The Effects Of Nitrogen Addition On The Growth Of Phragmites Australis In A High Marsh Of The Yellow River Delta

Nitrogen is one of the most important nutrient elements needed by plant growth and reproduction,which is also the limiting factor of coastal wetland ecosystem.Currently,with the increase of nitrogen emission from industrial and agricultural production and atmospheric nitrogen deposition,the nitrogen input in coastal wetlands shows an upward trend year by year.The increase of nitrogen content may result in changes in soil physical and chemical properties,bacterial community structure and plant growth and reproduction traits in coastal wetlands.The Yellow River Delta is an important estuarine-coastal wetland in China,which also encounters the environmental problem of nitrogen enrichment in water and soil.Therefore,under the background of global change,exploration on the influence and mechanism of nitrogen addition on Phragmites australis growth in coastal wetland of the Yellow River Delta can provide theoretical basis for ecological protection in the lower reaches of the Yellow River.In this study,the field control test sample-plot was set up in the P.australis vegetation area of the high marsh,Dawenliu Management Station,Yellow River Delta National Nature Reserve in mid-May,2017.Four nitrogen addition levels(N0:0 g·m^-2·year^-1,N1:5 g·m^-2·year^-1,N2:20 g·m^-2·year^-1,N3:50 g·m^-2·year^-1)were set up in the experiment.Urea(CO(NH₂)₂,containing 46%nitrogen)was added in mid-May and mid-July each year,and the effects of different nitrogen levels on soil physical and chemical properties,soil bacterial community structure and diversity,and P.australis growth were analyzed.The research results mainly include:(1)The experimental study for three consecutive years(2018-2020)showed that most soil physical and chemical properties did not show significant differences among the four nitrogen levels(p>0.05).Except for conductivity and total sulfur in September2018 and ammonium nitrogen in September 2020,there were significant differences among different nitrogen addition levels(p<0.05).However,there were no significant differences in water content,p H value,EC,AP(Availabe P),TP(Total P),NH₄⁺-N?NO₃^--N?AN(Available N)?TN(Total N)?TC(Total C)?TS(Total S)among different nitrogen addition levels in other years(p>0.05).(2)Based on the data analysis in 2018,it was found that there were no significant differences in the number of soil bacterial OTUs and soil bacterial?-diversity index(shannon,simpson,Chao1,ACE,goods coverage)among different nitrogen addition levels(p>0.05).Nitrogen addition had no significant effect on the relative abundance of soil bacterial Phylums,but it had significant effect on the relative abundance of soil bacterial Classes,Orders,Families,Genera and Species.We found that the soil bacterial community structure showed significant differences among nitrogen addition levels by the NMDS and MRPP analysis(p<0.05).Soil physical and chemical properties had no significant influence on the changes of the structure of main Phylum and Class of soil bacteria,but they had significant influence on the changes of the structure of main Orders,Families,Genera and Species.The content of soil TC was the main influencing factor of the structure of the dominant Species of soil bacteria(p<0.1);the content of soil TS was the main influencing factor of the structure of the dominant Orders,Families and Genus of soil bacteria(p<0.1);the content of soil TP was the main influencing factor of the structure of the dominant Genera and Species of soil bacteria(p<0.05).(3)Three-year research shows that the basal diameter,plant height,leaf length,leaf width and other traits that directly reflect the growth and reproduction of P.australis are the highest at N1 or N2 levels.There were no significant differences in P.australis density,leaf number,internode length,leaf TN,leaf TC and leaf TS content among different nitrogen addition levels(p>0.05).(4)The SEM structure model of nitrogen addition level-soil physical and chemical properties-bacterial diversity-P.australis growth was constructed using2018 data.The results showed that nitrogen addition had a direct negative impact on P.australis growth(standardized path coefficient was-0.25,p<0.05);however,nitrogen addition had a significant direct positive effect on soil physical and chemical properties(standardized path coefficient is 0.35,p<0.05).Soil physical and chemical properties had a significant positive impact on bacterial diversity(standardized path coefficient was 0.88,p<0.001).The standardized path coefficient of bacterial diversity on P.australis growth was 0.84 and reached a significant level(p<0.001).Nitrogen addition had no significant effect on bacterial diversity(standardized path coefficient was 0.06,p=0.63).Therefore,in terms of different nitrogen addition levels,the direct effect of nitrogen addition on P.australis growth was negative;however,nitrogen addition indirectly promoted the growth of P.australis by affecting the physical and chemical properties of soil,and then affecting the diversity and structure of bacteria.In a word,nitrogen addition has a direct negative effect on the growth of P.australis,that is,excessive nitrogen addition inhibits the growth of P.australis.However,nitrogen addition can indirectly promote the growth of P.australis,of which the mechanism is that nitrogen addition positively affects the physical and chemical properties of soil,indirectly and positively affects the diversity and structure of bacteria,thus promoting the growth of P.australis to a certain extent.Therefore,nitrogen input should be properly controlled in the future protection of P.australis vegetation in tidal freshwater marshes of the Yellow River Delta.