| As farmers generally accept the efficient and rapid performance of chemical fertilizer in increasing crop yield,the irrational application of nitrogen fertilizer in many fields is widespread in China.This not only reduces soil fertility,but also affects soil microbial environment.Based on long-term field experiments,this study adopted the experimental design of chemical nitrogen fertilizer and organic fertilizer combined application.Nitrogen fertilizer was the main treatment,and three nitrogen levels（N0,N1,N2）were set,and the nitrogen application rate was 0,150,300 kg N ha-1,respectively.Organic fertilizer was used as a side treatment,and two levels（-M,+M）were set.The organic fertilizer was cow dung compost,and the dosage was 30 t ha-1.Two growth cycles in dryland wheat farmland soil organic inorganic fertilizer was studied in greenhouse gas emissions,the influence of the physical and chemical properties,enzyme activity,and calculated the agricultural border carbon footprint,soil chemical metrology characteristics of extracellular enzymes,after further to clarify organic fertilizer into the soil environmental change,to determine the best ratio of organic and inorganic fertilizer,to provide theoretical basis for reducing greenhouse gas emissions.The main results are as follows:（1）The contents of soil organic carbon（SOC）,available phosphorus（AP）and soil microbial biomass carbon and nitrogen（MBC and MBN）did not change significantly under different nitrogen application levels,while the contents of total nitrogen（TN）and soil ammonium nitrate nitrogen increased.The contents of SOC,TN and AP were significantly higher than those of single fertilizer application,and the content of nitrate nitrogen was significantly increased,while the content of ammonium nitrogen was not significantly changed.After combined application of organic fertilizer,the average increment of soil SOC and TN was more than 30%,and the average increment of soil nitrate and ammonium nitrogen was more than 15%.There was no significant difference in soil physical properties with the increase of nitrogen application level.After applying organic fertilizer,soil bulk density decreased.Field water capacity,soil porosity,water content,soil pore water content and saturated water conductivity increased.（2）Soil urease,invertase and catalase activities increased during the 2017-2018growing season after increasing nitrogen application rate.However,there was no significant difference in the 2018-2019 growing season;The increment of enzyme activity was between 1%and 16%after combined application of organic fertilizer,which was not significantly different from that of single application of chemical fertilizer.The extracellular enzymeβ-glucosidase（βG）and fibrinogen（CBH）activities of C were significantly increased with the increase of nitrogen fertilizer dosage.TheβG activity of organic fertilizer was higher than that of chemical fertilizer alone.The CBH activity of adding organic fertilizer was lower than that of fertilizer alone at tillering stage and filling stage,but it was opposite at jointing stage and maturity stage.Extracellular enzyme acetyl-glucosaminidase（NAG）and extracellular enzyme alkaline phosphatase（AKP）were obtained from N,and the activities of these two enzymes were also significantly increased with the increase of nitrogen application rate.There was no significant difference in the activities of these two enzymes under the combined application of organic and inorganic fertilizers and the single application of chemical fertilizers.（3）Soil enzyme activities can reflect the relationship between microbial nutrient requirements and environmental nutrient availability.The results of this study showed that soil LAP enzyme activity was decreased and the activities of soil carbon,nitrogen and phosphorus cycles were not significantly increased under the combination of organic and inorganic fertilizers in dryland soil.The activity of carbon acquisition enzyme was negatively correlated with nitrogen acquisition enzyme,but positively correlated with phosphorus acquisition enzyme.With the advance of crop growth period,the limiting effect of microbial C and P on microbial metabolism became stronger and stronger.Redundancy analysis（RDA）further revealed that soil total nitrogen and microbial biomass nitrogen contents were the main factors affecting soil enzyme activity,which could better explain the variation characteristics of soil enzyme activity.（4）In the single fertilizer application,the CH4 absorption flux and CO2 emission flux did not increase significantly with the increase of nitrogen application level.N2O emission flux and ammonia volatilization increased with the increase of nitrogen application level.After combined application of organic and inorganic fertilizer,N2O emission flux was higher than that of chemical fertilizer alone,and ammonia volatilization only increased significantly at N0 level.After the application of organic fertilizer,the average increment of soil greenhouse gases CO2 and N2O emissions was more than 30%,while the average increment of CH4 emissions was less than-15%.The results showed that organic fertilizer could increase the content of soil available nitrogen,improve soil fertility and reduce the emission of greenhouse gas CH4.（5）The greenhouse gas warming potential（GWP）increased with the increase ofnitrogen application level.The emission intensity（GHGI）at N0 level was higher with the addition of organic fertilizer.The total carbon footprint（CF）of agriculture increased with the increase of nitrogen application rate.N0+M treatment had the highest carbon footprint per unit yield.With the increase of chemical nitrogen application level,the carbon footprint of unit yield decreased.The increase of total carbon footprint and carbon footprint per unit output decreased significantly.Correlation analysis shows that CO2 and CH4 emissions are significantly negatively correlated,and N2O emissions are significantly positively correlated,while CH4 emissions are not significantly correlated with N2O emissions.The indirect path coefficient analysis shows that ammonium nitrogen,AKP and MBN have great direct effects on N2O emission flux,while other indirect effects are small and cannot be considered.In conclusion,with the increase of nitrogen application level,soil greenhouse gas N2O and ammonium nitrate nitrogen increased significantly,while total organic carbon,total nitrogen,greenhouse gas CO2 and CH4 emission fluxes did not change significantly.Soil enzyme activity did not show a certain increase trend.After combined application of organic fertilizer,N2O,total organic carbon,total nitrogen and ammonium nitrate nitrogen increased significantly.The activities of enzymes related to carbon and nitrogen cycling were also increased.The results showed that the application of organic fertilizer provided soil microorganisms with macronutrients and substrate for soil enzyme activities,which increased the sensitivity of these indexes and enzyme activities to nitrogen levels.Reasonable combination of organic and inorganic application can reduce greenhouse gas emission flux while ensuring yield increase.At N1 level,the effect of organic fertilizer application was the best. |
