| Inland wetlands have declined dramatically in the past century in China due to human activities like wetland drainage and agricultural intensification.Soil salinization has become one of the key factors that changing the physiological and ecological characteristics of individual plant,species distribution,structure and ecological functions of vegetation community in the wetlands under the influence of climate change and human activities.The Songnen Plain,as one of the important meadows and saline-sodic area of China,plays an indispensable role in maintaining biodiversity of this region.However,the natural wetlands are facing with the vegetation degradation caused by soil salinization and farmland reclamation at the same time.,As a reservoir for replenishment and renewal of aboveground vegetation in wetlands,soil seed banks play a significant role in restoring plant community in degraded or damaged wetland,maintaining diversity and balance of the ecosystem.In the context of much attention to wetland conservation and restoration,the technique of restoring vegetation by using the soil seed banks in the wetlands that disturbed by agricultural activities is widely applied.To explore the vegetation change in the natural wetlands and recovery potential of degraded wetlands of the western Songnen Plain,we studied the structure and composition of soil seed bank in the natural sedge meadows,soda-saline wetlands,and their adjacent farmed fields,and then further compared the responses of saline-alkaline stress on soil seed banks through soil collection and vegetation investigation in the fields,and seed germination and control experiment in the greenhouse.We studied the characteristic of spatial distribution of soil seed banks in the natural wetlands,the effects of agricultural reclamation and saline-alkaline stress on soil seed banks and their associated potential of restoration vegetation in the degraded wetlands,which provided reference for wetland conservation and restoration.The main results were as follows:(1)Soil environment of natural wetlands in the western Songnen Plain changed significantly on the gradient from non-salinized to severe salinization.From Nenjiang River to Momoge National Nature Reserve,soil salt content increased from 0.39 g/kg to 6.40 g/kg,exchange sodium percentage increased from 0.72%to 49.3%,and soil p H increased from 5.72 to 10.1,all of which increased significantly(p<0.05).while the soil water content,soil organic carbon,total nitrogen,ammonium nitrogen and available phosphorus were significantly decreased at the same direction(p<0.05).The results showed that the soil physicochemical properties deteriorated gradually on the gradient of soil salinization in the natural wetland of the Songnen Plain.At the same time,the structure and species composition of aboveground vegetation changed significantly on the intensified gradient of soil salinization.The dominant species of aboveground vegetation in the natural wetlands changed from Carex spp.to Phragmites australis,and the abundance and species richness of plant community were 504individual/m~2 and 12 species/m~2,respectively,reduced to 148 individual/m~2 and 2species/m~2,both of them showed a significantly decreased trend with the increase of soil salt content,p H and exchange sodium percentage(p<0.05).(2)Soil salinization significantly affected the structure and species composition of soil seed banks in the natural wetlands.The seedlings number and species richness of soil seed banks in the natural wetlands were decreased from 124 individual per pot and16 species per pot to 2 individual per pot and 1 species per pot,respectively,and the similarity index of species composition between soil seed bank and aboveground vegetation decreased from 0.51 to 0 on the gradient of soil salinization in the natural wetlands.The seedlings number,species richness and the similarity of soil seed banks and aboveground vegetation significantly reduced with the increased of soil salt content,p H and exchange sodium percentage(p<0.05).A total of 39 species were germinated from soil seed bank of Carex-dominated community,and 19 species were germinated of Phragmites-dominated in the wetlands,thus the seedlings number and species richness of soil seed banks in Carex-dominated community were higher than in Phragmites-dominated.(3)The main factors affecting the structure and species composition of soil seed bank in natural wetlands were soil physicochemical properties and aboveground vegetation.The species abundance and richness of aboveground vegetation were significantly positively correlated with the seedlings number and species richness of soil seed bank(p<0.05).Soil organic carbon,total nitrogen,total phosphorus,ammonium nitrogen and available phosphorus were positively correlated with the seedlings number and species richness of soil seed bank(p<0.05),while soil bulk density and nitrate nitrogen were negatively correlated with them(p<0.05).The effects of soil salinization on the structure and species composition of soil seed banks were mainly as follows:1)Soil salinization could directly affect the seedlings number and species richness in the soil seed banks by limiting seed germination;2)It also indirectly affect the structure and species composition,as well as the natural regeneration potential of aboveground vegetation by directly affected the nutrient availability of soil and species composition and richness of plant community.(4)Agricultural reclamation resulted in seeds losing that dominated in the natural wetlands,making it difficult to achieve vegetation restoration based on the natural potential of soil seed banks.The seedlings number germinated from soil seed banks of natural Carex-community wetland,dryland and paddy fields were 72,34 and 7individual per pot,respectively,and the species richness were 13,9 and 5 species per pot,respectively.The seedlings number germinated from soil seed banks of natural Phragmites-community wetland,dryland and paddy field were 12,11 and 11 individual per pot,respectively,and the species richness were 5,3 and 7 species per pot,respectively.The results showed that agricultural reclamation significantly reduced the seedlings number and species richness of soil seed bank in the Carex-dominated community wetlands,however,it was not significant in Phragmites-dominated community(p<0.05).But the species composition of soil seed banks in the wetlands were changed after reclamation.Carex schmidtii and Phragmites australis that respectively dominated in the natural sedge meadows and salt marshes were seldom present in the soil seed bank after farming.Instead,other non-wetland species like Echinochloa crusgalli,Typha angustifolia and Draba nemorosa dominated in the farmed fields.Therefore,the dominated species lost in the soil seed banks caused by wetland drainage,agricultural reclamation and other activities,which was the main factor of the difference in the structure and species composition of soil seed bank between natural wetlands and farmed fields.It was difficult to restore aboveground vegetation using soil seed bank after farming in the Songnen Plain.(5)Agricultural reclamation also changed characteristics of soil environmental in the wetland.The soil salt content,soil p H and exchange sodium percentage of dryland and paddy field in Carex-community were 1.87 g/kg and 1.72 g/kg,7.70 and 7.47,4.55%and 2.66%,respectively,and soil water content and soil organic carbon were 29.3%and 17.2%,1.90%and 1.62%,respectively.Thus,agricultural reclamation reduced soil moisture and nutrients and aggravated soil salinization in the Carex-dominated community wetlands(p<0.05).The soil salt content and exchange sodium percentage of dryland and paddy field in Phragmites-community were 5.23 g/kg and 2.76 g/kg,17.6%and 13.3%,respectively,and soil p H and soil water content were 7.68 and 7.24,7.8%and 26.7%,respectively.Therefore,soil salinization degree changed from mild and moderate to moderate and severe salinization,respectively after farming in the Phragmites-dominated community wetlands.Meanwhile,the changed environmental factors caused by farming had a significant impact on seed germination in the soil seed bank.Soil water content,salt content,p H and total nitrogen were the main environmental factors that caused the difference of species composition in the soil seed bank between natural wetlands and farmed fields.Soil salinity,alkalinity,soil moisture and nutrients were abiotic factors by limiting potential of seed germination that affect the structure and species composition of soil seed banks of natural wetlands and farmlands in Carex-and Phragmites-dominated community,respectively.(6)The response of saline-alkaline stress on soil seed banks was related to type,species composition and structure of current plant community.The seedlings number germinated from soil seed banks in the natural Carex-community wetlands under the contrast,mild,moderate and severe saline-alkaline stress conditions were 82,60,32and 18 individual per pot,respectively,and the species richness were 13,11,7 and 7species per pot,respectively.Sedge and forb as the main function groups,dominated in the seedlings germinated from soil seed banks,and both of them were significantly decreased with the increasing saline-alkaline stress(p<0.05).Therefore,saline-alkaline stress significantly affected seed germination of soil seed banks in the natural wetlands dominated by Carex-community(p<0.05),but there were differences in tolerance to saline-alkaline stress among species.About 80%of freshwater species could not germinate under moderate and severe saline-alkaline stress.The seedlings number germinated from soil seed banks in the natural Phragmites-community wetlands under the contrast,mild,moderate and severe saline-alkaline stress conditions were 7,7,3 and 2 individual per pot,respectively,and the species richness were 3,2,1and 1 species per pot,respectively.Forb and grass as the main function groups,dominated in the seedlings germinated from soil seed banks.However,there had no significant effect of saline-alkaline stress on the seed germination of soil seed bank in natural wetlands dominated by Phragmites-community(p<0.05).The results showed that soil seed banks had higher tolerance to saline-alkaline stress and greater potential of vegetation regeneration of natural Phragmites-community wetlands.(7)Saline-alkaline stress limits the regeneration potential of soil seed banks to aboveground vegetation.The average number of seedlings in the soil seed bank of drylands and paddy fields that transformed from natural Carex-community wetlands under four concentrations of saline-alkaline stress were 23.4 and 5.6,23 and 5.4,12.8and 1,5.2 and 0 individual per pot,respectively,and the species richness were 6.6 and4.2,5.8 and 2.6,3.4 and 1,1 and 0 species per pot,respectively.The average number of seedlings in the soil seed bank of drylands and paddy fields that transformed from natural Phragmites-community wetlands were 8.6 and 12.8,3.4 and 2.7,1 and 1.3,1and2.7 individual per pot,respectively,and the species richness were 2.8 and 7.8,1.8 and1.7,0.8 and 1.3,0.8 and 1.7 species per pot,respectively.The results showed that moderate and severe saline-alkaline stress had negative effects on seed germination in soil seed banks of farmed fields,and the seedlings number and species richness were significantly decreased compared with the control group(p<0.05).The seedlings number and species richness in the soil seed banks of farmed fields were not affected in Carex-community(p>0.05),but decreased significantly in Phragmites-community under mild saline-alkaline stress(p<0.05).The species with strong tolerance of saline-alkaline stress,such as Typha angustifolia and Echinochloa crusgalli,dominated in the farmland soil seed banks,so that saline-alkaline stress may affect species composition and distribution of plant communities in the wetland by limiting seed germination in the soil seed banks. |
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