| Soil organo-mineral associations play key roles in the long-term persistence of soil organic carbon（SOC）.The SOC bound to active Al and Fe minerals is thought to play an important part in carbon（C）stability by limiting microbial and enzymatic activities.However,these associations seem to be sensitive to biotic products（e.g.root exudates）and soil parameters（e.g.soil pH and cations）.In alpine mountain ecosystems,the different microclimates caused by topographic and considerable changes in environmental factors with the seasons will significantly affect the soil biogeochemical processes,and then affect the spatial and temporal distribution of soil organo-mineral associations.In this study,3 elevation gradients（3 800 m,Low;4 000 m,Mid and 4200 m,high）and 2 slope aspect（NE,semi-shady and SW,semi-sunny）were set to study the spatial distribution characteristics of soil organo-mineral associations in alpine mountain soil.Both in situ（vegetation removal）and field experiments were used to study the short-term dynamic characteristics of soil organo-mineral associations.Na-pyrophosphate（PP）,HCl-hydroxylamine（HH）and dithionite-HCl（DH）were used to quantify organo–metal complexes,SOC associated with short-range order（SRO）phases and crystalline phases,respectively.The main results were represented as follows:（1）Elevation and slope aspect significantly affects the distribution of soil physical and chemical properties.Compared with semi-sunny slope,semi-shady slope soils have higher soil water content,higher nutrients,more abundant vegetation types and lower pH.With the elevation increasing,soil clay content trends to increase and aggregate content trends to decrease.Besides,SOC,total nitrogen and total phosphorus showed an increasing trend with elevation,while the vegetation types trended to decrease.（2）In the two alpine mountain soils（alpine meadow soil,AM and mountain podzolic soil,MP）,the extracted C and their percentages of SOC are similar.The crystallizing degree of Al and Fe minerals in horizon A is lower than horizon B.The organo-metal complexes are the dominant fractions of organo-minetal associations in Horizon A.With the increase of soil depth,the crystallinity of Al and Fe minerals increased,and the content of organo-metal complexes decreased,while the content of amorphous and crystalline minerals associated C increased,especially in MP soil.（3）Elevation and slope aspect significantly affected the distribution of soil organo-mineral associations.Compared with the semi-sunny slope,the content of organo-minetal associations,their percentage of SOC and Fe minerals in the semi-shady slope are higher than semi-sunny slope,which related to higher soil water content and lower pH.With the elevation increasing,the organo-metal complexes and its percentage of SOC increased,while the amorphous and crystalline minerals associated C decreased.In addition,there is a significant positive correlation between HH and PP extracted C and Fe,but no significant relationship with Al.The DH extracted C was positively correlated with Al,but not Fe.（4）Soil freeze-thaw event significantly affected soil chemical properties.Soil pH and exchangeable cations significantly increased after freeze-thaw event.The increased soil pH weakens the adsorption of organic carbon by minerals,CDH and C/MDH in AM decreased by 7.72%and 14.10%after freeze-thaw,respectively,which was negatively correlated with soil pH.The CDH decreased by 7.72%after freeze-thaw event,and correlated with a significant negative correlation with increased soil pH.The increased Al3+and Fe3+after freeze-thaw event can directly react with the formed organo-metal complexes,resulting in a significant decrease of 14.56%in C/MPP after freeze-thaw event.However,in MP（low pH）,freeze-thaw event had no significant effect on the content and structure of organic mineral complexes.（5）In AM,the content of CHH in August was significantly lower than that in October and May.In MP,the content of CPP in May was significantly lower than that in August and October,while the content of CHHin October was significantly higher than that in August and May.Although the C content of other components did not show significant differences between seasons,the metal content and C/M both varied in different degrees,indicating that the organo-mineral associations were dynamic among seasons.The HH extracted C in the two soils showed consistent trend with SOC,reaching the highest value in October,reflecting the significant role of amorphous minerals in SOC preservation and accumulation.（6）All the C/SOC fractions in AM reached the highest value in May.In MP,the CPP/SOC and CHH/SOC gradually decreased from October to May,while the CDH/SOC increased,indicating the mineral protection of SOC is periodic.In addition,the dynamic characteristics of C/SOC in the two soils were different with the seasons,which indicated that the dynamic characteristics of organo-mineral associations varied with soil types.In soils with less microbial substrates（e.g.MP）,organic carbon in soil organo-mineral associations was more easily degraded.Our study showed that elevation,slope aspect,soil types and seasonal freeze-thaw events significantly affected the spatial and temporal distribution characteristics of soil organic carbon and iron and aluminum mineral associations.In addition,although organo-mineral associations are assumed to be stable carbon fractions,the content and structure of organo-mineral associations due to seasonal variations in soil properties（e.g.soil pH,exchangeable cations,ECEC,etc.）and biological processes（root exudates）.Therefore,the protection of SOC by minerals is not absolute,but the dynamic balance of organic carbon under certain environmental and biological factors. |
