Effects Of Water-salt Conditions On The Distribution Of Iron Forms In The Yellow River Estuary Wetland Soil

The Yellow River Estuary wetland is an important node of ecological protection in the Yellow River Basin.It is one of the important carriers in the biogeochemical processes of iron,sulfur,carbon,nitrogen and phosphorus,and plays a key role in regulating nutrient elements and heavy metals.Because it is located at the interchange of river and marine ecosystem,the tidal hydrological conditions show periodicity under the influence of tidal action,resulting the surface submergence-exposure and salt accumulation-washing.Under the background of sea level rise,the intensity of seawater intrusion in estuarine wetlands increases,thus the soil salinity and redox conditions will change significantly.As one of the main redox substances,iron oxides are widely distributed in soil and easy to be transformed by environmental factors,which plays an important role in indicating the redox of estuarine wetland ecosystems.However,it remains unknown what would happen to the iron oxides under the effect of water-salt conditions.This study takes the Yellow River estuary wetland as the research object,using the methods of field in situ observation,translocation culture and lab salt simulation,to measure total iron（Fe _T）,complexed iron（Fe_p）,amorphous iron（Fe_o）,free iron（Fe_d）and reduction rate（RFe）.The aims are to clarify the distribution of iron forms and variation of RFe in different tidal zones,through the layer-by-layer analysis of the relationship between water-salt factor and iron forms,to reveal the water-salt mechanisms of iron oxide transformation in estuarine wetland.The main results are as follows:（1）The contents of Fe _T,Fe_p,Fe_o and Fe_d in the supratidal zone（Stz）,the upper of high tidal zone（Uph）,the high tidal zone（Htz）,the middle tidal zone（Mtz）and the low tidal zone（Ltz）soil（0-40 cm）across different seasons ranged from 16.18-39.88,0.02-0.56,0.26-3.67 and 3.80-13.63 g·kg^-1,respectively,varied with tidal zone,soil depth and season.The values in Ltz,Stz and Uph were higher and significant differences in different soil depths.Fe _T and Fe_p were higher in autumn,Fe_o and Fe_dwere in spring and summer.The ratio of iron oxides（Fe_p/Fe_d,Fe_o/Fe_d and Fe_d/Fe _T）are higher in Uph and Htz,showing a trend of decreasing activity with increasing salinity.Correlation analysis results showed that the content of iron oxides was positively correlated with total organic carbon（TOC）,dissolved organic carbon（DOC）,total nitrogen（TN）and Clay,and negatively correlated with electrical conductance（EC）value.（2）The ranges of RFe across different tidal zones in the Yellow River Estuary wetland was 0.11-0.43,0.08-4.27,0.10-0.37,0.11-0.65 and 1.15-4.21 mmol·kg^-1·d^-1,respectively.RFe significantly varies with tidal zone,soil depth and season,with an order of Ltz>Uph>Stz>Mtz>Htz,autumn>winter>summer>spring,and showing a decreasing trend with the increase of depth.The mean values of the rate of organic carbon mineralization contributed by iron reduction（RFe-C）were 0.20,0.35,0.10,0.12 and 0.52 mmol C·kg^-1·d^-1,and their contribution rates to carbon mineralization（RFe-C%）were 31.6%,51.5%,47.7%,43.3%and 65.4%,respectively.The total abundance of iron-reducing bacteria in different tidal zones varied from 1.4 to 5.3%.Among them,the relative abundance of Pseudomonas was higher in soil,Thiobacillus,Bacteroides and Vibrio were higher in Uph,and Geothermobacter was higher in Mtz.The activity of iron-reducing bacteria decreased with the increase of salinity.Correlation analysis results showed that RFe was positively correlated with soil water content（SWC）,TN,TOC and DOC,and negatively correlated with soil salinity.（3）During translocation culture,after the freshwater marsh soil was transferred to the salt marsh,the change of water-salt conditions had a significant effect on the iron and its oxides in the freshwater marsh soil.The contents of Fe _T,Fe_p,Fe_o and Fe_d in freshwater marsh were significantly higher than those in salt marshes,and the lowest values were found in Htz.After 23-month culture,Fe _T,Fe_p,Fe_o and Fe_d were 31-39%,85-89%,55-71%and 7-22%lower than those in Stz,respectively,indicating that the activity of iron oxides decreased significantly after the change of salinity.RFe will decrease significantly,and this degree will raise with the increase of tidal influence intensity.During the 23-month culture,the RFe ranged from 0.07 to 1.60mmol·kg^-1·d^-1,contributing from 12 to 82%of the carbon mineralization.After translocation,RFe decreased by 17%,44%and 58%,respectively,compared with Stz.（4）In the salinity gradient experiment,the content of soil iron oxides was significantly different under different water-salt treatments.In general,the contents were higher at the salinity level of 10‰and 5‰,and lower at the salinity level of 0‰and 20‰.The activity of iron oxides under anaerobic treatment was higher than aerobic treatment.Compared with the initial value,Fe_p,Fe_o and Fe_d changed by33.33-133.33%,2.15-88.17%and-17.79-9.49%,respectively.RFe was higher at the salinity level of 10‰and 5‰than 0‰and 20‰,and the RFe and CO₂-C mineralization rate（Cmin）were highly consistent.The contribution at the level of 0‰,5‰,10‰and 20‰was 21%,24%,20%and 15%,respectively.Pseudomonas,Geobacte and Geothermobacter were the dominant genera.The increase of salinity reduced the number of bacteria,species diversity richness and the number of common species in freshwater marsh.To sum up,iron oxides and RFe in the Yellow River Estuary wetland soil vary with the tidal hydrological conditions,and are significantly affected by soil salinity.The spatial heterogeneity of iron forms is closely related to soil organic matter,grain-size and salinity level.The increase of salinity can reduce the activity of iron oxides by affecting the microbial community structure.The results indicate that,under the background of sea level rise in future,the difference of soil properties caused by salt input may lead to the spatial heterogeneity of iron in estuarine wetlands,and affect the migration and transformation of iron oxides in freshwater marsh by changing soil electrochemistry and organic matter cycling.