Salinity Effects On Key Nitrogen Conversion Pathways In Water Column And Sediment In Jiulong River Estuary

Estuary communicates land with sea.It is a natural habitat for thousands of species and a treasure trove of resources for human development as well.Changing salinity is the most prominent feature of the estuarine system,but it is also a relatively neglected environmental regulation factor.In fact,the sea level is gradually rising due to the climate change and it is reported that more and more fresh water environments have been affected by saline water intrusion.Salinity plays a crucial role in regulating microbial physiological metabolism and niche differentiation.Therefore,the effect of salinity on the nitrogen cycle should not be underestimated.Most of the existing related studies focus on the impact of salinity on sewage treatment efficiency.In-situ environmental studies are rare and mainly focus on the changes of microbial community structure,which remained the influence of salinity on rates of the key nitrogen conversion processes(nitrification,denitrification,anammox)rather unclear.Since these rates regulate not only the distribution of various forms of nitrogen but also the releasing of greenhouse gas N2O,it is very much needed to be explored.In July and December 2018,two cruises were carried out in Jiulong River,the second largest river in Fujian Province.We conducted salinity control experiments on sediments of different salinity sites and on water column from the freshwater site.Based on stable isotope addition method and combined with molecular biological techniques,we explored this subject from perspectives of water,gas,sediment and microorganisms.Through elaborate salinity regulation,we observed strong influence of salinity on the nitrogen cycle from water body and sediment incubation results.We also found the potential laws and connections through the analysis of the data.Our results showed that 1)At the freshwater side of Jiulong River,both the ammonia oxidation process and the nitrite oxidation process were inhibited by salinity.The higher the salinity was,the stronger the degree of inhibition.2)Nitrite-oxidizing microorganisms were more sensitive to salinity than ammonia-oxidizing microorganisms.3)The rates of ammonia oxidation were higher than the nitrite-oxidizing rates at each salinity,which was the main cause of nitrite accumulation in this area.4)Increased salinity stimulated ammonia-oxidizing microbes to produce more by-product N2O.5)At the freshwater side of the Jiulong River,the N2O contributed by ammonia oxidation was not significant.The high concentration of N2O in the in-situ water mainly came from the denitrification process of sediments.6)Denitrifying microorganisms in freshwater environment were very sensitive to changes of environmental salinity.Up-regulation of salinity inhibited the rate of denitrification and the activity of N2O reductase.7)Denitrifying microorganisms in sediments with altered water salinity showed a wide range of salinity adaptation.8)Anaerobic ammonium oxidizing microorganisms were more sensitive to changes in salinity than denitrifying microorganisms.9)The anaerobic ammonium oxidation at different salinity stations in Jiulong River had a different contribution to denitrification,and the ability of denitrifying microorganisms to produce N2O was also different.This subject filled the gap of the related research,deepened our understanding of the influence of salinity on nitrogen cycle and also furthered our awareness of the salinity effect in field study.It may also give clues on further exploring the mechanisms of salinity affecting the key nitrogen conversion processes of water bodies and sediments in estuaries.