Influenceing Mechanisms Of Intertidal Wetland Reclamation On Soil Nitrogen Transformation And N₂O Production Pathways

Intertidal wetlands are key transitional zones between terrestrial and marine ecosystems,which play important roles in marine ecosystems.Over the past few decades,the contradiction between human and land has become increasingly serious due to the rapid economic development and continuous growth of population in coastal areas.Intertidal wetland reclamation is an important way to alleviate the contradiction between human and land in coastal areas.Therefore,a large number of intertidal wetlands have been converted to agricultural land,aquaculture ponds,and construction land during the past few decades.Large scale of intertidal wetland reclamation can greatly destroy coastal wetland ecological function,increase nitrogen pollution in coastal areas,and promote global warming.Therefore,comprehensive understanding of the effects of intertidal wetland reclamation on soil nitrogen transformation processes,N₂O emission,and source pathways as well as revealing associated driving mechanism are significant to alleviate nitrogen pollution and to reduce N₂O emission in coastal areas.In this study,intertidal wetlands in different climatic zones and their reclamation areas along the China coast from 1984 to 2020were selected as a typical study area to examine the characteristics of nitrogen transformations,N₂O emission and production pathways in different climatic zones,and to reveal the driving mechanism.The main results obtained in this study are as follows:(1)Significant spatial distribution differences between subtropical climate zone and temperate climate zone are observed in ammonification rate,anaerobic ammoxidation rate and denitrification rate(P<0.05),but there were no significant differences in the spatial distribution of nitrification rate and dissimilatory reduction of nitrate to ammonia rate(DNRA).The rates of ammonification rate,anaerobic ammoxidation rate and denitrification rate in subtropical climate zone were significantly higher than those in temperate climate zone.Intertidal wetland reclamation significantly changed soil ammonification rate,nitrification rate,denitrification rate,anaerobic ammoxidation rate and DNRA rate.Ammonification rate,denitrification rate,DNRA rate were significantly higher in paddy land and aquaculture land than in intertidal wetland,and increased with reclamation chronosequences.Rates of anaerobic ammoxidation and nitrification in paddy land were significantly higher than intertidal wetland,and increased with reclamation chronosequences,while nitrification rates in aquaculture land were significantly affected by soil water filled pore space(WFPS).Intertidal wetland reclamation significantly increased soil contents of Fe²⁺,TOC,sulfide,NH₄⁺and NO₂^-,and increased with reclamation chronosequences,which greatly changed the abundance of nitrogen transformation functional genes,thus leading to the change of nitrogen transformation processes.(2)Sediment N₂O emission in subtropical climate zone was significantly higher than that in temperate climate zone(P<0.01),with emission rates of 0.47-8.42 ng g^-1h^-1and 0.37-2.35 ng g^-1h^-1,respectively.Soil N₂O emission in paddy land and aquaculture land under 200%WFPS condition was 0.89-2.18 ng g^-1h^-1and 0.19-1.49ng g^-1h^-1,respectively.Under 90%WFPS condition,soil N₂O emission in paddy land and aquaculture was 2.13-3.60 ng g^-1h^-1and 1.02-3.65 ng g^-1h^-1,respectively.In paddy land,soil N₂O emission at large scale was increased with reclamation chronosequences both in 200%and 90%WFPS conditions,N₂O emission in aquaculture land increased with reclamation chronosequences under 90%WFPS condition,while no significant difference was observed in coastal wetland sediments under the condition of 200%WFPS.(3)Contribution of heterotrophic denitrification pathway to N₂O emission was significantly higher in subtropical climate zone than that in temperate climate zone,while the contribution of nitrifier denitrification and nitrification-coupled denitrification pathways to N₂O emission was significantly higher in temperate climate zone than that in subtropical climate zone.The availability of sediment substrate significantly affected sediment nitrogen transformation processes in different climate regions of China,and then affected the N₂O emission and production pathways.Contribution of heterotrophic denitrification pathway to N₂O emission increased with reclamation chronosequences both in paddy and aquaculture land,especially under 200%WFPS condition.Contributions of nitrifier denitrification and nitrification-coupled denitrification pathways to N₂O emission were significantly higher in paddy and aquaculture land than in intertidal wetlands under 90%WFPS conditions,and their contributions increased with reclamation chronosequences.Changes of soil nitrification rate,denitrification rate,water content and temperature were the key factor driving the changes in soil N₂O emission and production pathways both in aquaculture and paddy land.(4)For the influences of typical agricultural managements,chemical fertilizer application significantly promoted the soil nitrification rate.Soil N₂O emission after chemical fertilizer application in July,August and October was 11.30 ng g^-1h^-1,4.73ng ng g^-1h^-1and 1.78 ng ng g^-1h^-1,respectively,which increased exponentially.Application of chemical fertilizer significantly increased the contribution of ammonia oxidation pathway(nitrifier nitrification,nitrifier denitrification and nitrification-coupled denitrification pathways)to N₂O emission.Increase of soil water contents significantly increased the contribution of heterotrophic denitrification and nitrification-coupled denitrification pathway to N₂O emission,and decreased the contribution of nitrifier denitrification and nitrifier nitrification pathway to N₂O emission.(5)In the past 36 years,total reclamation area of intertidal wetland in China was up to 1.48×10~6hectares.Potential annual nitrogen removal amounts in intertidal wetland sediments of China in 1984,1995,2007 and 2020 were 1.15×10~7,1.07×10~7,9.69×10~6and 8.52×10~6t yr^-1,respectively.Under the inertial development scenario,the total amount of potential nitrogen removal in intertidal wetland will be lower than the total amount of nitrogen transported from land to coastal area.Under emission reduction scenario,the total amount of potential nitrogen removal in intertidal wetland will be similar with the total amount of nitrogen transported from land to coastal area.