The Influence Of Mildly Saline Groundwater On Soil Secondary Salinization And The Physiological Characteristics And The Yield Of Cotton Under Drip Irrigation Condition

Because of shortage of fresh water, the exploitation of underground-water were gradually becoming main way of irrigation in the northern Xinjiang cotton-growing areas. However, unreasonable utilization of groundwater resources lead to the soil secondary salinization, which seriously influence the productivity and sustainable utilization of cotton yield in northern xinjiang oasis. In this study, the groundwater's quality, chemicals type and distribution were investigated in Mosuo region, in order to determined the representative cotton yield which long-term irrigated with mildly saline water. Furthermore, the soil physical, chemical and biological characters and the character of accumulation and distribution of salt in soil-crop systems which consequently affect the growth and yield of crop on cotton yield under drip irrigation conditions, mulched with plastic film, in Northern Xinjiang were evaluated. The results obtained provide a promising option for develop saline water irrigation in arid and semiarid areas. Major finds as follows:1. In Mosuowan reclamation region, the groundwater hydrochemical types mainly are low salinity HCO₃--Cl^--K⁺-Na⁺ or Cl^--HCO₃--K⁺-Na⁺, which are obviously differences in different Mosuowan region. Therefore, there are lessly high salinity groundwater which are more than 1000 mg/L, but the sodium adsorption ratio is higher. After irrigation with the higher sodium absorption ratio(SAR) groundwater due to soil secondary salinization risk.2. Compared with the canal water irrigation, soil salt significantly accumulated and pH , ESP significantly increased with the continuing of applying saline water in different siol profiles depths by under-film drip irrigation. The soil organic matter, total nitrogen and the potassium content decreased, but the CEC significantly increased in the top layer of the soil (0～40cm). After restoring irrigation with canal water three years later, soil salt was leached to deeper layer , the pH and ESP significantly decreased in the every soil profiles . In contrast, the soil water-stable aggregates were broken and the soil caly increased lead to the soil CEC significantly increased.After irrigation with mildly saline water, mildly groundwater irrigation not only caused to increase soil exchangeable sodium percentage (ESP>15%), but it also resulted in a change of the composition of the soil salt ions, especially increased soil harmful Na⁺ and Cl^- concentrations and decreased soil beneficial K⁺ and Ca²⁺ concentrations. Furthermore, the law of ions distribution was different in soil profiles. Such as, K⁺,Na⁺,Cl^-,HCO₃-accumulated in deep layer of soil profiles, becasues of the easier movement with irrigation water, but Ca²⁺,Mg2+,SO42-decreased or increased in all layer of soil profiles. Soil total salt content increased from topsoil to deep layers and built up less at topsoil, but soil salt mainly distributed below the 0.6-0.8m soil depths layer in 3 kind irrigation fields.Soil aggregation stability, soil bulk density and porosity were significantly affected by soil Sodicity. Finally, the changes of soil physical properties accorded to decrease in soil infiltration rate. Furthermore , application of mildly saline water can result in negative impacts on soil microbial biomass, cellulose, urease, invertase and catalase enzymes. Howevery, there have a significant positive linear relationship between soil salinity factors and enzyme activities of soil alkaline phosphatase and polyphenol oxidase, meanwhile according to enzyme activities increase.3. Long time mildly saline groundwater irrigation caused deleterious effects on nutrients of cotton stem, leaf and petiole in cotton boll formation stage.The nitrogen content and K⁺ concentration of different organs of cotton significantly decrease, but phosphorus mainly accumulated in tender leaf and petiole and decreased in aging leaf and petiole. Continuous irrigation with mildly saline groundwater caused cotton plant Na⁺ concentration to increase significantly in different organs of cotton, whereas the K⁺/ Na⁺,Ca²⁺/ Na⁺ ratio significantly decreased. The results indicated that ion compartmentation in cotton which broken the balance of ion of cotton could accumulate in aging leaf and petiole, then the Na⁺ been carry off the cotton though the abscission of the aging leaf and petiole. Restoring canal water for irrigation, the balance of the cotton nutrient and ion were improved three years later.As a result, the higher accumulation of soil salt affected significantly the cotton growth and yield. Compared with the canal water irrigation, continuous saline water caused the cotton leaf potential decreased rapidly between 10:00 to 15:00, then increased rapidly between 15:00 to 20:00 and reach to the level of canal water irrigation field. Continuous saline water irrigation reduced the number of cotton and available bolls, cotton plant height significantly, consequently declined cotton yield by 11.3% dramatically . And the salt stress only affected cotton fiber length and specific breaking strength, and no effects other quality indexes.