Temporal-spatial Distribution And Transformation Of Nitrogen In The Min River Downstream And Estuary

Min River is the largest river of Fujian Province,but there weren't much research about this important basin for the South-East coastal areas of China.As one of the most important nutrient elements,the research about nitrogen at the Min River area was also limited.So we sampled the water?suspended solids and sediments for four seasons during 2014-2015 at the Min River downstream and estuary,determined basic parameters such as temperature,transparency,pH,salinity and turbidity,aided parameters sch as suspended solids,DO,COD,and main parameters including NO₃--N,NH₄+-N,NO₂--N,TN of suspended solids and sediments.Then discussed the distribution,seasonal variations,composition,transformation,flux of different forms of nitrogen,and draw a conclusion about the geochemical cycle of nitrogen at the study area.Results show:The distribution of NO₃-N was different in different seasons,while the distribution of NH₄-Nand NO₂-N in NT?north tributary?and ST?south tributary?were basically the same in different seasons and the concentrations varied in a small range in ET?estuary tributary?.In terms of DIN composition,the percentages of different N species was similar between NT and ST,but the average percentage of NH₄+-N in ST was far higher than NT due to the different terrestrial input;the composition o.f DIN in NT and ST didn't reach the thermodynamic equilibrium.In ET,the composition of DIN has reached the thermodynamic equilibrium.In terms of DIN transformation,the correlations between different N species were insignificant and the correlations between DIN and DO were varied a lot from season to season in NT,such phenomenon indicated the relationships between different N species were effected by complicated nitrogen sources,human activities and biochemical processes.In ST,the correlations were more significant but more complicated than NT,it indicated that the relationships between different N species were controlled by plant uptake,ammoniation and nitrification.In ET,the seasonal differences of the correlations were significant.The vertical distribution of DIN at the estuary was affected by the ebb and flow,and the impact was more significant during dry season compared to the wet season.Moreover,the excess and annual increase of DIN concentrations was quiet serious in the Min River downstream and the estuary.The distribution of TN of SS was similar during summer and autumn,the high-value area was focused on the ST in spring while ET and ST in winter.The distribution of PN was also similar during summer and autumn,and the high-value area was focused on the maximum turbidity zone in summer and autumn.The seasonal variations of TN of SS were significant in different areas while the seasonal distribution otherness was non-significant compared to TN.Significant positive correlations were discovered between TN and TC for both seasons and areas,which indicated the homology for nitrogen and carbon in suspended solids.The high-value area of TN of sediments was focused on the NT area and the low-value area was focused on the ET area.The differences of TN between sediments from different seasons and sampling locations were non-significant,which indicated the relatively stable depositional environment at Min River downstream and estuary.Combined with the annual variation and environmental assessment,the sediments nitrogen pollution was pretty serious at Min River estuary for recent years.According to the correlation analysis of different forms of nitrogen,the correlations between DIN and TN of suspended solids and sediments were more significant at ST area than NT and ET areas,more significant in summer and autumn than spring and winter,and NH₄-N was the most relevant one with TN of suspended solids and sediments.The input flux analysis of nitrogen at estuary area showed that Min River estuary was a source for NO₃-N and PN,while a sink for NH₄-N and NO₂-N.According to the C/N ratios,the terrigenous sources were more significant at the ET area than NT and ST areas.There was a feature of terrestrial nitrogen pollutants retention at the Min River estuary,which indicated the need for a land-sea overall management for nitrogen.