| Sediment microorganism is the main participants in the material cycle and energy flow in the wetland ecosystem.Sediment microbial nutrient limitation is the important index for measuring and judging wetland ecosystem nutrient status.In-depth study on the sediment microorganism will help to understand the structure and function of wetland ecosystem and its evolution process and influencing factors more clearly.In the context of increasing human disturbance and global change,such research shows important scientific significance and application value.In this study,the microbial nutrient limitation of sediments in Dongtan wetland of Chongming Island was evaluated by analyzing the activities of extracellular enzymes and their stoichiometric characteristics as well as their controlling factors.In this paper,sediments at different depths of 5 major habitat types at high,middle and low tidal flat levels in 4 seasons.We focused on several major habitat types of physical and chemical indicators of sediment and 11 types of sediment extracellular enzyme activity relating to carbon,nitrogen,phosphorus and sulfur cycle,and their relevant stoichiometry indexes were used to analyze the main controlling factors of sediment enzyme activities（EEAs）and microbial nutrient limitation changes.The variation characteristics of sediment nutrient status under different tidal flat,community or habitat conditions were discussed,and the effects on microbial nutrient limitation in wetland sediment were also discussed,so as to identify habitat types that could alleviate nutrient limitation.Finally,the differences of sediment microbial nutrient limitation under different sediment depths of different tidal flats,communities or habitats were evaluated,in order to provide a certain scientific basis for the sustainable development of wetland ecosystems and wetland habitat restoration or planting methods.The results of the study might be of great significance in-depth understanding of the microbial nutrient limitation characteristics of Dongtan wetland sediments in Chongming Island,and might also provide certain technical support for ecological conservation,restoration and management practice in this area.The main results were as follows:1.Based on the multi-response displacement method（MRPP）,the characteristics and main controlling factors of sediment enzyme activities in the Dongtan wetland of Chongming Island were comprehensively analyzed.The results showed that the sediment depth had no significant effect on sediment enzyme activities,while the habitat type and tidal flat level had significant effects on sediment enzyme activities.The physical and chemical properties of sediments,sediment depth,tidal flat level and habitat type together explained 28.2%of the variable of enzyme activity in sediments.The physical and chemical properties of sediment explained 17.5%of the sediment enzyme activity variable（EEAs）.Sediment depth,tidal flat level and habitat type mainly affected sediment enzyme activities by changing sediment physical and chemical properties,and these three variables explained 3.6%of the variation in EEAs.Sediment NH4+-N,C/N and BD were the most important factors affecting EEAs.This study quantified the effects of sediment depth,habitat type,tidal flat and sediment physical and chemical properties on sediment EEAs changes at a small regional scale in Dongtan wetland of Chongming Island.The results showed that the physical and chemical properties of sediments in the small-scale habitat could explain the main reason for the variation of sediment enzyme activities.2.Based on the activities of 11 types of enzymes and 5 stoichiometric indexes of enzyme activity of the wetland sediment,the main controlling factors and the variation characteristics were analyzed comprehensively.According to the variance partitioning analysis,physical and chemical indexes,habitat and season explained 41.8%of the sediment enzyme activities and stoichiometric variation,among which physical and chemical indexes explained 32.3%of the sediment enzyme activities and stoichiometric variation,and seasonal factors explained 29.1%of the sediment enzyme activities and stoichiometric variation.Habitat factors explained 9.8%of the sediment enzyme activities and stoichiometric variation.There were interactions between these three groups of variables.When the factors such as tidal flat,sediment depth and sediment physical and chemical indexes were integrated together for variance partitioning,the results changed.The physical and chemical indexes,tidal flat,sediment depth,habitat and season explained 45.3%of the variation of sediment enzyme activities and stoichiometric variation,but only increased by 3.5%.Through t value test,it was found that pH,ammonia nitrogen,nitrate nitrogen and total organic carbon content had a high proportion of explanation on the sediment enzyme activity and stoichiometry.Therefore,it was concluded that tidal flat and sediment depth had no significant effect on enzyme activities and stoichiometry,while season had significant effect on enzyme activities.Tidal flat,sediment depth,habitat,season and sediment attributes together explained49.6%of the variation in enzyme activities and stoichiometry.The physical and chemical properties of sediments were the main factors affecting the change of enzyme activities,accounting for 32.3%.Sediment pH,total organic carbon and inorganic nitrogen（ammonia nitrogen and nitrate nitrogen）were the main controlling factors affecting the sediment enzyme activities and stoichiometric variations.The results showed that in the small spatial scale of Dongtan wetland of Chongming Island,the sediment enzyme activities were mainly affected by the physical and chemical properties of sediments.At present,the main controlling factors of the sediment extracellular enzyme activity and stoichiometry were not well studied,especially in the regions with neutral and alkaline sediment.This study provided new results for the related research in this field.At the same time,this study also quantified the influence of tidal flat,sediment depth,habitat,season and sediment attributes on the changes of sediment enzyme activities and stoichiometry in Chongming Island for the first time,which would have important reference value for the study of sediment enzyme activities and stoichiometry in this area.3.In the study of P.australis communities distributed in the high,middle and low tidal flats and sediments in unvegetated habitats in different seasons,it was found that the sediment nutrients in the low,middle and high tidal flats were mainly limited by microbial carbon and phosphorus,but weakly limited by the microbial nitrogen.There were significant differences in the patterns of microbial carbon and phosphorus limitation in wetland sediments under different tidal flat levels in the unvegetated habitats and P.australis communities（P<0.05）.These results indicated that tidal flat level changed nutrient resource limitation patterns of wetland sediment microorganisms.The results elucidated the general rule and the related mechanism of microbial nutrient limitation of P.australis community and unvegetated habitat in Dongtan wetland with changes of tidal flat level,i.e.,the change of sediment calcium content in different tidal flat levels under unvegetated habitat and P.australis community might lead to the change of sediment available phosphorus content.This resulted in the difference of microbial phosphorus limitation in sediments at different habitat types and tidal flat levels.Under different tidal flats,sediment C and P were the most limitated in December,and the lowest in June,and there was an interaction effect between microbial nutrient limitation in tidal flats and quarterly sediments.4.In the study of the sediments of P.australis community,S.mariqueter community and S.alterniflora community,it was found that the enzyme activities of sediments in different community types were different,and the sediment enzyme activities were closely related to the physical and chemical properties of sediments.The results of stoichiometry of enzyme activities in sediments of different community types showed that the degree of microbial carbon limitation in sediments of different community types was different.The nitrogen and phosphorus limits of sediment microorganisms were higher in P.australis and S.alterniflora communities.The degree of microbial carbon limitation in sediments was related to the content of organic matter in sediments,and the increase of organic matter content was the main reason to alleviate the microbial carbon limitation in sediments.The increase of sediment C/N ratio and C/P ratio was the main reason for the enhancement of microbial N and P limitation in wetland sediments.The microbial carbon and phosphorus limitations in December were relatively serious,while the microbial carbon and phosphorus limitations in March were low.The results of this study indicated that community types changed the pattern of microbial nutrient limitation in wetland sediments,and also affected the degree or state of microbial nutrient limitation in wetland sediments.5.In the study of sediments at different depths（0-5 cm,5-10 cm,10-15 cm,15-20cm,20-25 cm and 25-30 cm）,it was found that sediments at different depths were weakly limited by microbial nitrogen,and sediments at different depths were jointly limited by microbial carbon and phosphorus.Within 0-30 cm sediment depth,15-20 cm sediment had the greatest microbial carbon limitation（P<0.05）,the microbial carbon limitation of sediments deeper than 20 cm was weak.The microbial phosphorus limitation in the deeper sediments was relatively low.Microbial phosphorus limitation was significant in surface sediments.This suggested that although sediment depth changed the availability of nitrogen in the sediment,it did not alter microbial N limitation patterns.The results of this study showed that sediment depth changed the state of microbial nutrient limitation in wetland sediments,and microbial nutrient limitation characteristic were a very effective nutrient indicator for sediments. |
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