Transformation And Migration Of Nitrogen In Farmland Soils During Freezing And Thawing

Farmland nitrogen is one of the most important pollution sources of the agricultural ecological environment.A large amount of nitrogen in agricultural production will not only inhibit the utilization rate of nitrogen fertilizer,but also affect the health of the underground ecosystem.The Sanjiang Plain is an important commercial grain base in China.The long-term agricultural development has led to the application of nitrogen fertilizer in some irrigated areas being higher than the demand for nitrogen by crops.In addition,the plain is located in a seasonally frozen soil area,and the freeze-thaw period of more than 6 months per year will directly affect the Change the physical and chemical properties and biological properties of soil and then affect the content and occurrence form of soil nitrogen.Relying on the Sanjiang Connectivity Project,this paper took the Puyang Irrigation Area of the Sanjiang Plain as an example,and systematically analyzes the effect of freezing and thawing in soil on the migration and transformation of nitrogen.This research can improve the efficiency of nitrogen fertilizer use in farmland and promote the sustainable development of agriculture.Provide scientific theoretical basis.This paper firstly obtained the overall characteristics of nitrogen pollution in soil and groundwater in Puyang irrigation area through field experiments and statistical analysis.Then,the indoor batch experiment was used to simulate the transformation process of soil nitrogen under freeze-thaw conditions,and the soil denitrifying bacteria and ammonia oxidizing archaea were detected in each experimental process.Factors influencing mineralization,ammonification and denitrification were analyzed using multivariate variance analysis.The dynamic soil column experiment was used to simulate the nitrogen migration process during the melting period.The nitrogen exchange and transport capacity in the soil under thawed conditions was quantitatively calculated using hydrus1 D.The specific conclusions were as follows:(1)Field monitoring data showed that the denitrifying bacteria involved in soil nitrogen transformation in the study area were mainly Proteus and a small amount of Actinomycetes.Ammonia-oxidizing archaea mainly included odd archaea and a small amount of archaea.Freeze-thaw increased the content of dominant populations such as Proteobacteria and Omoarcha,thereby enhancing nitrification and denitrification.The effective pore diameter of shallow soil increases under the influence of temperature.During the thawing period,soil nitrogen would migrate and transform under the action of freezing and thawing,and the nitrogen content would decrease with the increase of depth.The chemical characteristics of groundwater further confirmed that the nitrate nitrogen in it mainly comes from the soil.leaching,water-rock interaction,and ammonia nitrogen comes from the transformation of nitrogen.(2)The indoor freeze-thaw batch test showed that under the freezing conditions of continuous freezing or intermittent thawing,the soil ammonia nitrogen showed a downward trend,and the nitrite nitrogen content was low and changed little.The content of nitrate nitrogen showed a trend of increasing first and then decreasing,mainly due to the reproductive activity of microorganisms when it was not completely frozen,and the effect of microorganisms gradually weakened with the increase of freezing time in the later stage.The denitrifying bacteria in the laboratory experiment were mainly Proteus,a small amount of Green Bay bacteria and Firmicutes.The species diversity and distribution uniformity of denitrifying bacteria increased,resulting in a gradual weakening of the net denitrification rate.The rates of net mineralization,net ammoniation and net denitrification in the later stage tended to be controlled stably by soil nitrogen content,indicating that the existing form and content of nitrogen were the main controlling factors of soil ammoniation,denitrification and mineralization.Control experiments showed that the nitrogen content of frozen-thawed soil was slightly lower than that of frozen soil samples,indicating that continuous freezing would inhibit the activity of soil nitrogen-transforming bacteria,and the type of irrigation water had little effect on it.(3)The simulation results of nitrogen migration during thawing process showed that thawing promoted the migration of soil nitrogen.Nitrate nitrogen would eventually approach 0 as melting proceeds.The initial migration amount of ammonia nitrogen was high due to the high initial soil content,but as the melting progressed,its migration amount would decrease,but it would not drop to 0 like nitrate nitrogen,and could be maintained at about 1 mg/L.For nitrite nitrogen could only be detected at the effluent end in the initial stage of melting,indicating that the initial stage of melting is the main period for the diffusion of nitrite nitrogen pollution;the peak value of nitrite nitrogen in the initial effluent indicated that the transformation intensity was the highest when soil nitrogen is dominated by nitrate nitrogen.It should be dominated by denitrification,and the spatial variation characteristics of bacteria in the denitrification system further reflect that the transformation site was more concentrated in the deep soil layer,which would lead to a significant decrease in effluent p H.The nitrification and denitrification coefficients in different initial nitrogen form bars were retrieved using Hydras-1D.During the melting period,the nitrification intensities of the potassium nitrate column and the fertilizer column were consistent,and the denitrification intensities of the ammonium chloride column and the fertilizer column were consistent.The nitrification effect of the ammonium chloride column was the strongest,and the denitrification effect of the potassium nitrate column was the strongest,which was considered to be caused by the initial nitrogen form.