Effects Of Exogenous Nitrogen Enrichment On Sulfur Biological Cycle In Tidal Marsh Of The Yellow River Estuary,China

The marsh in the Yellow River estuary marsh is the most integrated,expansive and youngest coastal marsh ecosystem in warm temperate zone of China,which is formed by the interaction between the Bohai Sea and the Yellow River estuary.In recent years,the amount of exogenous nitrogen?N?imported into the Yellow River estuary showed an increasing tendency and always maintained a high level,which might have a significant impact on the cycling of nutrients and the succession of marsh community.In this paper,the Suaeda salsa marsh in high marsh of the northern Yellow River estuary was selected as the study object in order to explore the effects of exogenous N enrichment on sulfur?S?biological cycle status in marsh ecosystem.In situ experiment of N enrichments(N0,no extra N import treatment,6.0 gN·m^-2·a^-1;N1,low N import treatment,9.0 gN·m^-2·a^-1;N2,middle N import treatment,12.0 gN·m^-2·a^-1;N3,high N import treatment,18.0 gN·m^-2·a^-1)and incubation experiments were conducted,and the seasonal variations of S accumulation and allocation in plants of different exogenous N enrichments were investigated,the decomposition and S releasing mechanism of litters with different substrate qualities were determined,the temporal and spatial variations of inorganic sulfur fractions in marsh soils were illustrated,and the variations of the potentially mineralizable S pools?S₀?in soils with different substrate qualities were revealed.Finally,the S biological cycling compartment model for plant-soil systems with different exogenous N enrichments was established,and the status and change trend of S biological cycling status were evaluated.The main results were as follows:i)The contents of TS in different organs of S.salsa in the four treatments generally showed leaf>stem>root?P>0.05?,indicating that leaves were the key tissues for S accumulation.Although different N import treatments did not alter the patterns of S accumulation in different organs of S.salsa or the S nutrient supply relationships between aboveground and belowground tissues,the plant could adjust its growth traits and S allocations to adapt different N import conditions,and the adaptation was particularly evident in plants at N2 treatment.ii)As N was continuously imported,the alteration of substrate quality promoted litter decomposition and the promotion was particularly obvious at N1 treatment.The variations of TS contents in different decomposing litters showed an opposite trend with the corresponding C/S ratio,indicating that the C/S ratio was the main factor controlling the variations of TS contents in different litters during decomposition.As N was continuously imported,alterations in litter quality stimulated the release of S and the stimulation was predominant in the N3 treatment,indicating that continuous N input would promote S returning from decomposing litters,and,as a result,the S biological cycling rate in S.salsa marsh might be accelerated.iii)The distributions of TS contents in soils of different N enrichments were greatly altered.Except for topsoil,the contents of TS in other soil layers showed an increasing trend with increasing N loading,which was mainly dependent on the influence of plant growth rhythm and aboveground biomass variation since most plant roots were distributed in topsoil.The increased contents of TS in deep soil layers rested with two possible reasons.For one thing,it might be related to the decomposition of organic matters and the returning of S nutrient promoted by exogenous N input.For another,it might be due to the strong burial for litters and the actine activities of invertebrates in high marsh.iv)The contents of inorganic sulfur fractions in soils of different exogenous N enrichments were altered with varying degrees.The levels of inorganic sulfur fractions in soils of different treatments generally followed the sequence of H₂O-S>Adsorbed-S>HCl-Soluble-S>HCl-Volatile-S,and the proportions of total inorganic sulfur?TIS?content in total sulfur?TS?content in the four treatments ranged from 34.52%to 39.58%.In different N import treatments,the influences of pH and EC on variations of different inorganic sulfur fractions were particularly obvious.The import of exogenous N might affect the absorption and utilization of inorganic sulfur fractions for plants by altering the growth rhythm of plants and the acid-base status of soils,which indirectly changed the contents of inorganic sulfur fractions and the stocks of TIS in marsh soils.v)The import of exogenous N generally promoted S mineralization,and,to some extent,elevated the potential of mineralized S in marsh soils.However,as the soils were flooded,the mineralized S and the potentially mineralizable S in soils at N2 treatment not only were much lower than those at N3 and N1 treatments but also were much lower than those at N0 treatment,implying that,as the marsh soil was flooded at short-term period due to the influence of severe precipitation,spring tide or storm surge,the potentially mineralizable S in marsh soil would decline significantly and the decrease amplitude was particularly evident in N2 enrichment.vi)Besed on above studies,the S biological cycle compartment model for plant-soil systems of S.salsa marsh with different exogenous N enrichments was established,and the S stocks and turnovers among compartments were determined.According to the quantitative relationships in compartment model,the status of S biological cycling in S.salsa ecosystem as affected by different N enrichments was assessed.From the perspective of S biological cycling,the nutrient supply between N and S was inconsistent with increasing exogenous N loading,that is,the increased N might enhance the limitation of S nutrient.This paper found that,with increasing N loading in S.salsa marsh of the Yellow River estuary?particularly at N2 level?,the patterns of S accumulation and allocation in plants and the S supply relationships between aboveground and belowground tissues would be greatly altered and the S limited status in marsh ecosystem was unlikely alleviated.However,as the N loading reached a higher level??N3?,the limitation of S nutrient in marsh ecosystem might be alleviated obviously and a positive feedback mechanism might be formed between N and S.Thus,from the perspective of S biological cycling,the positive feedback mechanism would favor for maintaining the stability and heath of the S.salsa marsh ecosystem.