| Soil salinization in arid and semiarid area is one of the most significant environmental issues restricting the sustainable agricultural development.There is about 9.5×108 hm2 of salinized soil all over the world,and it tends to get larger and aggravated in recent years.It is critical to take scientific and reasonable measures to control or reduce the soil salinization.Therefore in this paper,biological improvement was taken as the breakthrough point and a typical salinized land in North Yinchuan,Ningxia was chosen as the experimental area to study the spatial variance and annual change of Medicago Sativa L.,Achnatherum splendens,Leymus chinensis and Onobrychis viciaefolia against the physical properties,chemical properties,and microbiological diversity of the saline-alkali soil through field experiment research using a qualitative-quantitative and theoretical-empirical analyzing method.Comprehensive soil quality evaluation indexes were used to evaluate the improvement effect and screen out forage grass species suitable in this region,and a research on the rotation with sorghum dochna snowden,Zea mays,and Helianthus annuus L.was made to clarify the improvement effect and mechanism of different grass and crop rotation modes against salinized soil,determine the suitable grass and crop rotation mode in this region,and provide theoretical reference for improving the soil quality in the saline-alkali soil and establishing a scientific cropping system in North Yinchuan,Ningxia.The main results are as follows:1.Effect on improving the saline-alkali soil by planting different forage grassesThe effect and mechanism of different salt resistant forage grasses on improving the saline-alkali soil were discussed,and the forage grasses suitable for this region were screened out.A two-year(2013-2014)positioned experiment was made by planting different salt resistant forage grasses Medicago Sativa L.,Achnatherum splendens,Leymus chinensis and Onobrychis viciaefolia in the saline-alkali soil in North Yinchuan,Ningxia,and a saline-alkali soil with no forage grass planted was used as the control to analyze the impact of different salt resistant forage grasses on the change of salt characteristics,physical properties and chemical properties in the saline-alkali soil.The result showed:(1)The salt content in each soil layer was decreased wavily,and salt returning degree appeared as 0-20cm>20-40cm>40-60cm.The total salt content was decreased as Medicago Sativa L.>Leymus chinensis>Achnatherum splendens>Onobrychis viciaefolia.(2)The four forage grasses distinctly improved the physical properties of the saline-alkali soil,as the soil bulk density was reduced year by year in 0-20cm>20-40cm>40-60cm;the soil water content responded to the rainfall in a single peak trend,and was increased more or less as Medicago Sativa L.>Onobrychis viciaefolia>Leymus chinensis>Achnatherum splendens after different forage grasses were planted.(3)The impact of the four forage grasses on the chemical properties of the saline-alkali soil appeared with increased organics in the soil as Medicago Sativa L.>Onobrychis viciaefolia>Leymus chinensis>Achnatherum splendens.The soil nutrient under different grass root systems was modified in fluctuation,Medicago Sativa L.generally concentrated the alkali-hydrolyzable nitrogen in the soil,Onobrychis viciaefolia consumed a lot of the alkali-hydrolyzable nitrogen,Achnatherum splendens promoted the transformation of the rapidly available phosphorus,while Leymus chinensis lightly improved the content of potassium.And meanwhile,the degree of alkalization was decreased wavily with the extension of improvement time.Generally the enzymatic activity in the surface soil layer planted with Medicago sativa L.was high and the catalase in the surface layer was significantly increased with the progress of cropping years.(4)The SQI(Soil Quality Indexes)of treatments with different forage grasses were analyzed and calculated with membership function and factor analysis methods,and they appeared in a similar rule among different treatments in 2013 and 2014 as Medicago Sativa L.>Onobrychis viciaefolia>Leymus chinensis>Achnatherum splendens in the soil layer 0-60cm in 2013 and Medicago Sativa L.>Onobrychis viciaefolia>Leymus chinensis>Achnatherum splendens in 2014.It meant Medicago Sativa L.brought the superior improvement effect followed by Onobrychis viciaefolia,Leymus chinensis and Achnatherum splendens.In different years,the SQI of salinized land planted with different forage grasses changed in a similar rule.2.Effect and mechanism of improving the saline-alkali soil by different grass and crop rotation modesA positioned experiment was made with the crop and rotation system of Medicago Sativa L.to discuss the effect and mechanism of improving the saline-alkali soil by different grass and crop rotation modes including continuous cropping(AA),fallowing after plowing(AF),rotating with Helianthus annuus L.(AO),Zea mays(AC)and sorghum dochna snowden(AS).Through the two-year experiment(2015?2016),the impact of different grass and crop rotation modes on the salt characteristics,physical properties,chemical properties and microbiological diversity in the soil was analyzed,and the result showed:(1)The change of soil salt characteristics was analyzed with Medicago Sativa L.after continuous cropping,fallowing after plowing,and rotating with Helianthus annuus L.,Zea mays and sorghum dochna snowden,and the result showed:the total salt content in the soil layer 0?20cm was always lower than that in Mode AA,the differences among the treatments were significant,but in deeper layers,the total salt contents in different treatments were increased.(2)The change of soil physical properties was analyzed with Medicago Sativa L.after continuous cropping,fallowing after plowing,and rotating with Helianthus annuus L.,Zea mays and sorghum dochna snowden,and the result showed:all the soil bulk densities in the mode of rotating with Zea mays,rotating with Helianthus annuus L.and rotating with sorghum dochna snowden were higher than that in the continuous cropping mode in 2015.In 2016,all the soil water contents in different rotation modes were higher than that in CK,the highest soil water content is in the mode of rotating with Helianthus annuus L.,the soil water contents in different grass and crop rotation modes were generally increased with the treatment progress,and significantly higher than that in the continuous cropping mode.It meant in North Yinchuan,Ningxia,crop rotation was good for improving the structure,permeation effect,and physical properties of the soil.(3)The change of soil chemical properties was analyzed with Medicago Sativa L.after continuous cropping,fallowing after plowing,and rotating with Helianthus annuus L.,Zea mays and sorghum dochna snowden,and the result showed:the soil fertility could be improved and thesoil alkalization can be reduced by different grass land crop rotation modes,in the modes of rotating with Zea mays and rotating with sorghum dochna snowden,the soil enzymatic activities were higher than those in the modes of continuous cropping,fallowing,and rotating with Helianthus annuus L.,the activities of catalase,sucrase,urease and alkaline phosphatase were the highest in the mode of rotating with Helianthus annuus L.,and the soil enzymatic activity was improved distinctly in the soil layer 0-20cm.In deeper soil layers,the difference of enzymatic activity among soil layers were not significant(P>0.05),it meant the soil enzymatic activity played a significant role in the plowed layer,and the soil depth change had no significant impact on the soil enzymatic activity too.(4)The soil microbiological diversity was assayed and analyzed with Medicago Sativa L.after continuous cropping,fallowing after plowing,and rotating with Helianthus annuus L.,Zea mays and sorghum dochna snowden,and the result showed:The top ten advantageous bacteria with the highest relative abundance in different grass land crop rotation modes were Proteobacteria,Actinobacteria,Acidobacteria,Firmicutes,Gemmatimonadetes,Bacteroidetes,Chloroflexi,Verrucomicrobia,Planctomycetes and Thermomicrobia,and the abundance of these bacteria was 79.6-89.7%of all bacteria measured in each sample.Further statistical analysis showed that the relative abundance of Gemmatimonadetes and Verrucomicrobia was significantly improved in Mode AO(p<0.05).Analysis on the relevance between the soil microbiological diversity and soil environmental factors showed that:CAT and AK were respectively,extremely and significantly correlated to Gaiella(p<0.01),and significantly correlated to Sphingomonas(p<0.05).CAT and pH were respectively and significantly correlated to Haliangium(p<0.05),CAT was significantly correlated to Novosphingobium(p<0.05),and AK was significantly correlated to Ramlibater and Opitutus(p<0.05).The soil physical,chemical and enzymatic activity indexes comprehensively showed that:in soil layer 0?20cm under the crop rotation mode AO,the catalase activity was the highest and the pH value decreased the most significantly,they were correlated to the significantly improvement of Haliangium diversity in Mode AO,and it meant the soil microecological environment was significantly improved and the microbiotic community diversity was increased by the crop rotation mode AO.In conclusion,the soil organics and alkali-hydrolyzable nitrogen could be improved more or less by planting different forage grasses Medicago Sativa L?Leymus chinensis?Achnatherum splendens?Onobrychis viciaefolia on the saline-alkali soil,all physical and chemical indexes were significantly improved and the comprehensive soil quality index was highly improved by planting Medicago Sativa L.a research by rotating Medicago Sativa L.with other crops showed that,the soil physical and chemical properties,soil enzymatic activity,and soil microbiological diversity could be improved by rotating Medicago Sativa L.with Helianthus annuus L.,and this mode can be extensively generalized in North Yinchuan,Ningxia. |
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