Benthic denitrification, nitrification, and nitrate reduction rates in a range of grass carp polyculture enclosures

Abstract

We measured denitrification,nitrification,and ammonification rates in the pond sediment of several systems for the polyculture of grass carp(Ctenopharyngodon idellus) using AIT(acetylene inhibition).The rates of denitrification,nitrification,and ammonification ranged from 0?734.15 μmol/(m2?d),0?1209.20 mmol/(m2?d),and 0?41.25 mmol/(m2?d),respectively.The denitrification rates were relatively high,and were similar to those measured in estuaries and pools.The nitrification rates were either very low or undetectable during the mid-period of culture.The ammonification rates increased each month in all systems,but were highest in the triple polyculture system.A higher ratio of shrimp(Litopenaeus vannamei) to grass carp was associated with an earlier increase in denitrification.However,the system was unable to maintain this level of nitrification.Conversely,a higher abun-dance of grass carp resulted in a delayed increase in denitrification,but a more rapid,and sustained increase dur-ing the culture period.In general,cultures with fewer species had lower rates of nitrification and cultures with a higher proportion of silver carp(Hypophthalmichthys molitrix) had higher rates of nitrification.The polyculture group(shrimp,grass carp,and silver carp) outperformed the monoculture triple group(shrimp only) and the binary groups(grass carp and shrimp or grass carp and silver carp).We tested two stocking densities of the polyculture group,but it is unclear which of these groups performed best.The objective of the present study is to access the effects of different poly culture systems of grass carp on the dynamic changes of N at the ponds sediment-water interface,and to provide a certain basis for the further optimize of polyculture models.