Characteristics Of Soil Denitrification In A Bamboo Forest Riparian Zone In The Upper Reaches Of The Taihu Lake Basin,China

There is a serious nitrogen(N)pollution problem in the upstream agricultural area of Nantiaoxi River in the Taihu Lake Basin,and the woody riparian zone in this area has been basically replaced by the bamboo forest riparian zone.Under this circumstance,the soil denitrification in a bamboo forest riparian zone in the upper reaches of the Taihu Lake Basin during spring,summer and autumn was investigated by determining the abundance of denitrifying functional genes,denitrifying enzyme activity(DEA),and denitrifying rate(DR),as well as the factors mediating denitrification in this study.Incubation experiments of exogenous carbon and nitrogen addition were designed to analyze soil DR,nitrate nitrogen(NO₃^--N)removal ratio and their responses to carbon and nitrogen limitation.This study provides important insights into the assessment of ecological functions of bamboo forest riparian zone on reducing nitrogen pollution into rivers.The results showed that:(1)The abundance of soil denitrifying functional genes in summer was significantly higher than that in spring(except nosZ? gene).The abundance of nirS,nirK,nosZ? and nosZ? gene in each season decreased gradually with increasing soil depth,while the variation pattern of soil denitrifying functional gene abundance at the horizontal scale in one season differed from the other one.The temporal and spatial changes of soil denitrifying gene abundance were affected by the joint action of the other three functional genes and soil NH₄⁺-N concentration,and they possessed large temporal and spatial variations.(2)The mean value of spring soil DEA was significantly higher than that in summer and autumn,while there was no significant difference between latter two seasons.In each season,the riparian soil DEA decreased gradually with increasing soil depth and distance from the water in general.Soil NO₃^--N and soil organic carbon(SOC)jointly affected the spatial and temporal changes of DEA.In the study area,the riparian soil may lack relatively sufficient source of labile carbon(C),which might inhibit the soil DEA to some extent.(3)The mean value of soil DR in summer was significantly lower than that in autumn and spring,and the value in autumn did not differ from that in spring.The vertial and horizontal changes of soil DR were the same as DEA,and the maximum soil DR all appeared at the depth of 0 to 10 cm.Soil ammonium nitrogen(NH₄⁺-N)concentration,the abundance of nosZ? gene and SOC concentration jointly affected the spatial and temporal changes of DR.Soil labile C concentration in the riparian zone was also a limiting factor of DR.(4)In the experiment of simulating the addition of different exogenous nitrogen and the same exogenous nitrogen addition and different carbon-nitrogen ratio additions,the denitrifying rate(DR*)of soil in summer within 72 h of each treatment increased at the beginning and then decreased,and the maximum DR*all appeared in the 46 to 48 h after the addition of exogenous nitrogen.DR*and NO₃^--N removal ratio increased first and then decreased with the increase of exogenous NO₃^--N input,and gradually increased with the increase of exogenous C input,indicating that soil labile C was the key limiting factor for soil denitrification in the riparian zone,high level of NO₃^--N input might inhibit soil denitrification in summer.(5)Soil NO₃^--N removal ratio for 5 to 20 mg·L^-1 exogenous NO₃^--N input ranged from 9%to 70%,and the ratio for high level of NO₃^--N input(20 mg·L^-1)was ranging from 9%to 31%.The riparian soil in the study has a high potential of denitrification and nitrogen removal.The implementation on nitrogen pollution control of regional water in the future should take full advantage of the denitrification and nitrogen removal function of riparian soil.