| The water quality of shiwuli river is always at the level of grade V,and the nitrogen?especially ammonia?pollution is very serious.The shiwuli river connects the central and western urban areas of Hefei with the western half of chaohu lake basin,which is the main input channel of nitrogen and phosphorus load in the western half of the lake.Sediment and overlying water samples were collected seasonally from five different geomorphic structures?i.e.pools,riffles,gravel bars,point bars and runs?at urban construction area of Shiwulihe River.Both sediment potential and areal nitrification rates were measured and its seasonal and geomorphological variability were evaluated.Moreover,the denitrification potential of sediments in different geomorphic units was evaluated quantitatively and nitrogen/carbon content import levels were set for each of the treatment groups,and the denitrification rates of them were measured to analyze the limitation of carbon and nitrogen.The main research results are as follows:?1?The Shiwuli Stream is seriously polluted by nitrogen and phosphorus,NH4+,TN,PO43-and TP were severely than?of surface water quality standard,and the water in a significant reduction state at fall and winter.Based on analyzing the comprehensive water quality identification index method,water quality of rivers in the study areas are worse?the water and black smelly.And based on the assessment results of the aquatic water quality criteria for ammonia in water,the ecological toxicity risk of river water quality should not be neglected.?2?The evaluation of TP pollution in sediments showed that the studied river section presented a high ecological risk effect on the whole,and point bars was the most serious that its pollution index exceeded 1.5.Based on the evaluation of sediment organic pollution index,the organic nitrogen mass fraction and organic nitrogen reached the III standard as a whole,were still at a clean level.Moreover,geomorphological difference showed the point bars of organic pollution were most serious,followed by pools and runs,and the smallest in riffles and gravel bars.?3?Potential nitrification rates?PNRs?across the five geomorphic structures ranged from 0.002 and 0.079?mol·?g·h?-11 with a mean value of 0.024?mol·?g·h?-1.The ranking order of PNRs was pools>point bars>riffles>gravel bars>runs.Areal nitrification rates?ANRs?across the five geomorphic structures ranged between 0.140 and 13.543?mol·?m2·h?-1,with an average of 3.700?mol·?m2·h?-1.In general,the highest mean value was in riffles,followed by runs,and then by gravel bars and point bars,and the smallest in pools.In addition,the seasonal change rules of the two is basically the same,which is basically as follows:2017>summer of 2018>spring>autumn>winter?4?According to the difference analysis,there were significant differences between pools or riffles and the others in the PNRs.As for ANRs,extremely significant differences were observed between more than half of the geomorphic structures.?5?The denitrification rates of sediments in the five geomorphic features increased rapidly with the increase of the exogenous nitrogen concentration gradient?R2=0.80?,while the denitrification activity reached a peak at the N3 gradient and decreased slightly at the N4 gradient?R2=0.56?.However,the seasonal variation rules of them are basically consistent,which was generally characterized by summer>spring>autumn>winter.?6?In the case of nitrogen addition alone,cluster analysis showed that the denitrification rates of riffles and gravel bars in different seasons belonged to the same or the same sub-category,while the classification of other geomorphic structures was unstable.The overall denitrification growth rate by 45.09%,26.67%,15.79%and 11.43%under the gradient of exogenous carbon concentration.,that is,the growth rate of denitrification rate with high exogenous carbon concentration gradient is lower than that at low concentration level. |
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