| Along with the facility vegetables development, the continuous cropping obstacles has become the question which some facility vegetables production region urgently needs to be solved, has seriously restricted the sustainable development of facility agricultural production, also has adversely effected the agricultural product security and the ecological environment. Therefore study on the reasons and preventing measures of continuous cropping obstacles of different region and facility condition has become the urgent matter which realizes the sustainable development of facility agricultural production. Based on the study object of cucumber which has large scale cultivation area in facility vegetables and easily occur soil obstacles, this paper studied the effect of the four kind of continuous cropping years of 0a, 1a, 4a and 8a and the seven kind of rotation patter of different preceding crops of corn, tomato, cowpea, green bean, rape, black soybeans and fallow on the greenhouse soil environment and the cucumber output and the quality, illustrated the soil environment effect of continuous cropping cucumber, the evolution rule of soil physical, chemical and biological characteristics and the major reason and the occur mechanism of soil obstacles, elucidated the function mechanism of different preceding crops in overcome the soil obstacle and the suitable preceding crops in cucumber rotation patter which can prevent the soil obstacle. Simultaneously, this paper studied the allelopathy of root exudates of cucumber with the other nine kind of crops about corn, tomato, cowpea, green bean, rape, black soybeans, wheat, pakchoi cabbage and pumpkin, revealed the causes for continuous cropping obstacles and the interactional mechanism of root exudates of different crops, and also put forward the suitable preceding crops in cucumber rotation patter which can prevent the soil obstacle. The result indicated as below.1,The cucumber continuous cropping caused the soil respiration intensity and the soil actinomyce reduced, the soil bacteria and the fungi increased, the total quantity of microorganism slightly raised, but its relatively population structure was stable. Along with the continuous cropping year increases, the soil bacteria first raised and then felled, the soil fungi change remarkably, but the soil fungi remarkably raised by straight line. The continuous cropping years remarkably influenced the activity of catalase, urease and alkaline phosphatase of soil, but no unremarkably affect on the soil saccharase activity. With the increase of continuous cropping years, the activity of catalase and the urease was first raised and then felled, and the soil alkaline phosphatase activity was first felled and then raised. With the continuous cropping years increase, the content of soil total N, total P, available P, available K , organic matter, available Fe, Zn and Cu remarkably accumulated, the soil electricity conductivity, the saline ion enhanced and the soil pH value reduce, the soil available Mn content has no remarkable change. But the soil element appeared unbalance accumulation tendency, such as the soil P element increased prominently, the organic matter and the N element accumulation little, the available K smallest accumulated, and the soil tend to be acidification and salinization. Simultaneously, continuous cropping inhibited the physiological and the agronomical characteristics of cucumber plant, the leaf photosynthetic rates, the chlorophyll content and the protection enzyme activity, the plant height, leaf number of per plant, root number of per plant and plant dry weight was differently been inhibited, the yield reduces and the fruit quality changes poor. The soil of continuous cropping cucumber for four years, for which soil fungi quantity has remarkably increased and soil respiration intensity has obviously reduced, the soil nutrient has obviously unbalanced, the soil EC excessively higher then cucumber growth limit scope, the soil available Mn content tend deduced, and the physiological and the agronomical characteristics of cucumber plant felled, the yield and the quality dropped, soil continuous cropping barriers phenomenon occurrenced, was not suitable to continue plant cucumber.2,Compared the rotation patter of six different preceding crops with the fallow model, the result showed that the soil total quantity of microorganism, the soil fungi, the soil respiration intensity and the soil pH value has enhanced in various degree, and the soil element content of N, P, K, available Mn, available Zn and available Cu, the soil EC value has obviously reduced, the soil fungi and the soil bacteria quantity, the soil available Fe content has different change among each treatments. The corn and cowpea obviously reduced the soil fungi quantity, the soil available phosphorus content and the soil EC, and notably increased the soil actinomyce quantity, but no obviously effect to soil available potassium, organic matter quality and soil available manganese contents. Which also increased the soil pH value and obviously improved the photosynthetic rates, chlorophyll contents and protection enzymatic activity of cucumber leaf, and which has better agronomical characteristics and got higher yield and fruit quality, was the most suitable crop for cucumber in 6 kinds of preceding crops. The fallow and the green bean treatment was the secondly suitable front stubble for cucumber. The tomato and the rape increased soil fungi quantity, restrained soil actinomyce quantity and the accumulation of soil available Mn , Zn and Cu content and notably deduced the soil organic matter content, and the black soybean and the tomato obviously reduced the photosynthetic rates and the protection enzyme activity of cucumber leaf, which agronomical characteristics of plant not coordinated and the output and the fruit quality decreased, therefore the tomato was the most unsuitable preceding crop for cucumber, black soybeans and rape was also not suitable for cucumber rotation crops.3,The autointoxication effect of cucumber root exudates on its seed germination and its seedling growth was strengthened with the frequency increase of cucumber seedling cultivation. Which mainly suppresses the cucumber seedling root growth through reduced the pH value and the NO3--N content, enhanced the electricity conductivity and the saline ion of the culture medium. The culture medium of cultivate cucumber for three stubbles was not suitable to continues raise the cucumber for the appearance of continuous cropping barrier.4,The pH value of culture medium was remarkable enhanced by cultivating the different crops, but the seedling of pumpkin, green bean, black soybean, cucumber and wheat remarkably enhanced the electricity conductivity of culture medium. And the root exudates of black soybean, cucumber, cowpea, rape, green bean and corn reduced the NO3--N content, but the tomato, pumpkin and wheat root exudates increased NO3--N content of culture medium. The cultivation of cucumber seedling increased the electricity conductivity and saline ion of the root exudates solution of each crops, reduced the pH value of the root exudates solution of tomato, pumpkin and cucumber, enhanced the NO3--N content of green bean, wheat, tomato and black soybeans root exudates solution. Besides the root exudates of corn, other crops root exudates suppressed the radicle and plumule growth of cucumber seed. The corn, the rape, the pakchoi cabbage and the cucumber root exudates have promotion effects to the height and the stem diameter of cucumber seedling. The root exudates solution of the black soybeans, the green bean, the corn and the cowpea have no obviously affects badly to the cucumber seedling agronomic characters. But the rape and the tomato root exudates inhibited the agronomic characters of cucumber seedling. The root exudates solution of the black soybean, the corn, the green bean and the cowpea has obviously promotion function to the dry weight of cucumber seedling. The rape, the pumpkin and the wheat root exudates solution remarkable increased the root and the leaf dry weight, inhibited the stem dry weight and no obviously influenced on the plant total dry weight of cucumber seedling. The cucumber and the tomato root exudates solution had inhibitory effects on the cucumber seedling dry weight. Above research indicated that the corn was the most suitable preceding crop for cucumber to prevent and control the continuous cropping obstacles, next was the cowpea,furthermore was the green bean, wheat and the pakchoi cabbage. The tomato was the most inaptitude crop for cucumber preceding crop, the secondly unsuitable crop was the black soybean, the rape, the pumpkin and the cucumber. |
