Nitrogen Loss Through Anaerobic Ammonium Oxidation Coupled With Iron Reduction In A Mangrove Wetland

Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity,extreme tides,strong winds,high temperatures and muddy,anaerobic soils.Mangroves are the only trees that are capable of thriving in salt water,they form unique intertidal forests at the edge of land and sea.Mangroves form the foundation of a highly productive and biologically rich ecosystem which provides a home and feeding ground for a wide range of species.Mangrove can absorb excess nutrients and heavy metal of water and can therefore act to purify water and to reduce occurrence of red tides.Human beings are more and more concern about environmental problems due to the global climate change.Over the past decades,human activities lead to acceleration of eutrophication in rivers and lakes,especially the increase of nitrogen emission making the nitrogen recycle become the research hotspot.Mangrove wetlands which as the terrestrial-sea transition zone are widely distributed around the world play an important role of alleviating nitrogen pollution.Moreover,mangrove sediments are associated with special physicochemical properties such as high organic carbon,high salinity,flooded,anaerobic,and recycle between aerobic and hypoxic because the tidal influence.Therefore,mangrove is an ideal area for studying nitrogen cycle,they play an important role in biogeochemical transformations in coastal ecosystems.The study area is located in Jiulong River Estuary Mangrove Nature Reserve,Fujian Province.In this study,sediment slurry incubation experiments were combined with isotope tracing and acetylene inhibition techniques to analysis three nitrogen loss process(denitrification,anaerobic ammonia oxidation(anammox)and anaerobic ammonium oxidation coupled with iron reduction(Feammox))and calculate the contributions to nitrogen loss respectively.Moreover,the characteristic and mechanism of graphene as electron shuttles mediate Feammox were studied.Feammox was demonstrated in mangrove ecosystems in the first time,and also graphene act as an electron shuttle to promote Feammox was demonstrated.The major results are as follows:(1)Physicochemical characteristics of overlying water and sediments.There was no statistically significant difference in salinity of overlying water between winter and summer;The pH of overlying water was 7.53 because the dilution of freshwater;The Eh of overlying water in winter(282.3 mv)was significantly higher than that in summer(212.9 mv)because the primary productivity of estuary in summer was higher than that in winter result in more dissolved oxygen was consumed in summer;The mangrove sediments were reduction environment,the Eh of sediments were lower than that in the overlying water because of the decomposition of TOC and high water content;The reduction environment was conducive to the reduction of Fe(?),therefore,HCl-extractable Fe(?)was the major form,accounting for 94%(bare flats(mud flats without mangrove))and 86%(mangrove sediments)of the total HCl-extractable iron respectively;The concentration of Fe(III)in mangrove sediments(1.04(±0.10)g Fe kg-1)were significantly(p<0.01)higher than that in bare flats(0.57(±0.02)g Fe kg-1)due to the flooded time in bare flats were longer than mangrove sediments.In winter,the pH of mangrove sediments were 7.70(±0.01)which were higher than that in bare flats(7.49(±0.01)).However,in summer,the mangrove sediments were weakly acidic(pH=6.39)because of the anaerobic decomposition of TOC produce H+;The water content of the bare flats were greater than that in the mangrove sediments,and the bulk density of bare flats were lower than that in the mangrove sediments.The TOC of mangrove sediments were significant higher(2.16%(±0.13))than the TOC of bare flats(1.37 5(±0.21))due to mangrove forest accumulate a large number of capillary dead roots in sediments;NH4+ was the major form of exchangeable dissolved inorganic nitrogen in sediments,accounting for up to 98%,which mainly because the high denitrification rate in sediments,(2)Using labeled 15NO3-isotope tracer technique,the rates of denitrification and anamuox were simultaIeously determined based on the 29N2 and 30N2 production rates.The productions of 29N2 and 30N2 increased linearly durilg the cultivation process,which proved that denitrification and anammox occurred in mangrove sediments.The rate of denitrification in mangrove sediments(3.50(±0.10)mg N kg-1 d-1)was significantly higher than that in the bare flats(3.16(±0.07)mg N kg-1 d-1);The rate of anammox in bare flats(1.65(±0.09)mg N kg-1 d-1)was significantly higher than that in mangrove sediments(1.44 mg(±0.04)N kg-1 d-1).Denitrification was significantly higher than anammox,and was the major nitrogen loss pathway in mangrove sediments.Meanwhile,anammox was an effective pathway to remove nitrogen in mangrove sediments.The contribution of anammox to nitrogen loss was 29%(bare flats)and 34%(mangrove sediments)respectively.The contribution in mangrove sediments was higher than that in most of other wetlands.(3)Feammox with dinitrogen,nitrite or nitrate as end-products was the most recently discovered nitrogen cycling process.This reaction has been observed in tropical forest soils,paddy soils and intertidal wetlands.However,Feammox has not been measured in mangrove wetlands.In this study,sediment slurry incubation experiments were performed combined with isotope tracing and acetylene inhibition techniques.Feammox was detected in mangrove sediments and bare flats with potential rates of 0.48(±0.03)mg N kg-1 d-1(accounting for 6.4%of the total nitrogen loss through N2)and 0.38(±0.02)mg N kg-1 d-1(accounting for 6.7%of the total nitrogen loss through N2),respectively.Microbially reducible Fe(?)(FeCl3)was added,which significantly(p<0.01)increased the Feammox rate in contrast to no addition of iron(III).It was estimated that a loss of 12.33 t Na-1 was associated with Feammox in mangrove sediments of the Jiulong River Estuary,accounting for 0.04%of the total external inorganic nitrogen transported into the estuary.Overall,these findings demonstrate that Feammox can act as a nitrogen loss mechanism in mangroves.(4).Graphene and 9,10-anthraquinone-2-sulfonate(AQS)were both investigated using a nitrogen isotope tracing technique(15NH4+-labelled)in mangrove sediment slurries.Graphene and AQS addition treatments increased the Feammox rates by 31%and 56%,respectively,in the slurry incubation experiments when compared with the control.It was estimated that potential nitrogen loss via Feammox(0.21-0.32 mg N kg-1 d-1)increased by 31-100%due to the graphene and AQS additions.Graphene also increased the Fe(?)reduction rate by 43%,but Feammox-associated Fe(?)reduction accounted for only 1.5-4.9%of the total Fe(?)reduction.Graphene could behave as a conductive bridge to promote direct electron transfer in the Feammox process,a functional role that was distinct from that of AQS.