| Land-use change is a key component of global changes and largely impacts ecosystem structures,processes and functioning.The water-source ecosystem consists of the terrestrial-aquatic interface and the waterbody of the reservoir.The terrestrial-aquatic interface has unique characteristics and is influenced by land use and flooding simultaneously.There are still few studies on the multi-scale effects of land use around water sources on water quality characteristics such as nitrogen and phosphorus concentrations.How to rationally allocate land resources on multiple scales and explore the impact of land use on the water quality of water sources is of great significance for ensuring the safety of water supply in reservoirs.In addition,the terrestrial-aquatic interface plays an important role in the removal of nitrogen.The study of the microbial structure,function and the influencing factors is of great significance for the removal of nitrogen in the terrestrial-aquatic interface and the reduction of the eutrophication level of the water body.This study was conducted in water-source ecosystems in Yangtze River Delta urban agglomeration by using Geographic Information System（GIS）,field sampling and experiments.The types and characteristics of the water-source ecosystem in the Yangtze River Delta urban agglomeration were first analyzed.Then,the impact of the land use on the water quality was studied.The effects of land-use types and flooding on soil microbial community and the abundance of functional genes,nitrogen cycle and its microbial characteristics in the ecotone were further examined.We tried to answer the following questions 1)How does land use affect water quality characteristics such as nitrogen and phosphorus of water-source ecosystems at different spatial and temporal scales?2)How does land use type and flooding affect soil microbial community and the abundance of functional genes in the terrestrial-aquatic interface of the water-source ecosystem?3)Whether flooding disturbance affects the stability of nitrogen cycle and its microbial characteristics in the ecotone of the water-source ecosystem?The results obtained are as follow.1.The types and characteristics of the water-source ecosystem in the Yangtze River DeltaBy combining remote sensing data with geographic information systems,a normalized difference water index（NDWI）model was established.Then,the types and characteristics of the water-source ecosystems in the study area were analyzed.The water-source ecosystems of the study area were mainly in the hilly area of the southeast,with large reservoirs and small and medium-sized reservoirs inlaid.There were also some small and medium-sized reservoirs along the main stream and tributaries of the Yangtze River,and the Taihu Lake Basin,located in the central and northwestern plains of the area.Medium-sized reservoirs were mainly the reservoir-type water sources,followed by large-scale reservoirs and small-scale reservoirs.Reservoirs accounted for 60%in Zhejiang Province;17.8%in Jiangsu Province and 22.2%in Anhui Province.From 1985 to 2015,the water area of large-scale reservoirs tended to decreased first and then increased,while the water area of medium-sized reservoirs and small-scale reservoirs did not change obviously.2.Effects of land use on water quality of water-source ecosystemsHow land use affects water quality of the water-source ecosystems within Zhejiang Province was analyzed by using self-organizing map（SOM）neural network from land use data and water quality data and correlation analysis.The results showed that land use types（woodland,grassland,cropland,and construction land）around water sources had a significant effect on the water quality of the waterbody and the effect was scale-dependent.The land use intensity（LUI）and the proportion of construction land had a positive correlation with ammonia nitrogen（NH3-N）and chemical oxygen demand(CODMn)concentrations.Except for CODMn,the ratio of cropland land area had no significant impact on water quality.Differences in topography and seasonal differences leaded to different temporal and spatial distribution patterns of water quality.The lower the fragmentation degree of woodland in the catchment area,the higher the degree of accumulation in the buffer zone,the greater the improvement of the water quality of the water source.Land use had the most significant impact on water quality within the range of a 100 m to 200m radius around the reservoir waterbody in hilly areas;while for the plain areas,land use had the most significant impact on water quality within the range of a 1500m radius around the waterbody.3.Effects of land use and flooding on the soil microbial community and functional genes in ecotones of water-source ecosystemsThe study was conducted in the terrestrial-aquatic interface of a water-source ecosystem in the hilly area.By using high-throughput sequencing and quantitative PCR（q-PCR）,we tested the effects of land-use types（i.e.,forest,wetland,and grassland）and flooding conditions（i.e.,landward locations and waterward locations within the land-use types）on soil microbial communities and microbial functional genes.Land-use type but not flooding significantly affected soil microbial community composition at the phylum level,while both land-use type and flooding significantly affected the orders Nitrosotaleales and Nitrososphaerales.Alpha diversity was higher in the wetland and forest regardless of flooding conditions.Functional gene abundances differed among the three land-use types.Archaeal amo A（AOA）and nir S genes were more abundant in the wetland than in the grassland or forest.Bacterial amo A（AOB）,nir K,nir S,and nos Z genes were more abundant in the waterward location than in the landward location but only in the wetland.Soil p H,moisture,and concentrations of soil organic matter and total soil nitrogen were significantly associated with the composition of archaeal and bacterial communities as well with their gene abundances.4.Effects of flooding disturbance on nitrification and denitrification and the related microbial resilience and resistance in ecotones of water-source ecosystemsAs the wetland is significantly disturbed by flooding,the influence of flooding disturbance on the nitrogen cycle process and microbial stability was tested in wetland in the ecotones.The three gradients were designed as follow:no flooding,short-term flooding and continuous flooding.The results showed that the potential nitrification rate and the gene abundance of amo A（AOA）had the highest value in the recovery area with short-term flooding disturbance.Potential soil nitrification rate and potential ammonia oxidation functional gene amo A（AOA）both showed higher resistance and lower resilience.Potential soil denitrification rate had the highest value in the control area,while nir S and nos Z genes were more abundant in the recovery area.The alpha diversity of amo A（AOA）and nos Z genes showed higher resistance,while the alpha diversity of nir S and nir K genes showed higher resilience.The similar alpha diversity of amo A（AOA）,nir S and nir K genes in the no flooding area and the short-term flooding area supported the conclusion that ammonia oxidizing archaea communities and denitrifying bacterial communities can be better recovered from flooding disturbances. |
