Study On The Effect Of Different Valence Iron On Organic Matter Decomposition In Temperate Forest Swamp Soil

Wetlands account for about 5–8%of the global land surface area,but store 12–24%of global soil carbon,much more than the area they occupy.Forest swamp is a special type of wetland where redox environment feature is more typical and is an important factor affecting carbon storage.As one of the richest elements in the earth’s crust.Fe has high redox activity and plays a key role in biogeochemical processes.Therefore,research on the effect of Fe valence content on organic carbon decomposition in wetland soils provides an important complement to the theory and research on carbon sequestration and organic matter conversion mechanisms in wetland soils under global climate change,as well as the carbon cycle in soil ecosystems.This article selected the forest swamp of Gushan Tun,Tonghua City,Jilin Province as the study area,field monitoring and sampling were carried out in summer and autumn of 2021,and carried out experiments in the laboratory with different concentrations of exogenous Fe additions（0 g,27.8 g,55.6 g Fe SO₄?7H₂O）in order to investigate the effects of soil Fe valence content on soil organic matter decomposition under the background of different seasons,the effects of different concentrations of exogenous Fe addition on soil organic matter decomposition and a comprehensive analysis of the mechanistic relationship between Fe and carbon.The specific results of the study are as follows.（1）The community composition and abundance of Fe-reducing bacteria increased significantly with the increase of water content.Due to the insufficient supply of oxygen due to the increase of water content,Fe-reducing bacteria could make better use of electron acceptors and thus promote the Fe-reducing reaction,resulting in a significantly higher Fe（Ⅱ）content of the forest swamp study area in autumn soil than in summer soil.The dissolved organic carbon（DOC）content in soil showed an increasing trend with seasonal water level fluctuations.Fe（Ⅱ）content was positively correlated with DOC content,and Fe（Ⅲ）content was negatively correlated with DOC content.（2）Soil DOC content is closely related to the metabolism of soil microorganisms,and microbial activity and abundance have a positive effect on the stability of DOC content.And with the seasonal water level fluctuation,the fluorescence index（FI）and biological index（BIX）values and the percentage of protein-like fluorescent fractions were higher in autumn soils relative to summer soils.（3）Soil Fe content was significantly increased by the addition of exogenous Fe.SOC content was not significantly affected by the addition of exogenous Fe,but showed an increasing trend with the extension of incubation time.DOC content and soil respiration rate were significantly decreased by the addition of exogenous Fe.The fluorescence index,biological index,humification index（HIX）and the proportion of each component were all affected by the addition of exogenous Fe.The FI and BIX values of L2 soil were significantly increased,the HIX values were significantly decreased,and the tryptophan-like fraction of soil DOM was significantly increased.（4）The co-precipitation reaction of Fe（Ⅲ）with dissolved organic carbon led to a significant decrease trend of DOC content,the analysis of the infrared spectra of the original soil and the co-precipitates of Fe（Ⅲ）and DOC showed that the main functional groups contained in the soil were hydroxyl?OH,carboxyl-COOH,carbon-oxygen double bond C=O,hydrogen bond H,and amino NH₂,etc.In the co-precipitates of Fe（Ⅲ）and DOC,the absorption peaks of Fe（Ⅲ）and DOC co-precipitates are basically the same as those of the original soil,but the C-H of aliphatic carbon and the C=O of cellulose show a decreasing trend,while the iron-oxygen Fe-O bond shows an increasing trend.From the X-ray diffraction analysis,it can be seen that the addition of Fe（Ⅲ）caused a significant decrease in the elemental content of C and a significant increase in the elemental content of Fe in the sample.