The Effect Of Land-Use/Land-Cover Change On Change Of Carbon And Nitrogen Components In Coastal Wetlands

Coastal wetlands have a high accumulation rate of carbon（C）and store a large amount of nitrogen（N）,and thus play a crucial role in mitigating global climate change.However,under rising global population and economic growth,coastal wetlands around the globe had been intensively developed for constructed and agricultural lands for more than a century due to land-use/land-cover change（LULCC）,resulting in a great loss of coastal wetlands and its ecological functions.Generally,LULCC will cause the carbon and nitrogen loss in the original ecosystem,and promote the carbon and nitrogen-containing greenhouse gases（GHG）release to atmosphere,making LULCC become the second largest source of anthropogenic greenhouse gases（GHGs）.Therefore,LULCC in coastal wetland would profoundly alter the carbon and nitrogen dynamics of ecosystem,and lead to response variations of carbon and nitrogen dynamics and relevant edaphic factors following conversion duration,due to different hydrological and edaphic conditions of converted land-use types.In this study,we conducted meta-analysis and field observation to compare the soil carbon and nitrogen contents and stocks,GHGs emission and lateral fluxes of carbon and nitrogen in surface water between coastal wetlands and its converted land-use types（e.g.,reclaimed/restored wetlands,agriculture lands,pastures and aquaculture ponds）,and to identify the crucial environmental factors in relationship to the changes in carbon and nitrogen.The main results were listed as follows:（1）Based on a global meta-analysis with database of 178 sites,we explored how the contents and stocks of soil total carbon（STC）,soil organic carbon（SOC）and soil inorganic carbon（SIC）and relevant environmental factors in coastal wetlands response to LULCC,such as conversion to reclaimed/restored wetlands,agriculture lands,pastures and aquaculture ponds.The results showed that the conversion of coastal wetlands to agriculture lands and aquaculture ponds decreased STC contents,SOC and STN stocks by 20.0%～69.5%,but the change degree of soil carbon and nitrogen in reclaimed/restored wetlands and pastures are relatively low.Loss of soil C would result in a CO₂ emission potential of 15.04±1.40、9.21±0.36 and-3.52±0.55 Mg CO₂e ha^-1yr^-1 for agriculture lands,aquaculture ponds and reclaimed/restored wetlands,respectively,indicating conversion to agriculture lands and aquaculture ponds enhancing the greenhouse effect.STC and STN contents in agriculture lands decreased with LULCC history,and the change rate of STC and SOC decreased with latitude.After LULCC,the response of soil carbon and nitrogen dynamics was sensitive to the changes in soil water conditions,soil bulk density,and soil p H and NH₄⁺-N content.（2）Furtherly,we conducted a field observation of 9 sampling sites of coastal wetlands and its converted land-use types in Chinese coastal region,to examine the effects of conversion to reclaimed wetlands,paddy fields,upland fields and aquaculture ponds on the soil organic,inorganic and labile carbon and nitrogen components contents and stocks（0～30 cm depth）.The results showed that LULCC significantly decreased the aboveground biomass and soil conductivity,but significantly increased the bulk density,particle size and soil phosphorus content.After LULCC,STC contents,SOC contents and stocks,STN contents and stocks,ecosystem carbon and nitrogen stocks and ratio of easily oxidizable carbon to SOC（EOC/SOC）were decreased by3.0%～41.2%,but soil inorganic carbon（SIC）content and stocks,NO₃^--N content and the proportion of labile organic carbon and nitrogen（microbial biomass carbon to SOC[MBC/SOC],dissolved organic carbon to SOC[DOC/SOC],microbial biomass nitrogen to STN[MBN/STN],dissolved organic nitrogen to STN[DON/STN]）in soil were increased by 0.9～80.3%.The increase of the proportion of labile organic carbon and nitrogen would stimulate the loss of soil carbon and nitrogen.The ecosystem C stocks in paddy fields and aquaculture ponds were significantly decrease with conversion time,while the ecosystem N stocks in upland field and paddy field were significantly increase with the reclamation time.We found that particle size,water content,soil conductivity,total phosphorus content and conversion time were the main environmental factors affecting the dynamics of soil carbon and nitrogen components.（3）In a subtropical salt marsh of eastern China,the seasonal patterns of dissolved carbon（DOC,dissolved inorganic carbon[DIC],CO₂,and CH₄）,particulate carbon（particulate organic carbon[POC]、particulate inorganic carbon[PIC]）,dissolved nitrogen（NH₄⁺-N、NO₂^--N、NO₃^--N、dissolved organic nitrogen[DON]、N₂O）,particulate nitrogen（PN）and the diffusive fluxes of GHGs（CO₂,CH₄,and N₂O）were compared between the natural tidal creeks and the enclosed tidal creeks.Due to notably changed hydrological and biological conditions in the enclosed tidal creeks,concentrations of DOC,DIC,CO₂,CH₄ and POC significantly increased by60.15%～288.25%,and NH₄⁺-N,NO₂^--N and PN significantly increased by121.5%～274.9%,while NO₃^--N,DON and N₂O decreased slightly,compared to the natural tidal creeks.The proportion of organic carbon and nitrogen components increased following reclamation.Significantly higher global warming potential(0.58±0.15 g CO₂-eq m^-2 d^-1)was found in the enclosed tidal creeks,making them hotspot of greenhouse effects,compared to the natural tidal creeks.Our results indicated that changes in flow velocity,salinity,chlorophyll-a（Chl-a）,p H and soil carbon and nitrogen content were the main factors controlling the carbon and nitrogen concentrations and consequent GHG emissions in creeks water after reclamation.Due to the blocking of tidal cycling,the lateral carbon and nitrogen fluxes in enclosed tidal creeks were significantly reduced by 71.5%and 99.9%,respectively.（4）Combining a global meta-analysis with a database of 108 sites and a field observation,we examine the effects of conversion to reclaimed/restored wetlands,agricultural lands,aquaculture ponds on the variability in CO₂,CH₄ and N₂O emissions from the coastal wetlands.Our results showed that the coastal wetlands were net sinks of atmospheric CO₂ and net sources of CH₄ and N₂O,exhibiting the capacity to mitigate greenhouse effects due to negative comprehensive global warming potential(GWP;-0.86±3.92 CO₂-eq ha^-1 yr^-1).Relative to the coastal wetlands,conversion to agricultural lands and aquaculture ponds decreased the net CO₂ uptake by 242.2%and 69.7%,respectively,due to a higher increase in ecosystem respiration relative to slight changes in gross primary production,while reclaimed/restored wetland slightly increased the CO₂ uptake.The constructed wetlands and aquaculture ponds significantly increased the CH₄ emissions,and agricultural land significantly increased the N₂O emissions compared to coastal wetlands.As a result,all converted land-use types led to remarkably higher GWPs by 7.8～29.5 times,enhancing the greenhouse effect.The variability in GHG fluxes with LULCC was mainly sensitive to changes in soil water content,water table,salinity,soil nitrogen content,soil p H and bulk density.In general,through significant altering hydrological conditions,vegetation growth,soil physical and chemical properties and microbial activities in soil,the LULCC in natural coastal wetland significantly reduced the contents and stocks of STC,SOC and STN in soil and ecosystem carbon and nitrogen stocks,and decrease net CO₂ uptake and promoted the emission of CH₄ and N₂O,enhancing the regional greenhouse effect.In addition,although the reclamation significantly decreased lateral carbon and nitrogen export of surface water,but the concentration of dissolved and particulate carbon and nitrogen components in water,GHG diffusion fluxes and consequent greenhouse effect were significantly increased.Therefore,LULCC weakened the abilities of carbon and nitrogen sink of natural coastal wetlands,resulting in the loss of carbon and nitrogen in ecosystems,making them hotspot of greenhouse effects with higher GHG emission.