Nitrogen Transformation Processes And Its Environmental Controls In Estuarine Tidal Flat Wetland

Nitrogen（N）is generally a limiting element for primary production,and N biogeochemical cycle plays an indispensable role in maintaining the health and sustainable development in estuarine ecosystems.Estuarine and coastal wetlands,as a transitional zone between seas and land,are multifunctional and complex,in which salinity gradient,saltmarsh plant and tidal wave are the most distinct environment characteristics.A large amount of anthropogenic reactive N has been delivered into estuarine ecosystems due to the overuse of N fertilizer and fossil fuels burning over the past several decades,causing urgent environmental issues.Microbial N transformation processes play a key role in mediating N loadings in estuarine and coastal zones.A comprehensive understanding of inherent gross N transformation mechanisms and corresponding environmental controls is thus critical for purging reactive N pollution in such ecosystems.At present,many researchers have focused on N transformations in estuarine and coastal wetlands,but these works mainly focused on one or two distinct processes.There is a lack of comprehensively and systematically inherent transforming mechanisms of N dynamics so far.Hence,the Yangtze Estuary was selected as a representative study area to estimate simultaneously gross N transformations and the underlying drivers in response to the changes in salinity gradient,saltmarsh plant and tidal flooding frequency via ¹⁵N isotope diluting technique combined with optimized¹⁵N tracing model and plant model.This research could not only deepen the systematical and comprehensive understanding of internal mechanisms on the N cycling,but also provide scientific reference for estuarine and coastal N management.The main results are as follows:（1）Along the Yangtze Estuary salinity gradient,compared to the high-salinity sites,the freshwater and low-salinity habitats exhibited higher rates of total mineralization,NH₄+immobilization,dissimilatory NO₃^-reduction to ammonium（DNRA）,denitrification,anammox,and adsorption of NH₄+except for nitrification and release of adsorbed NH₄+.Meanwhile,the functional gene abundances（amo A,Ure C,hzo,nir S and nrf A）exhibited a similar spatial distribution pattern to corresponding N transformation processes.Sedimentary total organic carbon（TOC）,nitrite（NO₂-）,ferrous iron（Fe（II））,and microbial functional gene abundances jointly mediated the gross N transformations.（2）NH₄⁺and NO₃^-uptake rates by native Phragmites australis（P.australis）and invasive Spartina alterniflora（S.alterniflora）were 4.62 to 5.38 mg N kg-¹ d-¹ and 1.29to 2.90 mg N kg-¹ d-¹,respectively,and the invasive S.alterniflora had a higher N uptake than the native P.australis.The presence of saltmarsh plants promoted N mineralization and DNRA,increasing the available NH₄⁺supply for the plants.Conversely,NH₄⁺immobilization and autotrophic nitrification rates were drastically reduced in the presence of the saltmarsh plants,indicating that the plants were able to outcompete soil microorganisms in NH₄⁺acquisition.Meanwhile,heterotrophic nitrification（organic N oxidation）,which accounted for 66-82%of the total nitrification,was stimulated by the saltmarsh plants.Increased heterotrophic nitrification in the saltmarsh plants helped to provide NO₃^-substrates to meet the needs of the soil microorganisms and the plants.The regulatory effect of the invasive S.alterniflora on soil gross N transformation was more pronounced than that of the native P.australis due to the higher N requirements of the former.Microbial carbon sources and energy sources,relevant gene abundances and exoenzyme activities were the main factors by which the saltmarsh plants regulated gross N transformations.（3）Along the elevation of tidal flat,the gross N transformation rates changed remarkably with the increase of tidal flooding frequency.Generally,the autotrophic nitrification rates decreased and organic N mineralization,microbial NH₄+immobilization,DNRA and rate of NO₃^-consumption firstly declined and then enhanced with the enhancement of tidal flooding frequency.However,the heterotrophic nitrification rates did not change significantly in response to the changes in tidal flooding frequency.In addition,most of the functional gene abundances exhibited a similar spatial distribution pattern to corresponding N transformation processes.Changes in soil redox conditions,TOC content and N availability related intimately to tide inundation frequency were important factors for spatial variations of N transformation processes.（4）Generally,organic carbon（Chlorella algae）treatments applied alone increased N transformation rates relative to the control treatment.Inorganic N treatments（NH₄NO₃）applied alone promoted the autotrophic nitrification and denitrification rates and inhibited the organic N mineralization rates compare to the control treatment.However,other N transformation rates did not change remarkably with N additions.Organic C and inorganic N treatments applied in combination generally increased N transformation rates except for the organic N mineralization rates.Compared to the control treatment,organic C treatments applied alone or organic C and inorganic N treatments applied in combination both increased the N₂O release rates,although the effects of inorganic N treatments applied alone on N₂O release rates were not generally obvious.The contribution of denitrification to N₂O emissions were the highest,following by autotrophic nitrification and heterotrophic nitrification.Organic C treatments applied alone increased the contributions of heterotrophic nitrification and denitrification to N₂O emissions.Inorganic N treatments applied alone enhanced the contribution of autotrophic nitrification to N₂O emissions,but decreased the contribution of denitrification to N₂O emissions.Organic C and inorganic N treatments applied in combination increased the contribution of the autotrophic and heterotrophic nitrifications to N₂O emissions.