Effects Of Iron On Lignin Photochemical Degradation In Subalpine Meadow Soil

Soil organic carbon（SOC）mineralization is a crucial subsurface ecological process influenced by various factors such as environmental,climatic,vegetation,and anthropogenic factors.It is closely related to the release and storage of soil nutrients and plays a significant role in the global carbon cycle.Lignin,the most abundant aromatic compound on Earth,is an important substance that influences the SOC cycle.Iron,which is abundant and redox-active in the Earth’s crust,exists primarily as oxides in soils and is widely distributed across different soil types.National and international studies have demonstrated that iron oxides bound to organic matter selectively adsorb phenolic and aromatic compounds derived from residual plant lignin.Furthermore,the addition of iron inhibits approximately 21%of lignin mineralization.Consequently,iron oxides in soils play a crucial role in lignin stability.Lignin is also sensitive to light,and UVB or solar radiation induces photochemical degradation of lignin in soils.The accelerated rate of lignin mineralization in surface soils due to increased global mid-wave ultraviolet（UVB）radiation is an important ecological process experienced by many terrestrial ecosystems,particularly in subalpine areas at higher elevations.Additionally,iron oxides in soils can regulate soil carbon cycling processes by influencing the stability of soil lignin through adsorption or co-precipitation.However,the influence of iron（Fe）on the photodegradation process of lignin remains unclear.Therefore,this thesis investigates the effect of exogenous iron oxide addition on the photochemical degradation of surface soil SOC by simulating enhanced UVB radiation conditions indoors.Furthermore,an indoor UVB radiation lignin mineralization incubation experiment is conducted to examine the effect of iron on the photochemical degradation.The primary research results are as follows:（1）Slope orientation significantly affects soil physicochemical properties.Surface soil p H and NO₃^--N content are significantly higher on the NW slope than on the SE slope.The C/V ratio of soil lignin is significantly lower,and（Ac/Al）v is significantly higher on the NW slope compared to the SE slope.Fe_o-pand Fe_pare significantly positively correlated with SOC（P<0.05）.Fe_pis significantly negatively correlated with C/V（P<0.05）and（Ac/Al）v（P<0.05）.（2）The surface SOC mineralization characteristics of subalpine meadows differ among slopes,with the cumulative SOC mineralization and mineralization rate being significantly higher on the NW slope compared to the SE slope.Under UVB radiation,the amount and rate of mineralization are significantly higher on the NW slope than on the SE slope,thus promoting the mineralization of soil organic carbon.（3）Exogenous addition of Fe oxides significantly reduces the accumulated mineralization amount and mineralization efficiency of soil carbon.Additionally,Fe oxides can mitigate the photochemical degradation of surface soil organic carbon to some extent.（4）Under UVB irradiation conditions,the exogenous addition of Fe（Ⅲ）stimulates the oxidative catalytic depolymerization of lignin,thereby increasing the rate of abiotic degradation of lignin.In contrast,the addition of exogenous Fe（Ⅲ）inhibits lignin degradation under dark incubation.（5）Exogenous Fe（Ⅲ）addition significantly inhibits the biological processes involved in the chemical degradation of lignin.UVB radiation and exogenous Fe（Ⅲ）addition do not significantly affect the total number,abundance,and diversity of soil bacteria and fungi but significantly reduce the Simpson index of bacteria.In conclusion,this study systematically investigates the effect of Fe on the photochemical degradation of surface soil organic carbon in subalpine meadows with different slope orientations.It further explores the mechanism of Fe on the photochemical degradation of soil lignin.The findings reveal that Fe has varying effects on the mineralization rate of surface soil organic carbon in different slope orientations.Fe can significantly reduce the photochemical degradation of surface SOC,and the exogenous addition of Fe oxides can inhibit the photochemical degradation of lignin to a certain extent.This work provides a theoretical basis for a deeper understanding of the carbon sequestration mechanism in the surface soil of subalpine meadows in the context of climate change.