Effects Of Nitrogen Loadings On Nitrogen Biological Cycle Process And Circulation In The Newly Created Marshes Of The Yellow River Estuary

As a marsh formed by the interaction between the Bohai Sea and the Yellow River estuary,the newbornness and vulnerability of the Yellow River estuary have drawn much attention in the world.In recent years,the amount of nitrogen(N)deposition in the Yellow River estuary has been constantly increasing and always maintaining a high level,which will have undeniable impacts on marsh ecosystems.In this paper,the Suaeda salsa marsh in the Yellow River estuary were selected as our study objects in order to further understand the effects of enhanced N loadings on the biochemical characteristics and circulation of N.In situ simulation of N loading experiments in the field,we collected samples about soil,plant(seed)and litter,and in-house incubation experiments,to study the characteristics of spatial and temporal distribution of N in soils and its chemical conversion and the characteristics of N decomposition in plants and release in litters under the enhanced N loadings(NO:No additional nitrogen treatment,6.0 gN·m-2·a-1;N1:Low nitrogen treatment,9.0 gN·m-2·a-1;N2:Middle nitrogen treatment,12.0 gN·m-2·a-1;N3:High nitrogen treatment,18.0 gN·m-2·a-1).As well as the germination and seedling growth of different N-substrate seeds.Finally,the N biological cycling compartment model of plant-soil system under the enhanced N loadings was established.The main conclusions are as follows:(i)Although the different N loadings did not change the dynamic changes of TN and NH4+-N contents in marsh soils,but the contents of TN and NH4-N increased with the increase of N loadings.Different N loadings treatment significantly changed the dynamic changes model of SOC and NO3--N in soils,the suitable N input(N1 and N2)significantly increased NO3--N contents in the soil,and the excess N input(N3)was not conducive to the accumulation of NO3--N.The SOC conversion of surface soils under continued N enhancement will be inhibited,and this will help to enhance its soil carbon storage function.(ii)The mineralization rate and accumulative mineralization of differentN-substrate soils are related to culture temperature and soil matrix quality.Higher temperature and lower C/Nr are beneficial to the N mineralization of soils.Under different temperatures,soils mineralization in the process of pH and cumulative mineralization changes in the pattern of significant consistency.Within a certain range,the increase of pH is beneficial to the mineralization of soil organic nitrogen,but excessive pH will inhibit the mineralization process.In comparison,N1 soil had the lowest mineralization ability,and N3 soil had the highest mineralization ability.The soil under N2 treatment had the highest mineralization and the most stable mineralization during the continuous mineralization.(iii)Different N loadings did not change the seasonal variation pattern of aboveground biomass,but changed the variation pattern of underground biomass.The biomass increased with the increase of N input,and N prolonged the peak growth plant(20d).The contents of TN in various parts of S.salsa also increased with the increase of N input,and its ability of regulating its nutrient supply and distribution was the most obvious under N2 treatment,The maturation time of seeds under N2 treatment may be about a month ahead of N0,N1,and N3 treatments.The reason may be that the N2 treatment can significantly affect the carbon allocation ratio in S.salsa and the amount of N input in S.salsa.It is related to special adaptation measures of the environment.(iv)The increase of N input not only increased the TN contents of the litters,but also decreased the C/N ratio and the matrix quality,and also increased the decomposition rate of the litters,especially the decomposition of the litters under N2 treatment.In addition to the net N release at the initial stage of decomposition,the residual accumulation under different N loadings showed a net accumulation of N at other decomposition stages and the cumulated amount increased with the increase of N loadings intensity.During the decomposition process,the litters will continue to hold N from the environment.In fact,the N held by litters will be continuously taken away by the tidal water,resulting in a loss of N in the marsh ecosystem,but this loss is the least loss in the N2 treatment.(v)The N contents of mature seeds under the different N loadings showed S2>SO>S1>S3(S0,S1,S2 and S3 represent the mature seeds produced by S.salsa under the treatment of N0,N1,N2 and N3,respectively.),and the proper N input(N2)was more conducive to the accumulation of N in seeds,while the high N input(N3)isn't conducive to the accumulation of N in seeds.Although the interaction of different salt and N concentrations had no significant effect on the germination rate of four kinds of N-substrate seeds and fresh weight,dry weight and growth of different organs.However,the germination rate of S2 seeds were higher overall,and the seedling of its development had the strongest adaptability to environment with increasing N concentration under different salt stress,and the germination rate and the seedling growth status of S3 seeds were the worst.(vi)From the point of view of N biological cycle,N2 treatment is not only beneficial to the mineralization and continuous N supply of soil organic nitrogen,but also to the growth and development of plants,the seeds germination and seedling growth,and more conducive to the decomposition of litters and N return,and N2 treatment under the S.salsa plant marsh plant-soil system,the highest N cycle factor.With the increasing supply of N in the S.salsa marsh of the Yellow River Estuary,it will be more conducive to nutrient supply to the marsh,seeds germination,seedling and plant growth,and decomposition of the litters when the nutrient status reaches a high level(N2),and the bio-cycle rate of N in the S.salsa marsh will be accelerated and the marsh ecosystem will be stabilized.When the nutrient status reaches a higher level(?N3),the circulation of N in the S.salsa marsh may be greatly reversed(unfavorable),and in the long term,it will not be conducive to the stability of the marsh ecosystem.Based on the above assessment results,we can enhance the dynamic monitoring of N in marsh ecosystems and control the critical path of N cycle so that the N in marsh ecosystems is at a relatively stable leve,that is the N2 treatment level set in this study.