Studies Of The Effects Of Portected Cultivation Years And Fertilization On Soil Salts And NO₃^--N Accumulation

This thesis focused on the effects of the different cropping systems, cultivation years andfertilization treatments on the changes of water-soluble salts, NO₃^--N, and pH in greenhouse soils andon the distribution and mobility of water-soluble salts and NO₃^--N in soil profiles via the fieldinvestigations, field plot experiments in Liangzhou District of Wuwei City, Gansu Province. Theobtained results will provide insight into the sustainable development of local protected agriculture. Themain results were showed as follows:1. The average total salt content in0-20cm soil layer of greenhouse soil was1.4,1.9,1.9and2.3times higher than that in the wheat fields for short than3years,4-6years,7-9years and more than10years cultivation respectively. The total salts gradually increasing with the extension of cultivation yearswere also found. The samples which total salt content was higher than4.00g·kg-1took21.1%of thetotal95greenhouse soil samples, while samples which total salt content was higher than8.00g·kg-1took4.2%. The salt accumulation in greenhouse soils might had adverse impacts on the plant growthand soil environment.2. According to the results of field plot experiments for3.5years, the successive application ofchemical fertilizer or manure all might lead to the accumulation of water-soluble salts in greenhousesoils. Furthermore, the salts accumulation upwards to the topsoil was also found. The total saltscontents in0-10cm soil layer for NPK（chemical fertilizer）, MNPK（chemical fertilizer+manure）,1/2MNPK （half of chemical fertilizer and manure） and M（manure） treatments were60.3%，87.6%，53.7%，68.2%higher than that in10-20cm soil layer respectively. The total salt content in0-10cm soillayer for MNPK treatment was remarkably higher than that in1/2MNPK treatment. It indicated that thetotal salt content increased significantly with the increase of fertilizer application rate. Under thecondition of same fertilization rate, the total salt content in0-10cm soil layer of NPK treatment was by2.17g·kg-1and most significantly higher than that in1\2MNPK which was by1.81g·kg-1and in Mtreatment which was by1.55g·kg-1. Salts accumulated more easily in the topsoil layers in the NPKtreatment than that in the manure or fertilizer+manure application treatment. The total salts contents in20-150cm soil layers followed the order of NPK>MNPK≈1/2MNPK>M>CK. The obvious leaching ofsalts in soil profiles was observed, and the chemical fertilizer application could more easily result to thesalt leaching than other treatments with the same application rate. The combined application of chemicalfertilizer and manure could weaken the leaching of salt in soil profiles. Chemical fertilizer brought moreCa2+and SO42-into soils because of large application of calcium phosphate and potassium sulfate,however, manure put more K+, Na+, and Cl-into greenhouse soils. The reasonable combined applicationof chemical fertilizer and manure was conducive to the decrease of secondary salinization in greenhousesoils.3. Based on data taken from six monitored greenhouses, NO-3-N contents in0-10cm and10-20cmsoil layers were significantly higher than those in wheat fields, The NO-3-N content in0-10cm soil layers was7.76,7.68,4.91,3.84,6.44, and5.41times higher than that in the wheat fields. The NO₃^--Ncontents in greenhouse soils were also increased with increasing of cultivation years.4. A field plot experiment was conducted to study the effects of different application rates andfertilizer types including chemical fertilizer, manure, and chemical fertilizer+manure on NO₃^--Naccumulation and leaching in greenhouse soil profiles. The results showed that NO₃^--N accumulationand leaching in greenhouse soils were affected by the application rates and types of fertilizer. Nitratecontent in soil profile increased with increasing of N fertilizer application rates, especially in0-20cmsoil layer. NO₃^--N accumulation in40-150cm soil layers of NPK,1/2MNPK and M treatments with theequal level of N application, were267.33,211.94, and125.72kg·ha-1respectively. It indicated thatchemical fertilizer application could easily result to the leaching and accumulation of NO₃^--N in deepersoil layers by contrast with the manure or fertilizer+manure application. Furthermore, no effect onvegetable yields and significant decrease in NO₃^--N accumulation in greenhouse soils were obtained byhalving fertilizer application amount in local farmers’ practice. It could be inferred that the highapplication of fertilizer in greenhouse soils in Wuwei City might lead to the lower utilized coefficient offertilizer, high cost of production and the possible environmental pollution risk.5. In the investigated areas, the pH values in0-20cm soil layer of greenhouse soils were alllower than that in the wheat fields for short than3years,4-6years,7-9years and more than10yearscultivation respectively. The soil pH declined gradually with cultivation years extended. In sixmonitored greenhouse soils, the same results were also obtained.6. The results of field plot experiment showed that the successive application of chemical fertilizeror manure might resulted to the pH declining in greenhouse soils. The decrease of pH in topsoil waspositively related with the application rate of fertilizer and negatively with the soil depth. Furthermore,the significantly negative correlation were found between soil pH（y） and the contents of water-solubleSO42-, Cl-, NO₃^-, and NO₃^--N（x） in greenhouse soils. The dependent linear equations were y=-1.1813x+8.8896（r=0.6540）, y=-10.814x+8.9324（r=0.7285）, y=-1.6276x+8.9553（r=0.8605）, y=-0.0086x+8.9638（r=0.9130）, respectively. The stepwise regression equation was y=-3.2402x2-1.3274x3+8.9832（p<0.05, x2and x3were the contents of Cl-and NO₃^-）, and it indicated that the effectsof Cl-and NO₃^-contents on soil pH were larger, especially the Cl-. The substantial accumulation of saltsand NO₃^--N in greenhouse soils resulted from the unrational application of physiological acidic fertilizer,N fertilizer, or manure might be responsible for the decrease of soil pH.