| With the rapid development of industry and agriculture,a large amount of exogenous nitrogen enters the river,posing a huge threat to the river ecosystem.Nitrification and denitrification in sediments are one of the main nitrogen removal pathways in river ecosystems.However,there are relatively few studies on the roles and mechanisms of nitrification and denitrification in the headstream of agricultural watersheds.It is necessary to study nitrification and denitrification activities in headstreams and their nitrogen removal capacity in river ecosystems,which will provide a scientific basis for coordinating water ecological balance.In this study,three typical headstreams in the Tuojia small watershed were selected,and nitrogen concentration and physical-chemical indicators in the waters and sediments were monitored.The potential nitrification and denitrification rates and various functional genes abundances in the three sub-stream sediments were studied.The key environmental factors affecting nitrification and denitrification in sediments were revealed.The main results of the present study are as follows:?1?The total nitrogen?TN?concentration of the three sub-streams is Dongshan(1.8-107.1 mg·L-1)>Xinsha(0.93-7.8 mg·L-1)>Fuling(0.51-12.0 mg·L-1).The TN concentration of most sites in the three sub-streams exceeds the national surface water environmental quality class V.In the sites where the nitrogen pollution is serious,the proportion of NH4+-N in water is larger,while the site with relatively light nitrogen pollution is NO3--N more proportional.The contents of NH4+-N,NO3--N,TN and SOM in sediments of the three sub-stream showed the regularity of Dongshan>Xinsha>Fuling.The nitrogen pollution in the three streams is mainly in the form of NH4+-N.?2?The potential nitrification rate?PNR?in sediments of the three sub-streams is obviously different,and the whole is Dongshan(36.6?g·?kg·h?-1)>Xinsha(23.5?g·?kg·h?-1)>Fuling(16.3?g·?kg·h?-1),PNR of most of the sites were the highest in spring.The variation range of potential denitrification rates?PDNR?in sediments of the Fuling,Dongshan and Xinsha sub-streams was 0.17-4.7,0.06-3.9 and 0.19-3.0 mg·?kg·h?-1,respectively.There was no significant difference,which was basically the same.The environmental variables have different effects on the PNR and PDNR of the three sub-streams,and the main influence factors of the three sub-streams are different.?3?The total number of bacteria in the sediments of Fuling,Dongshan and Xinsha were2.7×1010-2.2×1011,4.3×1010-4.5×1011 and 5.2×109-2.4×1011 copies g-1,respectively.The performance is Dongshan>Xinsha>Fuling,most of the sampling sites are higher in spring and autumn than in winter and summer.The AOA abundance in sediments is basically represented by Dongshan(5.8×108 copies g-1)>Xinsha(5.0×108 copies g-1)>Fuling(3.0×108 copies g-1).The AOB also showed a similar pattern,the Dongshan(5.5×108 copies g-1)>the Xinsha(1.4×108 copies g-1)>the Fuling(8.3×107 copies g-1).In addition,at the higher pollution sites,the abundance of AOB is higher than AOA,and AOB is more advantageous,while the site with relatively low pollution is the opposite.The nirS genes in sediments of Fuling,Dongshan and Xinsha sub-streams were 3.0×107-9.6×108,4.8×107-6.8×108 and 3.4×106-5.9×108 copies g-1,respectively.The nirK gene abundance was2.6×108-2.1×109,2.5×108-1.3×109 and 4.8×106-2.2×109 copies g-1,respectively.And the abundance of nirS and nirK genes in sediments of the three sub-streams were not significant difference?P>0.05?.The nirK gene abundance is higher than nirS,which indicates that nirK plays a more important role than nirS in denitrification.The analysis found that there was no significant correlation between PNR or PDNR and the corresponding functional gene abundance,and environmental variables may be the main influencing factors of nitrification and denitrification rates.The environmental variables have different effects on the abundance of nitrification and denitrification functional genes in the three sub-stream sediments,and the differences among different streams are large.In a word,there are different levels of nitrogen pollution in the three sub-streams.The PNR and PDNR and their corresponding functional gene abundances in sediments of three sub-streams have large differences in time and space,and the response relationship with various environmental variables is complicated,the main influencing factors are different.This paper preliminarily reveals the relationship between nitrification and denitrification activities,functional gene abundance and environmental variables in three sub-streams,but the specific mechanism of the three interactions needs further and comprehensive exploration. |
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