| At present,soil salinization is a global problem,and China is one of the countries most severely affected by salinization.The agricultural production in saline alkali areas mainly faces the problems of secondary salinization of farmland soil,low fertilizer absorption and utilization efficiency,low organic matter content,and uneven spatial and temporal distribution of irrigation water resources.Aiming at the current situation of soil salinization barriers in Hetao Irrigation District,which restrict crop quality improvement and nutrient efficiency,this thesis studied the fate of nitrogen in fertilizer in the soil plant system through pot experiments,indoor culture experiments,field experiments,and 15 N tracer experiments by using different biomass materials and fertilizer amounts,and identified the impact characteristics of nitrogen application and biomass materials on fertilizer nitrogen utilization,residue,and loss.Based on indicators such as mineralization rate and nitrification rate,the mechanism of biomass materials on nitrogen conversion in saline farmland under different salt environments was analyzed.By exploring the effects of biomass materials,salinity,and other soil characteristics on microbial diversity,community structure,and enzyme activity characteristics,the changes in urease and nitrite reductase activities in saline farmland soil under the application of biomass materials were discussed.By clarifying the effects of biomass materials,salinity,and other soil properties on soil urease and nitrite reductase activities,this thesis revealed the ecological characteristics of nitrogen conversion microorganisms in saline farmland soil and the response of enzyme activity to the application of biomass materials.Based on the changes in soil enzyme activity and microbial community structure after sunflower harvest,this article clarified the response of soil salinity and nutrients to the improvement of saline farmland using biomass materials,as well as the farmland biological effects of biomass materials on promoting the reduction of nitrogen loss in saline farmland soil and regulating the transformation process.Finally,it proposed a regulatory mechanism for nitrogen retention in saline farmland soil and countermeasures for nitrogen fertilizer efficiency management.This article has formed a systematic research achievement on the regulatory mechanism of nitrogen loss and transformation in saline farmland and the ecological effects of biomass materials on farmland.The research results can provide technical support for nitrogen fixation and emission reduction,fertilizer and expenditure saving,high and stable yield,and development and utilization of reserve land resources in saline and alkaline farmland along the Yellow River.The results are as follows.1.The regulatory effect of biomass materials on nitrogen loss and reduction in saline farmlandUnder the condition of not using biomass materials,nitrogen reduction treatment improved nitrogen fertilizer utilization and recovery rate,reduced residual rate and loss rate on mild saline soil.However,on moderate saline soil,nitrogen reduction treatment reduced residual rate and recovery rate,and increased nitrogen fertilizer loss rate.However,on severe saline soil,nitrogen reduction treatment reduced nitrogen fertilizer utilization and residual rate.Under the condition of adding corn straw and applying Biochar,the nitrogen reduction treatment improved the utilization rate and recovery rate of nitrogen fertilizer on mild,moderate and severe saline soil,reduced the residual rate and loss rate on mild,moderate and severe saline soil,and promoted the reduction of residual rate and loss rate on mild and severe saline soil to a significant level.Under the condition of nitrogen reduction,in mild,moderate and severe saline soil,the application of biomass materials significantly improved the utilization and recovery rate of nitrogen,and significantly reduced the soil residual rate and loss rate.In mild,moderate and severe saline soil,the addition of corn straw had a greater promoting effect on the utilization and recovery rate of nitrogen than Biochar.In mild saline soil,the effect of adding corn straw on reducing soil nitrogen residual rate and loss rate was greater than that of Biochar treatment,while in moderate and severe saline soil,the effect of adding corn straw on reducing soil nitrogen residual rate and loss rate was less than that of Biochar treatment.Under the condition of conventional nitrogen application,the addition of corn straw in mild,moderate and severe saline soil has a significant promoting effect on improving the utilization rate and recovery rate of nitrogen.The addition of corn straw in mild,moderate and severe saline soil has a greater effect on reducing the residual rate and loss rate of soil nitrogen than the Biochar treatment.The addition of corn straw in moderate and severe saline soil has a greater effect on reducing the residual rate and loss rate of soil nitrogen than the Biochar treatment.2.The effect of biomass materials on nitrogen transformation in saline farmland soilWithin 0-3 days of cultivation,the nitrogen mineralization rate of various treatments in saline soil of different degrees decreased rapidly,and the decrease rate accelerated with the increase of salinization degree.However,the decrease rate of mineralization rate gradually slowed down within 3-35 days of cultivation.Adding straw and biochar can both increase the mineralization rate of slightly and moderately saline alkali soil in the early stage of cultivation,but it will inhibit the mineralization rate of slightly saline soil in the later stage,and has no significant effect on the mineralization rate of moderately saline soil in the later stage of cultivation.The addition of straw and biochar has a promoting effect on nitrogen mineralization in severely saline soil throughout the entire cultivation period,and biochar treatment has a better nitrogen mineralization effect.Within 0-7 days of cultivation,the nitrogen nitrification rate of mild saline alkali soil gradually increased,followed by a downward trend.Moderately saline soil showed a gradual decrease throughout the entire cultivation period,while the nitrogen nitrification rate of severely saline soil varied with different nitrogen application rates and biomass materials.Adding straw will inhibit nitrification in the whole incubation period of mild and severe salinized soil,and nitrification in the early incubation period of moderate salinized soil,thus increasing the soil ammonium nitrogen fixation.Adding biochar can promote nitrification in mild and severe salinized soil,inhibit nitrification in moderate salinized soil,and increase soil nitrate nitrogen content.Adding straw will inhibit nitrification in the whole incubation period of mild and severe salinized soil,and nitrification in the early incubation period of moderate salinized soil,thus increasing the soil ammonium nitrogen fixation.Adding biochar can promote nitrification in mild and severe salinized soil,inhibit nitrification in moderate salinized soil,and increase soil nitrate nitrogen content.3.Ecological characteristics and enzyme activity of nitrogen conversion microorganisms in saline farmland soil in response to biomass materialsThe bacterial community in mild saline soil without the addition of biomass materials and with the addition of corn straw is similar,forming a significant difference from the bacterial community with the addition of biochar.However,in moderate saline soil,the bacterial community structure with the addition of corn straw and biochar tends to be consistent.Soil nitrate nitrogen and ammonium nitrogen are the main factors affecting the bacterial community structure in mild,moderate,and severe saline soil.In mild saline soil,most biocontrol fungi coexist with a small amount of pathogenic fungi.Corn straw significantly increases the beneficial fungal community in mild saline soil,and soil nitrate nitrogen and p H are the main factors affecting the fungal community.In lightly saline soil,equal amounts of biocontrol fungi coexist with pathogenic fungi.Biomass material treatment significantly increased the beneficial fungal community in lightly saline soil,with nitrate reductase and ammonium nitrogen being the main controlling factors affecting the fungal community.In moderate saline soil,most biocontrol fungi coexist with a very small number of pathogenic fungi,and these fungi have high salt tolerance.Nitrate nitrogen,ammonium nitrogen,and urease are the main controlling factors affecting fungal communities.The effects of different nitrogen application levels and biomass material treatments on fungal species composition were not significant in mild,moderate,and severe saline soils,but had a significant impact on fungal quantity.Compared with fungi,bacteria have a stronger response to different improvement measures in soils with different degrees of salinity.4.Ecological Effects and Control Strategies of Improving Saline Farmland with Biomass MaterialsThe yield of sunflowers treated with three types of biomass materials is N1>N2>N0,among which the yield of sunflowers treated with corn straw and nitrogen reduction is the highest.The interaction between nitrogen application rate and biomass materials has a significant impact on sunflower yield.Under nitrogen reduction conditions,the order of sunflower yield treated with three different biomass materials is C1>C2>C0,while under conventional nitrogen application conditions,the order is C2>C1>C0.The addition of corn straw and nitrogen reduction treatment reduced the content of palmitic acid and linoleic acid,but increased the content of oleic acid,linolenic acid,and protein.After adding biochar,compared with no fertilization,nitrogen reduction and conventional fertilization significantly increased the content of palmitic acid,linoleic acid,and protein,while reducing the content of oleic acid,linolenic acid,and protein. |
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