Salt Movement Trends In Cotton Fields With Long-term Drip Irrigation Under Mulch In Typical Oasis And Irrigation Management

Drip irrigation under mulch has been widely practiced in the arid region of Northwest China especially the saline-alkali soils in Xinjiang. As, theoretically, drip irrigation under mulch only affects salt distribution in root zones and salts are not drained out of the soil baby, whether salts will accumulate in root zones with long-term drip irrigation under mulch is an important question deciding whether such an irrigation method can be sustainably applied in arid regions. Previous results have provided a lot on water and salt management for drip irrigation under mulch but little on the actual changing trends of salts in fields with long-term drip irrigation under mulch. Therefore, in this study, salt changes in five agricultural fields with2-16years of drip irrigation under mulch were monitored consecutively from2009-2013. These fields were located in Regiment121, Shihezi, Xinjiang, which is a typical oasis. Study area wasteland0～40cm soil salinity average25～70g/kg, groundwater depth2-4m, sandy loam soil with varying degrees of salinization mainly in cotton irrigation quota on average816.15mm, irrigation water salinity0.4g/L or so.The results revealed soil salt content evolution trends in agricultural fields with long-term drip irrigation under mulch under current irrigation regime. Corresponding irrigation management strategies were proposed. The results from this study are expected to provide theoretical basis for the sustainable application of drip irrigation under mulch in arid regions. And main conclusions were drawn as follows:(1) Water and salt distribution and changing characteristics in cotton fields with long-term drip irrigation under mulchDrip irrigation under mulch has significantly influenced and changed water and salt distribution in farmlands under natural conditions. Under current irrigation regime, for the0-140cm soil layer, soil moisture was high in fields with long-term drip irrigation under mulch. Soil moisture was high after irrigation for the whole field, inside the films, between the films, or for the different growth stages of cotton. And there were no significant differences in soil moisture between positions and times. In addition, interannual variation was small. Distribution and variation of soil moisture and salt to a depth of300cm or to the water table in cotton fields with drip irrigation under mulch were influenced by irrigation, crop water assumption and evaporation together. Soil salt content and distribution displayed fairly big spatial and temporal variation with application years of drip irrigation under mulch. Holistically, high salt contents with great variation were found between films in the horizontal direction. Salt contents within films were low especially in the0-100cm layer, which was suitable for cotton growth. In the vertical direction, great variation in salt content was found in the surface soil layer while the deeper the layer, the smaller the variation, and the smaller the differences in salt content between different horizontal positions with gradual decrease as a whole. Soil salt content as a whole exhibited decreasing trends during the whole cotton growing period within the year especially after the irrigation in cotton seedling stage in April. With longer years of drip irrigation under mulch, interannual salt content variation coefficient and difference decreased gradually.(2) Soil salt content evolution trends in cotton fields with drip irrigation under mulchUnder current irrigation regime, drip irrigation history had significant impacts on the distribution of soil salt content in the0-300cm soil layer. After a single irrigation, soil salts in cotton fields with drip irrigation under mulch displayed pronounced movements in both the horizontal and vertical directions. Soil moisture was the main factor affecting soil salt movement in cotton fields with significant convection in soil salt movement. After several times of irrigation, soil salts showed a holistic downward movement which was close to a one-dimensional movement. As a whole, average salt content displayed the same first-rapidly-then-slowly decreasing trend as a power function exhibits with longer drip irrigation history. The first3years of drip irrigation was the rapid desalinization stage when soil salt content decreased rapidly compared with those in surrounding uncultivated lands. The3rd-8th year was the stable desalinization stage when desalinization rate increased linearly and soil salt content dropped to below5g/kg after7years of drip irrigation. After drip irrigation had been practiced for8-16years, desalinization rate stabilized at80-90%and soil salt content dropped slowly with drip irrigation history. After16years of drip irrigation, average soil salt content was below3g/kg. According to the correlation between soil salt content at different depths and irrigation history under current irrigation regime, drip irrigation needs to be practiced for5.69,6.08and6.53years for the average soil salt content to drop below5g/kg in the0-60cm,0-100cm, and0-140cm soil layers, respectively. The longer the drip irrigation under mulch was practiced, the lower the soil salt content was, and the smaller the salt content dropped. And finally the soil salt content would be in a dynamic equilibrium state.(3) Appropriate irrigation quota and irrigation management strategy for cotton fields with long-term drip irrigation under mulchSoil salt content in root zone of cotton fields with drip irrigation under mulch significantly impacted cotton growth and yield. Current drip irrigation regime under mulch has important implications in salt leaching. With the decrease in salt content in root zone (0-60cm depth), irrigation quota on each application in the seedling stage and irrigation quota should be adjusted. When drip irrigation has been practiced for no longer than6years, soil salt content in root zone is relatively high with the average between5-24g/kg and flushing should be strengthened to suppress salts. Flushing quota in the seedling stage should be104.5-350mm, irrigation quota on each application in the seedling stage should be161.7-400mm, and irrigation quota should be855.0-1660mm. When drip irrigation has been practiced for6-9years, average salt content in root zone is3-5g/kg, which basically meet the cultivation requirements for a cotton yield of above5250kg/hm2. Irrigation quota on each application should be properly reduced and flushing should be weakened in strength to keep controlling salts. Flushing quota on each application in the seedling stage should be66.1-104.5mm, irrigation quota on each application in the seedling stage should be123.3-161.7mm, and irrigation quota should be733.9-855.0mm. After9-16years of drip irrigation, average soil salt content in root zone was below3g/kg, Cl-content was below0.12g/kg, and cotton yield was over6000kg/hm2. Flushing quota on each application in the seedling stage should be around34.5-66.1mm to keep controlling salts. Irrigation quota on each application in the seedling stage should be91.7-123.3mm, and irrigation quota should be637.0-733.9mm. In addition, irrigation frequency should be properly raised to make full use of the advantages of small amounts for many times of the drip irrigation technology.