irrigation water, then moved upward with evapotranspiration of soil moisture through capillaries, and finally precipitated on soil surface. This upward movement and precipitation in the soil surface of water soluble salts must continue as far as capillary water is held between the water table and soils surface. Profiles P103 and P104 showed water tables shallow enough to keep the capillaries. In contrast, no water table was observed within 120 cm in Profiles P108 and P105, where capillary fringe never reached the soil surface. Profile P109 showed some salt accumulation in a subsurface horizon, i.e. 20 to 60 cm depth, in spite of the fact that no water table was observed at the time of sampling. This suggests the fluctuation of the water table, which may come up to within a 100 cm depth to carry salts upward during some period in a year. Even so, it should be noted there is a clear difference in EC between the surface layer and subsurface layer. Textural changes may affect the movement of salts. The surface layer of P109 has a coarser texture than the subsurface layer, thus, capillaries may be cut off to avoid salt accumulation on the soil surface. This mechanism possibly works in Profile P110 as well. The low EC of Profile P111 may be due to seasonal leaching, laterally as well as vertically, produced by too high a water table. A similar mechanism seems to take place in the farm. Water-logging for rice cultivation for 2 cropping seasons out of 4 promotes the leaching of salts, resulting in a lower EC level in the farm than in the virgin land in the study site. This effect was also reported as the decrease in alkalinity due to mono-cultivation of rice (Tikov and Gusev, 1989). Table 1. Electrical Conductivity and Cationic Composition of Surface
Water in Selected
Table 2. Composition of Water-Soluble Salts in Salts Accumulation
Horizons in Selected Soil
The quality of surface water standing or running around the study site was summarized in table 1. All cation concentrations were the lowest in Kapuchagai reservoir and the highest in the drainage canals of the study area. This means irrigation water dissolved naturally accumulated salts in the subsoils of the farm and transported some of them to the drainage canals and the rest to the soil surface as precipitates. In other words, precipitated salts on the surface originated from subsoils but not from irrigation water. An analysis of water soluble salts of salt accumulated layers of selected soil profiles (table 2) in the study site and its surroundings indicated that sodium sulfates dominated in irrigated area, both in the farm and virgin land, and gypsum (CaSO4) dominated in non-irrigated areas. These results agree with the fact that sodium ions are more mobile than calcium ions. The dynamics of salts in the surface soils during the rotation system in the study farm was shown in figure 4. Salt contents designated by the sum of anions here were very low under rice cultivation, but started to rise with barley and alfalfa. The rate of salt accumulation is about 24 kmol/ha/year, corresponding to 3t/ha/year of gypsum which is equivalent to 0.2 to 0.3% of the total weight of furrow slices. This rate of accumulation is the first approximation obtained from the observation of one rotation cycle in the study area, which should be up-graded to second and third approximation to predict the amount of salts to be accumulated in future in the environs similar to Bereke. Long-term monitoring of salt dynamics of the representative sites under large-scale irrigation agriculture in Central Asia must be initiated and continued for at least a several cycles of crop rotations. Conclusions 1. Irrigation water has caused and is accelerating soil salinization in the farm and its surroundings in the study site. The extent of salinization depends on topography, the level of water table, soil texture and land use. 2. Salinization resulted from the dissolution of water-soluble salts from subsoils into soil solutions supplied with irrigation water, followed by the transportation of salt containing soil solutions through capillaries and the precipitation of free salts when soil water is evapotranspired. 3. When compared to the virgin land, regular water-logging for rice cultivation washed out the accumulated salts and reduced salinization. Salinization may be enhanced by changes in the method of land preparation, the type and sequence of crop rotations, and the amount, quality and the method of application of irrigation water. 4. The rate of salt accumulation was estimated 24 kmol/ha/year during the rotation system practiced in this study site. References: Khakimov, F.I. 1989: Soil melioration conditions of desertification of deltas. Puschino Moscow, p.218 Mainguet, M. 1991: Desertification, Springer-Verlag, Berlin, p.306 Rozanov, G.G. 1984: Principles of the doctrine on the environment, IGU, Moscow, p.273 Szabolcs, I. 1986: Agronomic and ecological impact of irrigation on soil and water salinity, Advances in Soil Sci., 4, 189-218 Titkov, A.A. and P. G. Gusev, 1989: Composition and properties of soils of rice rotations in the Sivash Region of Crimea, Soviet Soil Sci., 21(4), 110-116 Williams, W.D. and N.V. Aladin, 1991: The Aral Sea; recent limnological changes and their conservation significance, Aquatic conservation: Marine and freshwater ecosystems, 1, 3-23
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