Research Of Combined Application Organic And Inorganic Nitrogen On The Nitrogen Transformation And Yield Regulation Mechanism

Water scarcity,soil salinization and excessive application of inorganic nitrogen fertilizers are major constraints on the sustainable development of the Loop irrigation area.Therefore,under the condition of water restriction,saline irrigation areas need to seek a suitable fertilization mode to promote grain production in saline farmland,improve water and nitrogen use efficiency and reduce nitrogen non-point source pollution.In this study,nitrogen was converted into the main line,and the mechanism of"yield increase and emission reduction"of organic and inorganic nitrogen in saline maize farmland in Hetao irrigation area was revealed.Through indoor mineralization experiments,the nitrogen mineralization law of organic and inorganic nitrogen co-application in different degrees of salinization soil was revealed.Through field experiments,the response of maize yield,water and nitrogen conversion,nitrogen-related functional microbial abundance and function to organic and inorganic nitrogen compounding in light and moderate salinized farmland was explored,and a reasonable organic and inorganic nitrogen mixing mode was proposed.Finally,combined with the DNDC model,the optimal management measures for salinized maize farmland were determined.The main findings are as follows:（1）The response of soil nitrogen mineralization process to the degree of salinization and the combined application of organic and inorganic nitrogen was clarified.Increasing salinization significantly inhibits soil nitrogen mineralization.At low soil salinity(0.46 d S·m^-1),organic nitrogen replaced 50%inorganic nitrogen to produce a smooth nitrogen mineralization process during culture,and the mineral nitrogen content was also at a high level.The soil salt was divided into 0.98 d S·m^-1,and the amount of nitrogen mineralization in the early stage of the culture was small in the single application of organic nitrogen,and the amount of nitrogen mineralization increased in the late culture.As the salinity level rose above 1.55 d S·m^-1,the amount of soil mineralization caused by the application of organic nitrogen was significantly reduced and the mineralization cycle was significantly prolonged,while the amount of nitrogen mineralization in the single application of inorganic nitrogen was significantly higher than that of other treatments（P<0.05）.（2）The effects of combined application of organic and inorganic nitrogen on the conversion of water nitrogen in mild and moderate saline soils were discussed,and the mechanism of increased production of organic and inorganic nitrogen was revealed.Compared with the single application of inorganic nitrogen,the combined application of more than 50%organic nitrogen can significantly improve the soil storage capacity of the tillage layer and the mineral nitrogen content of the tillage layer after the fertilization period（P<0.05）,and promote the absorption and utilization of water and nitrogen in the deep soil by the maize root system,thereby improving the yield and water and nitrogen use efficiency of maize.The water use efficiency of light and moderate saline soils was greatest instead of 50%inorganic nitrogen and 100%inorganic nitrogen instead of organic nitrogen,which were increased by 11.84%-17.27%and 14.93%-30.90%（P<0.05）respectively compared with single application of inorganic nitrogen.At the same time,the yield,plant nitrogen uptake,nitrogen harvest index,nitrogen recovery rate in the current season,nitrogen fertilizer partial productivity and nitrogen fertilizer agronomic efficiency are also higher.（3）The responses of light and moderate saline soil ammonia volatilization,N₂O emissions and nitrate nitrogen leaching to organic and inorganic nitrogen application in the Hetao irrigation area were clarified.The accumulation of ammonia volatilization in the medium saline soils was increased by 16.42%to 38.32%（3-year average）compared with the mild saline soils.Mild saline soils with organic and inorganic nitrogen compounding ammonia volatilization losses were minimal,significantly lower than the rest of the nitrogen treatment（P<0.05）,while moderate saline soils with more than 50%organic nitrogen can significantly reduce soil ammonia volatilization（P<0.05）.The total N₂O emissions of moderate saline soils were 11.86%-47.23%higher than those of mild saline soils（P<0.05）.The emissions of organic nitrogen N₂O were minimally reduced by each semi-combined application of organic and inorganic nitrogen and single application of organic nitrogen in light and moderate saline soils,respectively,which were significantly reduced by 26.37%and 35.59%（2-year average,P<0.05）compared with single-application inorganic nitrogen treatment.Correlation analysis showed that soil N₂O emissions were significantly positively correlated with NH₄⁺-N,but not significantly related to soil NO₃^--N,indicating that soil N₂O emissions were mainly generated during soil nitrification.The amount of nitrate nitrogen leaching in moderate saline soil was 36.93%-64.84%higher than that in mildly saline soil（3-year average,P<0.05）,and the combined application of organic nitrogen could increase the soil nitrogen retention,which could significantly reduce the amount of soil nitrate leaching,and the amount of organic nitrogen nitrate leaching loss in light and moderate saline soil was reduced by 39.70%and 52.69%（3-year average）respectively compared with the single application of inorganic nitrogen treatment.（4）In order to better understand the response mechanism of nitrogen conversion to organic and inorganic nitrogen application,the effects of organic and inorganic nitrogen combination on the microbial abundance and function related to nitrogen conversion in light and moderate saline soils were discussed.From the perspective of soil microbial biomass and microbial activity,light and moderate saline soils were superior to organic and inorganic nitrogen and single application of organic nitrogen,and the soil microbial biomass carbon,microbial biomass nitrogen and soil respiration were increased by 48.44%,42.50%,31.74%,68.07%,48.99%and 45.19%（2-year average）respectively compared with the single application of inorganic nitrogen treatment.Correlation analysis showed that the soil mineral nitrogen content was significantly positively correlated with the soil microbial biomass carbon nitrogen and soil respiration（P<0.05）,indicating that the combined application of organic fertilizer could improve the soil microbial biomass and activity,and more nitrogen could be stored and recycled for crop absorption and utilization.In mildly saline soil,the nitrification potential of each semi-compound application of organic and inorganic nitrogen was the largest,and the increase in inorganic nitrogen increased by 18.59%and 15.87%,respectively,which was mainly due to the significant improvement of gene abundance of ammonia oxidizing bacteria under this treatment.The various nitrogen application treatments in moderate saline soil had no significant effect on soil nitrification potential,because the high salinity reduced the activity of soil nitrous oxide reductase and inhibited the conversion of NO₂^--N to NO₃^--N.Correlation analysis showed that compared with single application of inorganic nitrogen,the combined application of organic nitrogen significantly improved the abundance of soil ammonia oxidation bacteria and increased the soil nitrification rate,indicating that the combined application of organic nitrogen reduced soil ammonia volatilization and N₂O emissions were mainly attributed to the increase in ammonia oxidation bacterial activity and soil nitrification rate.Compound application of organic nitrogen increases soil denitrification gene abundance and denitrification capacity（nir K and nos Z）without causing significant soil nitrogen loss.（5）The effects of different management practices on maize yield and nitrogen loss were studied by DNDC model simulation,and the optimal organic and inorganic nitrogen compounding mode in irrigation area was determined.The calibration ground model can better simulate crop yield（standard rms error less than 5%）,ammonia volatilization loss,N₂O emissions and nitrate nitrogen leaching loss（standard rms error less than 15%）.In addition,the model was used to simulate and evaluate the yield,N₂O emissions and nitrate nitrogen leaching amount of maize in different scenarios,and found that under the nitrogen application rate of 240 kg·hm^-2,with the increase of the proportion of organic nitrogen application,the yield of maize,NH₃ volatilization and N₂O emissions showed a trend of first rising and then falling,and the nitrate nitrogen leaching loss showed a gradual decrease trend.On the whole,when the organic and inorganic nitrogen mixing ratio was 3:2,the crop yield reached the highest value(12578 kg·hm^-2),the NH₃ volatilization(11.38 kg·hm^-2),the N₂O emission(4.49 kg·hm^-2)and the nitrate nitrogen leaching loss(15.7 kg·hm^-2)were also acceptable,which could be determined as the optimal organic-inorganic nitrogen compounding mode for mild saline soils.