| With the development of agricultural modernization and the adjustment of planting structure, facility agriculture has become the most powerful industry in modernized agriculture, and is also an effective change means from traditional agriculture to modern intensive agriculture. In the recent years, facility agriculture has gained great development in China. In 2008, the facility cultivation area reached over 330 million hm2 in China, which accounted for about 85% of the total area of facility agriculture in the world. The planting area of facility vegetable reached 347 thousand hm2 in China. In 2010, the total output of greenhouse vegetable was more than 170 million tons, accounting for 25% of the vegetable production in China. However, that the over irrigation and fertilization and soil continuous cropping obstacle in facility agriculture appear commonly, and the nutrient imbalance was serious. Moreover, the salt ions in soil move upward with evaporation, and finally gather in the surface layer of soil. These caused soil secondary salinization, soil hardening and other environmental problems to happen frequently, and eventually led to the decrease of the yield and quality of products. Thus, the sustainable development of facility agriculture was severely hampered. Among them, soil secondary salinization is the most prominent problem. At present, the soil area of secondary salinization reachs 77 million hm2, accounting for more than 30% of the agricultural soil, of which 58% occurred in irrigated agriculture, and appears an increasing tendency when the year goes. Soil secondary salinization has become one of the main factors to hinder the development of facility agriculture in our country. Thus, restoring the secondary salinization soil is an urgent evironmental problem to be solved in facility agriculture.The causes of soil secondary salinization are various, mainly induced by environmental factors, blind fertilization, unreasonable irrigation, improper planting patterns, and etc. Secondary salinization can not only damage soil nutrient balance, decrease soil fertility, but also hinder the plant to absorb water, and do harm to the plant, which cause the decrease of the output and quality of the plant. In addition, secondary salinization may influence on soil microbial growth and community structure, and inhibit the microbial activity. At present, the common methods to remedy saline soil mainly include engineering, chemical, biological and agricultural measures. Although these measures have achieved some success, but there are still some defects, such as high construction investment, high running cost, easy to cause sencondary pollution, poor remediation effect, and great human influence. Especially, the remediation capacity of single measure is very limit, it is essential to combine with multiple measures.Halophyte can be divided into the three types, such as salt-dilution halophyte (or euhalophyte), recretohalophyte, and salt-rejection halophyte (or pseudohalophyte), which have good desalting ability. It attracts great attention to screen, develop and utilize halophyte. As a salt-dilution halophyte, Sesbania cannabina Pers. has been widely used to remedy the saline soil, Sedum aizoon L. is a halophyte plants found in the recent years. The two halophytes not only have certain tolerance to the saline soil, but also can essectively absorb the salt ions, such as Na+, K+, Ca2+, Mg2+and NO3-. On the other hand, gamma-poly glutamic acid (γ-PGA) has been widely applied in the field of removing metal ions in waste water due to the good water solubility, strong chelating ability, good biodegradability and environment friendly. However, few report on the application of y-PGA in soil restoration can be found.In this study, the salinization degree and salinity characteristics of the greenhouse soils in the main horticultural farms with apparent obstacles for crop growth in Shanghai suburb were investigated and analyzed, and the main types and salt ions of the secondary salinization soils were attributed by statistical analysis. The commonly distributed secondary salinization with Ca2+-Mg2+-NO3-type in Shanghai was selected, and y-PGA was used to chelate Ca2+and Mg2+, and thus decrease the bioavailability of the two ions. The absorption characteristices of Sedum aizoon L. and Sesbania cannabina Pers. to Ca2+, Mg2+and NO3- were investigated by pot experiment. In addition, the removal efficiacies of Ca2+, Mg2+and NO3- in the secondary salinization soil were examined by the combination with γ-PGA and the two halophytes. The main results are summarized as follows:1. The salinity contents of the salinization greenhouse soils showed a trend of first increase and then decrease with the cropping duration increasing. The salinized soils mainly included slightly salted, mildly salted and salted soils, which account for 17.39%,56.52% and 13.04%, respectively. Most salt ions showed a good correlation with salt content. Among the salt ions in greenhouse soils, the cations were mainly Ca2+and Na+, and then Mg2+, while the anions mainly NO3- and SO42-, and then Cl-. NO3- was the main salt ion acumulated in greenhouse soils. The degree of salinity was influenced by fertilization mode, cropping duration, crop type and management level. According to the canonical correspondence analysis (CCA), the contents of Ca2+, Mg2+ and NO3- in greenhouse soils were greatly affected by cropping duration. Long-term application of chemical fertilizer and organic fertilizer caused soil acidification and basification, respectively, and also aggravated soil secondary salinization. The degree of salinity attenuated for mixed application of chemical fertilizer and organic manure, which were rarely affected by cropping duration.2. Based on the relationship between salinity ions and sampling sites, the greenhouse soils in Shanghai suburb could be classified as four patterns. Pattern I, which covered one part of Chongming Island, was mainly influenced by Na-, Cl- and HCO3-. Pattern Ⅱ, which covered the other part of Chongming Island, were mainly influenced by K+, Ca2+, Mg2+and NO3-. Pattern III, which covered Fengxian, Minhang, Qingpu, one part of Nanhui and Jiading, were mainly influenced by Ca2+, Mg2+, NO3-and Cl-. Pattern IV, which covered Songjiang, and the other part of Nanhui and Jiading, were mainly influenced by SO42-. Among them, most sencondary salinization soils belonged to Pattern Ⅲ, and need be primarily controlled.3. The optimal removal conditions for Ca2+and Mg2+were obtained by using y-PGA, i.e., the initial concentration of y-PGA was 1000 mg·L-1, the initial pH of the mixed system was 7, and the temperature was 25℃. Under the optimal conditions, the maximal removal percent of Ca2+and Mg2+were 51.59% and 68.03%, respectively. The removal process of Ca2+and Mg2+could be well described by second-order kinetic model, and the rate constants of Ca2+and Mg2+were obtained as 0.00282 and 0.0166 (mg·L-1)-1·min-1, respectively. The main removal mechanism was the chelation of amide groups and carboxylic acid anions in y-PGA with Ca2+and Mg2+.4. The maximal removal percents of Sedum aizoon L. for Ca2+and Mg2+were 64.35% and 53.35%, while those of Sesbania cannabina Pers. were 82.87% and 45.55%, respectively. Sedum aizoon L. also has strong ability to remove to NO3-, the maximal removal percent was 51.78%. Due to the nitrogen fixation of rhizobium in Sesbania cannabina Pers. and the decomposition of plant residue, NO3- concentration in the soil mantained at 1.038 g-kg-1, and increased 26.02% compared with the original saline soil.5. Pot experiment found that applying y-PGA not only decreased the bio availability of Ca2+and Mg2+and the salt stress for the halophyte seedlings, but also promted the growth and resistance of halophytes. From the physiological parameters of the halophytes, the best removal efficiacy could be found by Sedum aizoon L. applied with 1000 mg·L-1 of y-PGA (J3), the maximal removal percents of Ca2+, Mg2+and NO3- were 93.25%,94.79% and 93.25%, respectively. The good removal efficiacy could also be obtained by Sesbania cannabina Pers. applied with 200 mg-L"1 of y-PGA (T1), the maximal removal percents of Ca2+and Mg2+were 90.26% and 69.92%, respectively. Due to the nitrogen fixation of rhizobium in Sesbania cannabina Pers. and the decomposition of plant residue, NO3- concentration of in the soil mantained at 0.868 g·kg-1, and increased 5.36% compared with the original saline soil.Comparing with the remediation efficiacy of single treatment with y-PGA, single treatment with halophytes and combined treatment with y-PGA and halophytes, the most suitable and effective method to restore this kind of saline soil dominated by Ca2+, Mg2+ and NO3- was Sedum aizoon L. applied with 1000 mg·L-1 of y-PGA (J3), and the final removal percent of the salt ions was obtained as 74.71%. |
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