Divergence Of Soil Microbial Biomass Carbon And Nitrogen And Its Influencing Factors In Chongming Dongtan Wetland

Soil microorganism is one of the main influencing factors in the wetland ecosystem,which directly participates in various biochemical processes and has an important impact on the transformation and cycling of the soil nutrients.Soil microorganisms transform the component in the organic matter in soil into their own bodies and form the microbial biomass,which serve as the driving force of the organic matter and nutrient cycling,and the storage pool of the plant nutrients in soil.Coastal salt marsh wetlands with variable environmental conditions and complicated relationship between the organisms and environment are strongly influenced by the external physical forces,such as tides,waves and sediment discharge.In the coastal salt marsh of the Dongtan of Chongming Island,apart from the unvegetated site(Us),a variety of vegetation types have been successfully colonized,such as the Spartina alterniflora community(Sa),Phragmites australis community(Pa),Scirpus mariqueter community(Sm)and ruderal community(Rd).So far,the effects and the mechanisms of these communities on the soil nutrients and microorganisms have been still unclear.Therefore,it is necessary to carry out in-depth research,so as to further reveal the soil conditions under the different communities or habitats,and povide a certain theoretical basis for the restoration of native vegetation in this area.Through the investigation and analysis on the soil environment of the five communities or habitats,the main results were as follows:1 Soil p H increased with soil depth,and the soil organic carbon(SOC),total nitrogen(TN),nitrate nitrogen(NO₃^--N),carbon:nitrogen(C/N),microbial biomass carbon(MBC),the ratio of microbial biomass carbon to organic carbon(MBC/SOC)and the ratio of microbial biomass nitrogen to total nitrogen(MBN/TN)decreased with the increase of soil depth,and there were significant differences among soil depths.The ratio of soil microbial biomass carbon to nitrogen(MBC/MBN)was 19.44,indicating that the soil microbial structure was mainly dominated by fungi,but besides MBC/MBN,there were no significant differences in soil salinity,moisture,ammonium nitrogen(NH₄⁺-N)and available phosphorus(AP)among soil depths.2 Different communities or habitats showed complex effects on the soil environmental factors and soil microbial indices.The changes of soil salinity and moisture were consistent,and both showed that the values in Pa>Sa>Sm>Us>Rd;soil p H showed that the values in Rd>Sm>Us>Sa>Pa;the changes of soil NH₄⁺-N and p H were opposite.Soil NO₃^--N content in S.alterniflora community was significantly higher than that in P.australis community and unvegetated site,and the soil AP in the ruderal community was significantly lower than that in the S.alterniflora community,P.australis community and unvegetated site.Soil SOC,TN,MBC and MBN showed the same distribution characteristics,i.e.,the values in Pa>Sa>Sm>Us>Rd,which indicated that there was abundant organic matter and strong nutrient cycling capacity in the soil under the P.australis community,which was beneficial to stabilize and maintain the material cycling of the ecosystem in this area.In the low tidal flat,soil MBC/MBN under the S.alterniflora community was significant higher than those in the P.australis community in the autumn,suggesting that the invasion of S.alterniflora affected the microbial community structure to some extent.Except for the tidal level and season,community or habitat types had significant effects on the soil MBC/SOC.3 Soil salinity,moisture,p H and NH₄⁺-N all decreased with the increase of the tidal level.Soil NO₃^--N,MBC,MBN and MBN/TN were(0.21±0.02)mg/L?(43.67±2.01)mg/L?(2.12±0.01)mg/L and(0.32±0.01)%in the middle tidal flat,respectively,which were significant higher than low and high tidal flat.Soil C/N and MBC/MBN were 9.91±0.25 and 24.58±1.71 in the high tidal flat,which were significant higher than low and middle tidal flat.But soil SOC,TN,AP and MBC/SOC showed no significant differences among the different tidal flats.4 Soil p H,TN,NH₄⁺-N and AP were lower in the spring and higher in the autumn.Soil salinity and soil microbial indices showed the trend of values in the spring>summer>autumn,and showed significant differences among the different seasons.During spring,summer and autumn,soil moisture content changed in a trend of a"V"shape,while soil NO₃^--N changed in a trend of an inverted"V"shape.There was no significant difference in soil SOC among different seasons.5 Among the five variables,including the soil environmental factors,soil depth,habitat types,tidal levels and seasons,soil environmental factors had the highest explanatory degree for the variation degree of the microbial indices,while other factors changed soil environment indirectly,thus affecting the structure and activity of the soil microbial community.Among the soil environmental factors,soil water,salt conditions and p H,as well as their interactions affected the soil microbial indices,which explained the variation of the soil microbial indices more than the soil nutrient indices.Soil salinity,moisture,p H,SOC,TN,AP and C/N were the main factors affecting the soil microbial indices,among which the soil p H was the main factor affecting the MBC,and soil TN had more obvious effects on the MBN,MBC/SOC and MBN/TN.Soil AP was the main influencing factor of the soil MBC/MBN.