| In the context of global change,the frequency of extreme rainfall events is increasing.In the past 100 years,the climate change in the Loess Plateau has generally shown a warming and drying trend.In this context,how drought will affect plant growth and the changes in soil microbial characteristics have become the focus of attention and a scientific problem that needs to be solved urgently.This paper selects Syringa oblata,a typical shrub species in the Loess Plateau,as the research object.It uses potted water control experiments for three years of observations and studies 3 water treatments,namely,normal water supply(CK,water content is the field water capacity(FC)75±5%,moderate drought(55±5%FC)and severe drought(35±5%FC))affect plant leaves,stems,very fine roots(0-1mm),fine roots(1-2mm)and thick roots(>2mm)biomass,soil physical and chemical properties,enzyme activity and soil microbial characteristics.The main conclusions are as follows:(1)Compared with the control with normal water supply,moderate and severe drought stress caused the total biomass of Syringa oblata to decrease by 9.65%and 10.00%,the above-ground biomass by 25.81%and 14.29%,and the root biomass by 0.36%and 7.53%.The root-to-shoot ratio increased by 29.79%and 10.30%,and the very fine roots increased by 6.84%under moderate drought stress.It shows that under drought stress,Syringa oblata will distribute more carbon to the root system,especially to the very fine roots,to promote water absorption.(2)Under moderate and severe drought stress,the nitrogen(N)content of each organ of Syringa oblata showed an increasing trend.The leaf N content increased significantly by7.11%and 20.87%,respectively,and the leaf phosphorus(P)content decreased significantly by 38.70%and 41.76.%,but the P content of very fine roots and fine roots tends to increase under moderate drought stress.Leaf C/N decreased significantly by 2.65%and 12.17%,leaf C/P increased by 58.36%and 70.50%,leaf N/P increased by 68.69%and 101.29%.The average N/P of Syringa oblata organs was 3.08,and the growth of Syringa oblata was more severely restricted by N.(3)Under drought stress,soil nutrient content showed an increasing trend.Under moderate and severe drought stress,soil N content increased significantly by 15.61%and8.83%,nitrate nitrogen content increased by 53.53%and 44.16%,ammonium nitrogen content increased by 119.28%and 72.54%,water-soluble carbon(DOC))content increased by 14.61%,12.76%,all reached the maximum under moderate drought stress.SOC/TN,SOC/TP,DOC/AN and DOC/SAP all decreased significantly under drought stress;TN/TP and AN/SAP showed an upward trend,and both reached their maximum under moderate stress.(4)Soil enzyme activity under drought stress showed a trend of first decline and then rise.?-1,4-glucosidase(BG)and L-leucine aminopeptidase(LAP)activities decreased by24.66%and 1.45%under moderate drought stress,and increased by 21.22%and 25.60%under severe drought stress;Alkaline phosphatase(AP)activity decreased by 54.92%and41.68%under moderate and severe drought stress,and?-1,4-acetylglucosaminidase(NAG)enzyme activity increased by 48.39%and 36.44%.RDA analysis shows that soil enzyme activity is inversely proportional to soil nutrient content.SAP,NH4-N,and NO3-N are the main driving factors of soil enzyme activity changes under drought stress.(5)Microbial biomass nitrogen(MBN)and microbial biomass phosphorus(MBP)decreased by 25.54%and 44.64%respectively under moderate drought stress,and by 14.21%and 35.36%.under severe drought stress,MBC/MBN and MBC/MBP increased significantly,and MBN/MBP also showed a downward trend.The activity of C source enzyme BG and N source enzyme NAG+LAP decreased by 24.66%and 0.53%under moderate drought stress,and increased by 21.22%and 25.80%under severe drought stress.The activity of P source enzyme AP decreased by 54.92%and 41.68%under moderate and severe drought stress.ln(BG):ln(NAG+LAP)gradually decreased,while ln(BG):ln(AP)and ln(NAG+LAP):ln(AP)increased significantly.(6)Under drought stress,the bacterial community diversity and richness index showed a gradual increase trend,and reached the maximum value of 6.24(shannon index)and 2593(chao index)under severe drought stress.The dominant bacterial groups in the control group were Actinobacteria,Proteobacteria and Acidobacteria,accounting for about 67.50%of the bacterial community.With the intensification of drought stress,the relative abundance of most bacteria phyla decreased significantly,but the dominant bacterial flora remained unchanged under moderate drought stress.Under severe drought stress,the relative abundance of the cyanobacteria phylum increased by 1877.57%,and the cyanobacteria became the dominant flora.RDA analysis shows that SOC,TN,TP and NO3-N are the main driving factors of bacterial communities.(7)Under drought stress,the diversity and richness index of the fungal community showed a trend of increasing first and then decreasing,reaching the maximum value of 4.02(shannon index)and 367.10(chao index)under moderate stress.The dominant flora in the control group were Ascomycota,Basidiomycota and Zygomycota,accounting for 77.19%of the fungal community.With the intensification of drought stress,the relative abundance of Ascomycota,Glomeromycota and Chytridomycota showed an increasing trend,but the dominant flora remained unchanged under moderate drought stress.Under severe drought stress,the relative abundance of Zygomycota decreased by 95.95%,and the Glomus phylum became the dominant flora.RDA analysis showed that SOC,CH4-N,and NO3-N were the main driving factors of fungal community.The above results indicate that,moderate drought stress can help increase the species diversity and richness of soil bacterial and fungal communities.Under severe drought stress,bacteria are more adaptable than fungi.For cloves,moderate drought stress(55+5%FC)may be more conducive to accumulating vegetation biomass,improving the availability of soil nutrients,enhancing enzyme activity,and increasing the diversity and richness of soil microbial communities. |
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