Effect Of Different N Addition Forms On Soil Microorganisms Under Larix Olgensis Forests

With increasing industrialisation and urbanisation,environmental issues such as atmospheric nitrogen deposition have become one of the world’s most worrying problems.Nitrogen deposition can further affect soil nutrient health by altering soil physicochemical properties,soil enzyme activity,soil microbial biomass and community composition diversity,but the results of current studies on soil health response to nitrogen deposition are inconsistent due to different forms,different levels of nitrogen addition and forest type.In this study,the physicochemical properties and enzymatic activities of soils from a larch（Larix olgensis）understorey on the campus of Jilin Agricultural University were determined by simulating atmospheric nitrogen deposition in the field,with three levels(0,20 and 40 Kg N·hm^-2yr^-1)and three forms（CO（NH₂）₂,Na NO₃and NH₄Cl）applied for four consecutive years.The soil microbial community structure and diversity were analysed,and the response of soil microorganisms to different forms and levels of N additions was explored to investigate the coupling relationship between soil microorganisms and soil factors.This study provides data to further elucidate the response mechanism of soil nutrient cycling to nitrogen deposition in boreal forests under the background of global environmental change.The results of the study are as follows:1.All treatments with different forms of N addition decreased soil p H and caused an overall decreasing trend in TC content,an increasing trend in TN content,and an increase in NO₃^--N and NH₄⁺-N content,with little effect on TP content,and all high N additions had a more significant effect than low N additions.The effect of different forms of N additions on the 0-10cm soil layer was greater,with higher soil water content,p H,TC,TN,TP,NO₃^--N and NH₄⁺-N content in the0-10cm layer than in the 10-20cm and 20-40cm layers.2.The addition of different forms of nitrogen had a significant effect on the activity of urease and acid phosphatase only.The addition of different forms of N promoted the activities of urease,acid phosphatase and N-acetylaminoglucosidase and increased significantly with the addition of N.The activity of leucine aminopeptidase was inhibited and decreased significantly with the addition of N.Of the three forms of N,ammonium N had the greatest effect on soil enzyme activity.The results of the redundancy analysis showed that the main environmental factor affecting enzyme activity was NH₄⁺-N.All four enzyme activities were highest in the 0-10 cm soil layer.3.The different forms of N addition had a significant effect on soil microbial load carbon,N and P.The different levels of N addition significantly reduced the MBC content and increased the MBP content,with an overall trend of low level promotion and high level inhibition of the MBN content.The three forms of N additions only had a significant effect on the increase in MBP content,with ammonium N additions having the strongest effect,and MBC,MBN and MBP content all reached their highest levels in the 0-10 soil layer,decreasing with increasing soil depth.4.The addition of different forms of nitrogen increased bacterial community diversity,but did not have a consistent effect on bacterial community richness,with the addition of nitrate nitrogen suppressing bacterial community richness.The addition of nitrate nitrogen increased the diversity and abundance of the fungal community,while the addition of organic and ammonium nitrogen decreased the diversity and abundance of the fungal community.Bacteria were more sensitive than fungi to the sensing of nitrogen deposition.5.Nitrogen addition significantly increased the relative abundance of the dominant soil bacterial taxa in the phyla Aspergillus and Actinomycetes,and decreased the relative abundance of the phylum Greenbacter.Nitrogen addition significantly increased the relative abundance of Ascomycota,the dominant group of fungi,which are all beneficial microorganisms in the soil.This suggests that nitrogen addition increases microbial community diversity,increases soil microbial biomass,makes soil microorganisms more active and thus changes the health of the soil.In summary,nitrogen addition reduced soil p H,increased the effectiveness of nitrogen in the soil,increased soil enzyme activity,improved soil properties,moved soil health in a favourable direction,promoted beneficial microbial activity and made the soil more suitable for the growth of the acid-loving tree species,Larix olgensis larch.This suggests that N addition favours the development of soil microbial communities and the maintenance of soil nutrient status,and has a positive impact on the soil nutrient status of Larix olgensis larch.The results of this study may provide an important scientific basis for the adaptive management of forest ecosystems in the context of global nitrogen deposition.