Effects Of Reclamation On Nitrogen Cycle Of Coastal Wetland In China

The nitrogen(N)cycle is one of the most important chemical cycles in coastal wetlands.It not only affected the self-regulation mechanism of wetland,but also showed a special kinetics process which was correlated with a series of global environmental problems in the Earth Surface System.Reclamation,as an effective means to solve the problem of population and land resources shortage in China,could significantly alter the soil physio-chemical properties and the growth of plant in coastal wetland,finally affect the nitrogen cycle of ecosystem.For understanding the effects of reclamation on the nitrogen cycle in coastal wetland which can alleviate the adverse effects of reclamation activities,we conducted some field experiments in Jiangsu Yancheng Wetland National Nature Reserve and Dafeng Milu National Nature Reserve to test the differences of soil nitrogen forms and distribution,soil nitrogen mineralization and plant nitrogen storages between natural and reclamation areas in three typical wetlands(Phragmites australis,Suaeda salsa and Spartina alterniflora).Meanwhile,based on the above studies,we established the nitrogen cycling model of these three plant-soil systems in response to reclamation by using the compartment model.The results indicated that:(1)Reclamation could change a lot of soil physio-chemical properties in coastal wetland.In P.australis marsh,reclamation significantly increased water content,organic carbon(OC),microbial biomass carbon(MBC),carbon-nitrogen ratio(C/N)and microbial biomass carbon-nitrogen ratio(MBC/MBN)in surface soil,but decreased bulk density,pH,electrical conductivity(EC)and oxidation reduction potential(Eh).In S.salsa marsh,except for soil Eh,no effect of reclamation on soil physio-chemical properties was found.In S.alterniflora marsh,reclamation significantly increased soil bulk density,pH and Eh,but decreased water content,EC,OC,MBC,C/N and MBC/MBN.(2)Reclamation had different influences on the soil nitrogen of three typical wetlands.In P.australis marsh,reclamation significantly increased the total nitrogen(TN),acid nonhydrolysable nitrogen(ANN),acid hydrolysable nitrogen(AHN),ammonium nitrogen(NH4-N),nitrite nitrogen(NO2-N)and nitrate nitrogen(NO3-N)contents in surface soils while no effect was found in S.salsa marsh.On the contrary,in S.alterniflora marsh,the soil TN,ANN,AHN,soluble organic nitrogen(SON),NH4-N,NO2-N and NO3-N contents were all decreased by reclamation.According to the regression analysis,we found that in coastal wetlands,soil OC and MBC which were changed by reclamation were the main factors to impact the contents of soil organic N,and the main influence factors on inorganic N were soil water content and EC(salinity).Moreover,reclamation significantly increased the soil organic N and total N storages in P.australis marsh,and largely decreased the same values in S.alterniflora marsh,but didn't affect that in S.salsa marsh.And in all three wetlands,the storages of soil inorganic N were not altered by reclamation.This result indicated that reclamation altered the soil total N storage in coastal wetlands mainly by changing the soil organic N storage.(3)Reclamation significantly increased the soil nitrogen net mineralization rate(RN),soil ammonification rate(RA)and soil nitrification rate(RX)in P.australis marsh.At the same time,reclamation significantly decreased the soil RN and RA in S.alterniflora marsh,with no effect on the soil N mineralization in S.salsa marsh.According to the regression analysis,we found that in P.australis marsh,soil water content which was changed by reclamation was the main impact factor on soil nitrogen mineralization.In addition,we also found that soil Rx in all three typical wetlands was much lower than the soil RA,and it can be explained by the high soil moisture in coastal wetland.Therefore,the soil RN was mainly contributed by RA in our study.(4)Reclamation significantly increased the biomass and N storages of root,stem,leaf,seed and total in P.australis marsh,but decreased the litter biomass and N storage.And the biomass and N storages of each plant tissues in S.alterniflora marsh were all decreased by reclamation.According to the regression analysis,we found that soil EC(salinity)which was changed by reclamation was the main impact factor on plant biomass in coastal wetlands.Otherwise,reclamation also didn't clearly affect the plant biomass and N storages in S.salsa marsh.Because the N concentrations in three typical plants were not largely changed by reclamation treatment,the plant N storage was mainly contributed by plant biomass.(5)Reclamation largely increased the N flux in nitrogen cycle of P.australis plant-soil system,while significantly decreased the N flux in nitrogen cycle of S.alterniflora plant-soil system,and didn't affect the nitrogen cycle of S.salsa plant-soil system.Meanwhile,in these three typical plant-soil systems,reclamation areas all have higher nitrogen utilization efficiency and nitrogen re-translocation ability,but lower cycling speed,which could prevent the N from being lost easily.According to these researches,it is concluded that the reclamation can significantly change the nitrogen cycle of coastal wetland.Reclamation,by changing the soil physical and chemical properties,altered the soil and plant nitrogen content,and finally impacted the nitrogen cycling of coastal wetlands.Reclamation has different effects on the nitrogen cycle of different coastal wetlands.