The Coupling Effect Of Moisture, Temperature And Nitrogen On Soil Nitrate Accumulation In Greenhouse Culture And Its Forecast With BP Neural Network

The phenomena of soil secondary salinization,nitrate accumulation and soil acidification in protected culture are very serious,and have changed into a bottleneck problem to hinder the sustaining development of greenhouse production in China.To meet these problems in this paper,the interaction effects of temperature,moisture and nitrogen on its transformation,transport and accumulation of NO₃^--N,growth and nitrate uptake of plant were studied in the laboratory around the subject of soil nitrate accumulation in greenhouse culture.The conclusion has been drawn as follows:1.The investigation of soil salt and analysis of its physico-chemical properties in greenhouse cultureThe soil nitrate content,salt content and electrical conductivity were higher under perennial protected condition than those under open-field one.The salt contents were significant higher in 0～20cm surface layer soil than those in 20～40cm depth layer soil and the salt accumulation increased with increase of greenhouse age under protected condition. Various salt ionic contents were significant larger in protected-field soil than in open-field soil,and the main characteristic was relative to enrichment of NO₃^-,Cl^-and Ca²⁺.The salt accumulation was no significant in semiannual-cover plastic greenhouse because of eluviations of rainwater in rainy season,but it was apparent higher than that in open-field. The relationships between electrical conductivity of soil extract(EC_5:1) and chemical properties were analyzed.The results indicated that total soluble salt concentration(SSC) and concentration of Cl^-and Ca²⁺ were the most primary factor to influence EC_5:1.The direct effect of NO₃^-,K⁺ and Na⁺ to EC_5:1 was not significant,but they were enlarged by influence of other chemical factors,such as Cl^-,Ca²⁺ and SSC,making the correlation coefficient between EC_5:1 and these factors relatively higher.The ionic strength was related significantly to EC_5:1 which could be estimated by I=0.0176EC.2.The soil hydraulic and thermal properties in greenhouse cultureEffects of temperature and water content on the soil specific heat were studied by mixing method.The results indicated that the soil specific heat increased with increasing soil water content,and the increase rate was higher in low water content than that in high water content.The soil specific heat was also function of temperature,and its increase rate was lower under low temperature condition than that under high one.The linear function of soil temperature and water content had impact on soil specific heat in a highly marked level and their interaction had not attained a significant level.Effects of temperature and water content on the soil thermal conductivity were studied by heat-probe method.The results showed that soil thermal conductivity increased with increasing of soil water content and soil initial temperature in test level.The linear function of soil water content and initial temperature had impact on soil thermal conductivity in a highly marked level and square of soil water content in a significant level,but the square of soil initial temperature and their interaction in a no significant level.A pressure device was introduced to determine soil water retention curve.The results indicated that soil water content changed widely in 0～100kPa suction and it changed gently in 100～1500kPa suction.The change of water content resulted from change of unit suction was light,the mobility and conductivity of soil water were weaker but water retention capability of soil enhanced.A regression model for soil water retention curve was derived.3.Effects of moisture and temperature on water and NO₃^--N transport in greenhouse cultureVaporization temperature and initial soil moisture influenced significantly NO₃^--N transport from deep soil to top soil,and the transport rate increased with the increase of vaporization temperature and initial soil moisture.The effects on NO₃^--N transport rat were a descending order as initial soil moisture,vaporization temperature,and their interaction.The effects of initial NO₃-N contents in soil and its interactions with vaporization temperature and initial soil moisture on NO₃^--N transport rate were not significant,but it affected the absolute NO₃^--N contents of soils.After water was evaporated in soil for 5days,it showed that NO₃^--N content was higher in top layer soil and decreased with increase of soil depth,increased a little again and tended to an approximate steady value.The soil water distribution in vertical plane became low in surface layer and high in depth layer under different condition.Soil water contents decreased with increase of vaporization temperature and the soil depth to result form change of soil water gradient increased.Water contents increased after water was evaporated and soil water gradients decreased in surface layer soil with increase of soil initial water contents.Soil water contents increased with increase of soil initial NO₃^--N contents.4.Effects of moisture,temperature and nitrogen supply rate on NO₃^--N accumulation in greenhouse cultureAn incubation experiment was inducted to determined the changes of NO₃^--N accumulation with time during the process of nitrification under various soil moisture, soil temperature and nitrogen supply rate for greenhouse soil.The results indicated that equationâ–³(NO₃^-)=e^(a+b/Ï„) could be used to express the NO₃-N accumulation with time. The maximal rate of nitrification,delay period and maximal NO₃^--N mass of NO₃^--N accumulation were derived from the equation,and used to characterize quantitatively the nitrification process in different soil conditions.The effects on maximal rate of nitrification were a descending order as soil moisture,soil temperature,interaction between temperature and moisture,interaction of moisture between nitrogen supply rate, and nitrogen supply rate,and which was descending order of soil moisture,soil temperature,nitrogen supply rate and interaction of moisture and nitrogen supply rate on delay period.Maximal NO₃^--N mass of accumulation depended on nitrogen supply rate, moisture and temperature,and affected no significantly by their interactions.In greenhouse production systems,where moisture and temperature could be controlled,this model could be used to predict NO₃^--N accumulation and the effectiveness of N in the fertilizer management that considers both nitrogen supply rate and soil environment.5.Effects of moisture,temperature and nitrogen supply rate on pH and electric-conductivity in greenhouse cultureThe effects of temperature(T),moisture(W),nitrogen supply rate(N) and their interaction on pH and electrical conductivity of soil(EC) were studied in the laboratory incubation for the greenhouse culture soils.The results indicated that pH decreased with incubation time and first-order dynamic model could be used to express reduce rate.N,W, W×N and T×W sharply effected the pH reduce rate constant(k) which was descending order of N＞W＞W×N＞T×W.EC increased quickly to the maximum in the first week in the incubation then decreased lightly,and changed in an approximate constant EC_sty at last time.N,W,T×W and T had impact on EC_sty in a highly marked level and W×N in a significant level.Soil acidification and EC increment could be controlled by reduction of nitrogen supply rate and regulated by deficit irrigation.The regression equations of k and EC_sty were found and the equations could be used to predict pH reduce and EC_sty in the fertilizer management that considers both nitrogen supply rate and soil environment.6.Sensitivity responses of soil nitrate accumulation,pH and EC to soil moisture and N application rateSoil incubation was conducted to evaluate the sensitivity responses of nitrate accumulation,pH and electrical conductivity(EC) to soil moisture and N application rate in an open field soil(clay loam) compared with the same soil that in an adjacent greenhouse.The results indicated that the additional nitrate accumulation arising from the treatments imposed,pH descending and increased EC were greater in the greenhouse soil than in the open-field soil under the same soil moisture and N application rates.The nitrate accumulation rate and pH descending rate were increased with increasing of soil moisture,and most enhanced at 90%water holding capacity in the glasshouse soil.By contrast,the nitrate accumulation and pH descending rate was greatest at 70%water holding capacity in the open field soil.The nitrate accumulation rate and pH descending rate in greenhouse soil was more sensitive to N application rate compared with that in the open field soil.Increasing soil moisture and N application rate enhanced the EC in greenhouse soil and in the open field soil.The impacts of soil moisture and N application rate on the EC increment in the open field soil were more significant than that in greenhouse soil.the EC absolute increments in greenhouse soil with incubation time were far exceed that in the open field soil,which lead to serious salinization in greenhouse soil.7.Effects of moisture and nitrogen interaction on plant nitrogen uptake in greenhouse cultureA culture experiment was conducted with the soil in which nitrate was accumulated seriously.It was advantageous to tomato growth when soil moisture was 100%FC and N application rate was 150mg/kg.In this treatment,nitrate reductase activity(NRA) of plant was higher and nitrate accumulation was lower than that in other treatments.The residual soil nitrate in this treatment was lower than that in other treatments after tomato harvest. When soil moisture was 80%FC and N application rate was 300mg/kg,growth of tomato was delayed and tomato plant becamed fugacious.Nitrate reductase activity(NRA) of tomato in this treatment was lower and nitrate accumulation was lower than that in other treatments.The residual soil NO₃^--N and NH₄⁺-N was in a high level after tomato harvest. Sufficient irrigation is more beneficial to tomato growth and nitrogen uptake than deficit irrigation for soil of nitrate accumulation seriously.8.BP Neural Network model for soil nitrate accumulation in greenhouse culture An improved BP Neural Network(BPNN) with additional momentum and adaptive learning rate was applied to set up the predictive model to interaction of moisture, temperature and nitrogen on soil nitrate accumulation in greenhouse culture.The model was designed applying Matlab 6.5 and the check samples was simulated with the designed model.The calculated precisions between BPNN model and regression model were compared.The result showed that the model was of rapid convergent speed,high precision and better fit effect.The model could be applied to forecast the interaction of moisture,temperature and nitrogen on soil nitrate accumulation in greenhouse culture.