Coupling Response Of Iron Oxide Phase Differentiation And Phosphorus In Soils With Multi-environmental Gradients

The coupled geochemical cycle of iron（Fe）and phosphorus（P）is an important scientific issue in the origin and evolution of life.Fe oxides are climate-sensitive minerals that weathered by iron-bearing silicate in the surface of the earth,which are usually attached to the surfaces of primary and secondary minerals in the form of nano-colloids,dominating the optical and magnetic properties of soils and sediments.Soil magnetic and optical indexes have thereby become important parameters for soil classification and climate reconstruction.P is a life element of sedimentary cycle that plays a key role in the formation and evolution of biosphere.During terrestrial weathering and pedogenic process,primary phosphorus-bearing minerals are gradually leached and secondary Fe oxides are gradually enriched.Fe oxides can produce specific adsorption and physical occlusion of phosphate,thus retarding the leaching process of P and affecting its biological effectiveness.Conversely,the adsorption of Fe oxides to phosphate may also interfere with the crystallization and transformation of Fe oxides,thus affecting the optical and magnetic properties of soils.Therefore,we studied the coupling relationship between Fe oxides and P in Ferrisols and Ferralsols under different environmental gradients in subtropical area.This study is of great significance for the evolution of P and its related mechanism in soils under the background of global change.In this study,Ferrisols and Ferralsols,which derived from granite,carbonate and phosphate mining deposit in subtropical southern China,were collected.By the means of geochemistry,soil chemistry,spectroscopy and environmental magnetism,focusing on the distribution process of multi-scale Fe oxide phases caused by the change of water activity in natural processes,the relationship between iron oxides and phosphorus fractions was discussed from the gradient of terrain,climate and redox environment,respectively,and the effect of iron oxide formation path on phosphorus distribution was studied.Meanwhile,the effects of phosphate ligands on the Fe oxide free degree,crystallinity and enrichment of the magnetic minerals under high P/Fe gradient were discussed.The main results are shown as follows:（1）On the climate scale,from dry hot to cold wet environment,the content of free Fe oxides（Fe_d）,amorphous Fe oxides（Fe_o）,hematite（Hm）and their relative proportions in soils generally decreased,with Hm in particular decreasing mostly,while the content of goethite（Gt）and its relative proportion increased significantly.Meanwhile,the content of inorganic P and its related fractions decreased,but the relative proportions of Fe-bound P（Fe-P）increased,the proportions of occluded P（O-P）decreased,implying that Fe_otransformed mainly to Hm when the climate changed from cold wet to dry hot,and the Fe-P originally adsorbed on the surface of Fe_o was rapidly physically occluded by Hm to form O-P,making dry hot climates more enriched in O-P and cold wet climates more enriched in Fe-P.（2）On the terrain scale,from the upper slope to the lower slope,the free degree of Fe oxide（Fe_d/Fe_t）,the content and relative proportion of Hm decreased,in which the decrease of Hm was the most obvious,while the Fe oxide amorphous degree（Fe_o/Fe_d）increased slightly,and the content of Gt changed little.Inorganic P is the main P form,and O-P and Fe-P are the main fractions of inorganic P,but the change of O-P is not significant,while the content of Fe-P increased slightly with the decrease of altitude.Hm has an obvious negative correlation with Fe-P,but a weak correlation with O-P,indicating that it can reduce the Fe-P in the process of Hm formation.However,because Gt is the main crystalline Fe oxide,and the adsorption capacity of Hm for phosphate is weaker than that of Gt,the effect of rapidly decreasing Hm on O-P in the slope is not significant.（3）On the profile scale,from the top to the bottom,according to the change of soil water activity and redox condition,it can be divided into oxidation horizon,hydration horizon and reduction horizon.From oxidation horizon to hydration horizon,the content of Fe_d,Fe_oand Hm decreased,while Gt increased rapidly.Inorganic P is the main P form,and the contents of Fe-P,O-P and aluminum-bound P（Al-P）increased,among which the increase of Al-P was the most significant.There is a good positive correlation between Al-P and Gt,indicating that the isomorphous substitution of Gt by Al leads to the increase of the content of Al-P.At the same time,the structure of the substituted Gt was destroyed and its specific surface area increased,which increased the phosphate adsorption capacity of Gt,resulting in an increase in the contents of Fe-P and O-P.From hydration horizon to reduction horizon,the contents of Fe_d,Fe_o,Hm and Gt decreased significantly,but Fe²⁺/Fe³⁺increased rapidly.The contents of inorganic P and its related fractions decreased obviously,but the content of organic P increased and became the main P form,which implying that Fe²⁺could better complex with organic matter to form metal-organic complex,thus adsorbing phosphate to form organic P,and the increase of organic P content can partly offset the loss of total P caused by the decrease of inorganic P.In addition,according to the change characteristics of Fe oxides and P fractions in acid soil at different water activity stages,a three-stage conceptual model of P fractions changing with Fe oxides was established.（4）In the high P/Fe gradient sequence,from high P/Fe（HP）group to low P/Fe（LP）group,the Fe_d/Fe_t and Fe_o/Fe_d decreased,the Fe oxide crystallinity increased.Inorganic P is the main P form,the content of inorganic P and related P fractions decreased rapidly,but the proportion of O-P increased significantly,which was 50.38%in HP group and72.46%in LP group,while the proportion of Fe-P decreased significantly,39.1%in HP group and 13.07%in LP group.Therefore,the more P in HP group can inhibit the transformation of Fe_o to Fe oxides with higher crystallinity.Meanwhile,the strong adsorption capacity of Fe_o to phosphate increases the content of phosphate chemisorbed and decreases the content of phosphate physically occluded.The results showed that P/Fe has an important effect on the distribution of iron oxide phases and phosphorus fractions.（5）Ferrimagnetic particles have different enrichment and grain size growth patterns in different P/Fe ranges.When P/Fe>0.0275,Ferrimagnetic particles show an exponential enrichment,with fine superparamagnetic and single-domain particles dominating,and when P/Fe<0.025,the ferrimagnetic particles show a linear enrichment,with fine magnetic particles having an obvious grain size growth.Therefore,the adsorption of Fe oxides on phosphate at high content can inhibit the aging process of ferrihydrite to Hm,thus facilitating the rapid enrichment of ferrimagnetic minerals as the intermediate product.while the crystallinity of Fe oxides increases when the phosphate content is low,and the ferrimagnetic particles can grow rapidly and be transformed into Hm,an antiferromagnetic mineral that is more thermodynamically stable.This model successfully explained the decoupling of soil color and magnetism in highly weathered soils and sediments,laying the foundation for extreme climate reconstruction based on environmental magnetism and optics.