The Fatigue Of Greenhouse Soil And Its Affection Mechanism On The Growth Development Of Vegetable

The fatigue is the universal problem in greenhouse soil. It affects the sustainable use of greenhouse soil and also endangers vegetable security. The studies on the fatigue of greenhouse soil and its affection mechanism on vegetables are highly important for us to reasonably use soil, to scientifically regulate soil, and to maintain high land capacity. In this paper, the fatigues were studied by greenhouse soils of different planting years and uncovered soil; the physical fatigue of greenhouse soils was simulated using soil bulk density as index and its effects on growth and development, yield and fruit quality of cucumber were studied, and its affection mechanism were inquired. The main results were as follows:1. Acidification and salinization process proceeding at the same time was the main feature of the fatigue in greenhouse soils. These two processes have obvious stage characteristic.The acidification characteristics in greenhouse soil were as follows: There was different degree acidification in 0～40cm layer. Acidification was more serious in cultivation layer. The acidification fatigue process in greenhouse soils didn't last, but it occurred in primary stage of planting vegetable and its intensity was great, after this stage, the anti-acidification ability of soil rose and the acidification rate deceased with planting time prolonging of greenhouse soil, the changing of pH was small, the range of pH was 7.75～7.98.The salinization fatigue of greenhouse soils was obvious. In 0～40cm layer, total salt content of greenhouse soils was more than that of uncovered soil. In the same soil profile, the difference of salt content in different layer was significant, salt content was high in upper layer and low in lower layer, in 0～10cm layer, salt content was highest and salinization degree was medium. Salinization process of greenhouse soils was different from that of uncovered soil, and it has obvious space and time variation characteristic. In greenhouse planting 5 years, salt accumulation process mainly occurred in 0～10cm layer. After 5years, salt accumulation process and salt leaching process occurred at the same time, salt content in soil profile gradually increased and uniformed. Salt accumulation process in surface soil occurred again at high salt content level with greenhouse planting time prolonging. The characteristic of salinization process in greenhouse soils was that salt accumulation process in surface soil and salt uniformity process in profile occurred cross.2. NO3–-N accumulation was the main nutrition sign of the fatigue in greenhouse soils. In 0～40cm layer, NO₃^--N content was more in greenhouse soils than uncovered soil, NO₃^--N accumulation was obvious in greenhouse soils and it gradually increased with greenhouse planting time prolonging. In 0～10cm layer, NO₃^--N content was more in greenhouse soils of three planting time than uncovered soil. The feature of NO₃^--N accumulation process in greenhouse soils was obvious, that was: NO₃^--N accumulation process in surface layer occurred rapidly in primary stage of planting vegetable, and then occurred slowly after this stage, NO₃^--N accumulation rate lowered in 0～10cm layer, but soil layer depth of NO₃^--N accumulating gradually increased with greenhouse planting time prolonging.Water soluble Ca content was the intensity index of Ca supply ability and exchangeable Ca content was the capacity index of Ca supply ability in soil. The changes of Ca supply intensity and capacity were significant. Ca supply intensity gradually increased and Ca supply capacity gradually decreased with greenhouse planting time prolonging. In 0～40cm layer ,water soluble Ca content was higher in greenhouse soil than uncovered soil. The longer greenhouse planting time was, the higher water soluble Ca content was. But Ca saturation degree in soil colloid gradually lowered. Water soluble Ca content of 0～10cm layer and 30～40cm layer in profile significantly increased with greenhouse planting time prolonging. Water soluble Ca differentiation degree in profile showed that water soluble Ca increasing process was accompanied by the process of Ca moving and illuviating down deep layer. Water soluble Ca content increasing played an important role in rising resistance nutrition fatigue ability of soil. Water soluble Ca content increasing had close relationship with NO₃^--N accumulation and secondary salinization in soil.In 0～40cm layer , organic matter content was higher in greenhouse soils than uncovered soil, and it increased with greenhouse planting time prolonging. Organic matter content increasing not only established foundation for maintaining resistance fatigue ability of soil but also increased Ca supply ability of soil.3.In 0～10cm layer, bulk density was higher in greenhouse soils than uncovered soil. This showed that cultivation type greatly affected surface layer soil bulk density. Surface layer soil compaction was main feature of soil physic fatigue of greenhouse soils. The difference of bulk density in tested soil was no-significant in 30～40cm layer, it demonstrated that the effect of cultivation type on soil bulk density of the lower layer was small . In 10～20cm layer and 20～30cm layer, bulk density was higher in uncovered soil than greenhouse soils. 4. In 0～40cm layer, <0.01mm physical clay content and <0.001mm clay content in uncovered soil were highest in tested soils. The difference of layers in uncovered soil profile was no-significant. It showed that uncovered soil hadn't the case of <0.01mm physical clay and <0.001mm clay moving down. < 0.01mm physical clay content and <0.001mm clay content were lower in greenhouse soils of three planting time than uncovered soil. In the same soil profile, the difference of physical clay content and clay content in different layer was more and more great. Physical clay content and clay content were low in upper layer and high in lower layer. They moved from upper layer to lower layer. This case was called argillication of greenhouse soils; argillication was reinforced as greenhouse planting time prolonging. Physical clay and clay moving down had weakened the resistance fatigue ability of cultivation layer soil to a certain extent.5. According to evaluation of different diameter scale clay content measured by Dry-sieved and mean weight diameter (MWD), the machine stability was worse in greenhouse soils than uncovered soil. It mainly caused by secondary salinization of greenhouse soils.The water stability was better in greenhouse soils than uncovered soil according to evaluation of >0.25mm water stability aggregate content, MWD of water stability aggregate, destruction rate of aggregate, original stability coefficient and disintegration rate of aggregate. It was related to high organic matter in greenhouse soils. Organic matter was substance basis of soil resisting fatigue ability, so the increasing of organic matter content rose 0.25～10mm aggregate content and its water stability.6. Moisture retention characteristic of tested natural soils was fitted by Van Genuchten equation and moisture retention curve parameters of tested soils were estimated.θs and n were different among tested soils.θs was lower in uncovered soil than greenhouse soils ,but n was higher in uncovered soil than greenhouse soils. Main moisture constants were estimated by moisture retention curve of tested soils. The differences of wilting coefficient of tested soils were no-significant. Field water capacity and available water capacity were lower in greenhouse soils than uncovered soil and they cut with greenhouse planting time prolonging. Field water capacity maintained steady when greenhouse planting time was more than 5 years. The relationships of available water capacity of tested soils with <0.01mm physical clay content and <0.001mm clay content were close.After treating tested soils by wetting and drying cycle, the decreasing degree of saturated moisture content (θs) was more in greenhouse soils than uncovered soil. This suggested that water physical characters in greenhouse soils were weak and the ability of wetting and drying cycle destroying resistance in greenhouse soils was poor. The increasing degree of parameter n was more in greenhouse soils than uncovered soil. It meant that water loss rate increased and the abilities of water retention and drought resistance declined in greenhouse soils. The results of wetting and drying cycle made field water capacity decrease, wilting coefficient increase, and available water capacity decrease in greenhouse soils.7. Using soil bulk density as index, physical fatigue of greenhouse soil was simulated under laboratory condition and the effect of compaction on water evaporation process in soils was studied. Compaction made water loss rate increase, the abilities of water retention and fatigue resistance decline. Water evaporation rate and evaporation lasting time were more in compacted soil; therefore, water loss amount was larger.8. Using soil bulk density as index, physical fatigue of greenhouse soil was simulated by pot experiment. The effects of compaction on the growth development, yield and fruit quality of cucumber were studied. Compacted soil depressed height and stem thickness of cucumber plant, increased width and length of the forth leaf, lessened number of visible leaves, greatened number of yellow leaves; hampered root elongation, root activity, and total absorption of nitrogen phosphorous potassium, decreased root/shoot ratio; lowered the first female flower node; restrained fruit expansion, cut sugar/acid greatly, worsened fruit flavor; decreased biological yield, economic yield and economic coefficient. Compacted soil made cucumber plants occur a series of physiological response. The relative electrical conductivity and malondialdehyde (MDA) content of cucumber leaf rose, membrane permeability increased; the soluble protein content decreased, physiological function of leaf declined; the activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) increased, defense system in plants started; electron transport activity(Fm/Fo), maximal photochemical transformation efficiency(Fv/Fm) and potential activity (Fv/Fo) of photosystemâ…¡reaction center in leaves decreased; photosynthesis of leaves weakened, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (E) and specific leaf weight(SLW) decreased, but intercellular CO2 concentration (Ci) raised. These results were caused by higher CO₂ flux and CO₂ concentration in compacted soil (CS).