| Reservoir shorelines and riparian wetlands are the important barriers for excess nitrogen to input the water.Soil denitrification is the main driving force for removing nitrogen from wetlands.Soil conditions such as soil moisture,soil nitrogen and carbon content are closely related to the denitrification rate.Biotic factors including vegetation features and denitrifying communities also have a significant effect on the denitrification rate.At the beginning of the 21st century,more than 50000 dams have been built on the rivers all over the world,nearly 22000 of which were located in China.Large dams blocked the connectivity of the river,leading changes in the river flow which is the determined factor of the plant diversity and community structure.Hydropower station constructed on the rivers caused changes in vegetation characteristics of reservoir shorelines by altering the hydrologic regime,propagation process and geomorphic feature.The Han River basin is highly development in hydro-engineering project.While it is facing ecological problems such as pollution from agricultural,deterioration of water quality,and increased sediment in recent years.The concentration of total nitrogen in water of Danjiangkou Reservoir,which is the water source area for the middle rout of the South-to-North Water Transfer Projec,is exceeded the water standard II required by the water transfer project?TN<0.5mg/L?.In this study,we surveyed the reservoir shorelines and riparian wetlands in the upper and middle reaches of the Han River.We investigated the differences in vegetation characteristics between the twe types of Riparian,and researched the relationship between environmental factors such as vegetation characteristics and soil denitrification.Moreover,we explored the response of soil denitrification rate to vegetation restoration.Our s purposes are improving the theoretical basis for restoring the riparian vegetation and enhancing the nitrogen removal capacity of the wetland along the Han River.The main results are as follows:We seasonally surveyed in 5 reservoir shorelines and 15 riparian wetlands along the Han River,aiming to compare the species diversity and composition of reservoir shorelines and riparian wetlands and examined the correlation of plant distribution and environmental characteristics by investigating 5 reservoir shorelines and 15riparian wetlands along the Han River.A total of 78 species of plants were recorded in all plots,of which 28 species were found in reservoir shorelines and 75 in riparian wetlands.They belonged to 27 families and 77 genuses,including 49 species of hydrophyte,25 mesophyte and 4 emergent species.Our results indicated that the dominant species in riparian wetlands were Cynodon dactylon,Phalaris arundinacea,Conyza canadensis,Polygonum lapathifolium and Medicago lupulina.While Cynodon dactylon,Alternanthera Philoxeroides,Polygonum lapathifolium,Paspalum paspaloides and Polygonum hydropiper were the most common species in reservoir shorelines.Reservoir shorelines had a lower richness and species diversity than riparian wetlands.The species similarity coefficient was 48.5%between reservoir shorelines and riparian wetlands.The percentage of annual species in reservoir shorelines was higher than that in riparian wetlands.Riparian wetlands had a higher percentage of hydrophyte species than the reservoir shorelines.However reservoir shorelines supported higher emergent species.Temporal variation in vegetation distribution was obviously observed in reservoir shorelines.Hydropower station constructed on the rivers caused changes in vegetation composition of reservoir shorelines by altering the hydrologic regime,propagation process and geomorphic feature.Canonical correspondence analysis illustrated that topographic factors are critical to plant communities'distribution.Regression analysis showed that species richness is significantly positively correlated with altitude.To investigating the spatio-temporal patterns of soil potential and unamended denitrification rates and quantifying the relative contribution of abiotic and biotic factors to wetland denitrification rates in wetlands along the Han River,we examined seasonal variation in potential and unamended soil denitrification rates in 20 wetlands along the Han River.Potential denitrification rate in soils ranged between 0.06 and511 ng N g?1 h?1,with a mean value of 51.79 ng N g?1 h?1.Soil unamended denitrification rates varied from 0.03 to 105 ng N g?1 h?1 and averaged 10.56 ng N g?1h?1.Denitrification rates in lowland wetlands were slightly but not significantly higher than those in mountain wetlands.Reservoir shoreline wetlands supported greater denitrification rates than riparian wetlands.Sampling season had a strong influence on both potential and unamended denitrification rates.The abundance of nirK gene in wetland soils ranged from 0.31×105 to 84.50×105 gene copies g-1 soil,while the abundance of the nirS gene varied between 0.11×105 and 69.20×105 gene copies g-1soil.The mean abundance of nirK gene was approximately twice that of nirS gene.There was no significant difference in nirS and nirS gene abundances among seasons.Soil moisture was positively related to nirK gene abundance,plant species richness and plant cover,while soil NO3–?N concentration was negatively correlated with nirS gene abundance and plant species richness.Both potential and unamended denitrification rates showed significant positive relationships with denitrifier abundance,plant richness,soil moisture and STC.Stepwise regression analyses demonstrated that nirK abundance was the most important predictor of potential and unamended denitrification rates.Path analysis further revealed that edaphic conditions could regulate denitrification rates both directly and indirectly through their effects on denitrifier abundance.In order to prove the ubiquity of this conclusion,we seasonally surveyed and sampled in 17 riparian wetlands along the Qi River.We investigated seasonal variation in soil denitrification rates and functional gene abundances.Temporal variation in potential denitrification and unamended denitrification(59.04 ng N g-1h-1and 15.84 ng N g-1 h-1)were obviously observed.The abundance of nirK gene in wetland soils ranged from 0.003×105 to 47.24×105 gene copies g-1 soil,while the abundance of the nirS gene varied between 1.77×105 and 2922.5×105 gene copies g-1soil.The abundance of nirK gene and nirS gene showed significant seasonal dynamics.Pearson correlation analysis suggested that potential denitrification rate was significantly positively correlated with functional gene abundance,vegetation coverage,species richness,soil electrical conductivity,soil water content,and total soil nitrogen content.Multiple stepwise regression analysis indicated that the abundance of nirK gene is the most important predictor of soil potential and unamended denitrification rate and soil nitrogen content was the most important predictor of the abundance of nirK and nirS genes.Redundancy analysis?RDA?analysis showed that species richness,soil moisture,electrical conductivity,and nitrogen content were the determining factors of the abundance of functional genes.To prove the plant richness and diversity recovery could increas soil potential denitrification rate,we seleted 3 riparian species and carried out 8 different collocations and combinations in greenhouses.Studies have shown that the potential denitrification rate is 26.75 ng N g-1 h-1.In May 2017 the soil potential rate was highest.The other seven samples in the last three months are slightly higher than the control sample and the one with three species was significant greater than others.Obvious seasonal variations of soil properties such as total carbon,ammonia nitrogen,and nitrate were observed except soil total carbon.No difference found in soil properties among 8 samples other than soil total carbon in control sample.The average abundance of nirS gene(27.91×105 gene copies g-1 soil)was about 8 times higher than that of the nirK gene(3.31×105 gene copies g-1 soil,soil).The abundance of the nirS gene was highest in November 2016,and significantly higher after planting.There was no significant difference in denitrification functional gene abundance among treatments.Pearson correlation analysis showed that the denitrification functional gene abundance was closely related to the potential denitrification rate.The denitrification functional gene abundance was significantly correlated with soil properties.NirK gene abundance was significantly correlated with soil conductivity,density,moisture,total nitrogen and total carbon content.NirS gene abundance was significantly related to soil electrical conductivity,moisture,density,and soil pH,but did not show significant correlation with soil total carbon and nitrogen. |
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