Soil Extracellular Enzyme Activity And Its Influencing Factors In Four Typical Habitats Of Chongming Dongtan Wetland

Soil enzymes play an important role in the process of material cycling and energy flow in wetland ecosystem,and it is closely related to the mineralization and decomposition of organic matter,the cycling of mineral nutrients,energy transfer,and environmental quality.The organic matter needs to be depolymerized by extracellular enzymes of soil to generate small or single molecular organic matter,than microorganisms can absorb and use it,and the subsequent processes of soil carbon,nitrogen,phosphorus and other material cycles can proceed smoothly.Therefore,the step of depolymerization of soil macromolecular organic matter dominated by soil enzymes often becomes the limiting link of soil carbon,nitrogen,and phosphorus cycling.A deep understanding of soil enzymes is of great significance for understanding the biogeochemical cycle process and evaluating soil fertility.In wetland ecosystems,soil extracellular enzyme activities play a central role in the process of biogeochemical cycling.As a well-developed tidal flat wetland,Chongming Dongtan has a relatively complete series of vegetation succession from bare flat to the 98 dam.The physical and chemical characteristics of the sediments are different due to tidal influence,which affects the extracellular enzyme activity and material cycling.Taking the native dominant Scirpus mariqueter and the invasive Spartina alterniflora as objects,we studied the temporal and spatial distribution of extracellular enzymes and the relationship between the extracellular enzymes and environmental factors in the sediments of the tidal flat wetland in the Chongming Dongtan under different plant successional series.The purpose of this study was to investigate the characteristics and functions of extracellular enzymes under different vegetations in the wetland of the tidal flat.The main results were as follows:1.Soil bulk density and pH increased with increasing elevation,while soil water content and salinity decreased.The tide level had significant effects on soil organic carbon（SOC）,total nitrogen（TN）,and total phosphorus（TP）.SOC and TP were higher at low and high tide levels,and significantly higher than those at middle tide level,while TP had no significant difference between the tide levels.2.Soil bulk density and pH were significantly higher under weed habitats and bare flat than those of Spartina alterniflora and Scirpus trilobata habitats,and the soil water content and salinity of Spartina alterniflora and Scirpus mariqueter habitats were significantly higher than those of weed habitats and bare flat.The babitat types had significant effects on SOC,TN,TP.SOC in the habitats of Scirpus trilobata and Spartina alterniflora was higher,and significantly higher than those of weed habitats and bare flat.The soil TN in habitats of Scirpus mariqueter and Spartina alterniflora were significantly higher than that in weed habitat,while there was no significant difference in soil TP.3.There was no significant difference in soil bulk density,water content and salinity among different soil layers,only pH decreased with depth of soil layers,and there was a significant difference among different layers;the vertical profile distribution of SOC,TN and TP were not consistent,in which the SOC and TP in 20-30cm layer were significantly higher than those in 0-10cm and 10-20cm layers,while there were no significant difference in soil TN between soil layers.4.Soil water content and pH were lowest in June and September,respectively,showing a"V"shape trend of low in summer and autumn,while high in spring and winter.There was no difference in salinity in the four seasons;SOC,TN,NH₄⁺-N and NO₃^--N were the highest in March and the lowest in December,showing an inverted"V"shape trend of high in spring and winter,low in summer and autumn,while soil total phosphorus in autumn and winter were extremely significantly higher than those in spring and summer.5.Tide levels,habitat types,soil layers and seasons all had different effects on soil extracellular enzymes related to soil carbon,nitrogen and phosphorus cycling.The general trend of tide level was low<middle<high tide level;habitat types had complex effects on different ecological enzymes,the activities of four carbon enzymes were the lowest under the bare flat,and there was no significant difference between the Spartina alterniflora and Scirpus mariqueter habitats,but they were relatively high under the weed habitat.The activity of nitrogenase was higher in Scirpus mariqueter and Spartina alterniflora habitats.Different soil enzyme activities were different in weed habitats.There were no significant difference in TP between those in habitats.Four types of carbon enzymes and two types of phosphatase decreased trapezoidal with the increase of soil layer,while four types of nitrogen enzymes increased gradually（layer rising phenomenon）.Except for peroxidase,the carboenzymes showed the inverted"V"shape change of low in spring and winter,high in summer and autumn,and N and P related enzymes showed a change trend of high in spring and summer,in low in autumn and winter.6.There were significant differences in soil extracellular enzyme activities between different habitat types and tidal levels,while there were no significant difference in soil extracellular enzyme activities between different soil layers.Tide levels,habitat types and their interactions greatly influenced on the soil extracellular enzyme activities.Soil physical and chemical property,tide levels soil layer and habitat type factors accounted for 23.7%of the variation of soil extracellular enzyme activities,of which 21.1%of soil physical and chemical property factors was the highest.7.Soil N/P,TN,TP,salinity and pH had a great influence on soil enzyme activity.8.The contents of C,N and P in the soil of Dongtan wetland in Chongming were lower than national average.The study area was lack of C,N,but P content was relatively high.Soil C/N and C/P were lower than those of the national averages,while N/P was higher than that of the national average,which further indicated the lack of C,N and the accumulation of P.Tidal levels and habitat types had significantly influenced on the soil C/P and N/P.Soil C/N,N/P did not change significantly in the vertical section.Soil salinities and water contents were positively correlated with soil C/P and N/P,negatively correlated with C/N.Soil C/N/P was significantly different among seasons,C/P and N/P were significantly higher in March than those in December,while C/N was the opposite.9.Nitrogen deposition and phosphorus enrichment increased the availability of soil nutrients N and P,but led to the unbalanced measurement characteristics of soil N and P availability,and increased soil N/P.The increase of CO₂ concentration has a dilution effect on soil N,meanwhile,due to the structure and growth pattern of wetland plants,the carbon sequestration is low,resulting in a low C/N of soil,which leads to the gradual limitation of soilN.Soil C,N,P content and stoichiometry were significantly different between the bare flat and vegetation belt,which to some extent showed that wetland vegetation had a fertile island effect on the spatial distribution of nutrients.Soil N enzyme activity was relatively small and P enzyme activity was large.TN and TP were negatively correlated with N and P related soil enzyme activities.The results showed that nitrogen deposition had little effect on the increase of N related enzyme activities,soil was still deficient in N,while the increase of P related enzyme activities indicated that the soil was limited by P.10.lnBG:ln（NAG+LAP）:lnAP was approximately equal to 3:2:4.There was a certain degree of P limitation in soil nutrient cycling.BG:（NAG+LAP）was 1.28,lower than that of the global average of 1.41;BG:AP was 0.81;（NAG+LAP）:AP was0.70,higher than that of the global average of 0.44.It reflected that C and N were deficient,and soil was limited by P.The tide levels,habitats soil layers,and interaction of three factors all had significant effects on the soil C:N:P enzyme activity ratio,and the single effects of tide level and habitat were greater.TN,NH₄⁺-N,NO₃^--N,pH,and salinity were the deterministic factors affecting ln（BG）:ln（NAG+LAP）:ln（AP）.