Mechnism Of Water-salt Regulation For The Shelterbelts With Saline Water Irrigation Along The Tarim Desert Highway

Shelterbelts ecological project along the Tarim Desert Highway, which is cross the Taklimakan Desert from north to south. It is the large-scale eco-project by using the local underground saline water with drip irrigation. In the conditions of extreme arid climate, the sustainability of the shelterbelts is confronted with severe tests by a long-time potential threat of soil secondary salinization. In this paper, the Tarim Desert Highway shelterbelts as the study object. to the soil physics, plant ecology as a theoretical guidance, by measuring the woodland soil water-salt characteristics, growth characteristics of salt-tolerant plants, plants and soil water evaporation, soil water and salt balance and so on, by using the combining method of the located monitoring and the plot experiments, it is studied on drip irrigation technology by saline water and the mechanism of water and salt regulation and control of the shelterbelts in the desert area. It is to be provided a basis for sustainable development of the Tarim Desert Highway shelterbelts. The main results are as follows:(1) It is closely related between soil moisture status and irrigation scheduling in Tarim Desert Highway shelterbelt, Soil moisture depends on irrigation system. Under the stable irrigation quota, the dripper discharge and the volume of soil moist is changed larger with time increases. In the horizontal direction, the maximum radius is about 1.4m. And it did not increase with the irrigation amount; in the vertical direction, when the irrigation amount is greater than the water losses by vaporized, the soil moist had a gradual downward trend over time. Temporal dynamics of soil water is a type of irrigation cycle. Under the constant irrigation regime, the shelterbelt's soil moisture and soil water storage all had a decreasing trend by following the increase in irrigation years. According to the vertical variation of soil moisture, soil profiles could be divided into four layers, including water quick change layer (0cm-20cm), water moderate change layer (20cm-60cm), water weak layer (60cm-90cm) and water stable layer (90cm below).(2) Na-chloride is the main soil salinity in Tarim Desert Highway shelterbelt, and soil salt distribution and the cumulative impact of a large degree by the irrigation system. Salt accumulation at the surface soil layer is generally significant. In the deeper sites in the groundwater, Salt distribution in soil profiles in the vertical direction from top to bottom can be divided into three layers:one layer of salt leaching is low and relatively stable. The surface accumulation layer is about 2cm in general; leached layer and the accumulation layer thickness (depth) depended largely on irrigation quota. With the increase of irrigation years, soil salinity accumulation also increased. While the surface salt crust thickness is rapidly increased in the early period, is increa sed to a lesser extent in the late period, and the ultimate basically tended to stable state.(3) A significant difference of adaptability to salty water shows between three families plants, including Tamarix, Haloxylon and Calligonum, constituted the Tarim Desert Highway shelterbelt. Calligonum rapid grew in the first year; Tamarix relatively slowly grew in the initial 4-6 years; Haloxylon kept relatively stable growth rate. The distribution of shelterbelt plant roots presents two core layers:In the 20cm-60cm depth layer, these three plants'roots concentrated distribution. Below 100cm, there are only distributed to the roots of Tamarix and Haloxylon. Among them, Haloxylon roots' distribution depth could be reached to 2m below the surface and even deeper, but Calligonum is generally no more than 90cm.The impact on the three kinds of plant survival and growth to irrigate with saline water are as follows:With the irrigation salinity increased, the effect to inhibit plants growth is stronger. For Calligonum and Tamarix, their degree of influence by salt stress had a threshold mutation. To irrigation salinity as a standard, the survival threshold of Tamarix and Calligonum is 15g·L-1-20g·L-1, salt-resistance threshold are about lOg·L-1-15g·L-1 and 20g·L-1, and Haloxylon is higher than 30g·L-entirely To soil salinity as the standard, the survival and salt-resistance threshold of Calligonum are about 0.25ms/cm, those of Tamarix are about 0.3ms/cm, and those of Haloxylon are beyond 0.45ms/cm. The sequence of salt-resistance ability of 3 plants is Haloxylon ammodendron, Tamarix ramosissima and Calligonum arborescons. While soil salinity beyond Calligonum's salt resistance threshold, it would death, because the larger rainfall leached the salt to be accumulated from soil surface to the main root distribution layer. To solve this problem, a better approach is to supple irrigation without delay(4) Plant transpiration and soil water evaporation has a direct relationship with meteorological factors. During growing period, there are similar regulation for soil water evaporation and plant transpiration of the shelterbelt, which appeared obvious "single peak" inter-monthly variation. During the whole growing season, the maximum of soil water evaporation appeared in July(18.09kg·m-2·mon-1), the minimum appeared in March(5.40kg·m-2·mon-1), cumulative evaporation is 101.12kg·m-2, and the total soil evaporation of per-plant is 204.24kg. Plant transpiration of three plants had little difference from June to September, and the rate of annual water consumptions of every month is nearly 20%. The transpiration and water consumption of Haloxylon is the lowest, which of Tamarix and Calligonum are relatively large. In the growing season, for the plant diameter at 2.0cm of Haloxylon, Tamarix and Calligonum, their total transpiration are 351kg,305kg and 280kg, respectively.(5) Woodland salt input mainly depends on two factors which are irrigation amount and irrigation salinity. The input salt of woodland is mainly found in the soil layer which under the wet layer. Establishing reasonable irrigation scheduling, which could control the soil salt of deposition layer in the certain soil layer without threaten the growing of shelterbelt plant. At the same time, consumption water demand could be included in the irrigation regime, which involved soil available water content, and it is based on the distribution depth of root system(Haloxylon 120cm, Tamarix and Calligonum 60cm). Also this depth is the controlling layer of soil salt.(6) The Tarim Desert Highway shelterbelt project is confronted with a long-term threat of soil secondary salinization by irrigated with saline water. Water and salt to achieve scientific and reasonable regulation is the key to solving the problem. Through the establishment of rational irrigation scheduling, with assist monitoring and forecasting soil water and salt, use two branch pipes rotation irrigation, control woodland, reduce soil evaporation and other technical measures of water and salt regulation, soil secondary salinization problems could be an effective solution. And the sustainable management of protection forest can be ensured.