| Humus accounts for 70%-80% of total soil organic matter and is mainly derived from the humification process of litter residues,which is not only an important manifestation of soil fertility and quality,but also a key element in understanding the material cycling mechanism of natural ecosystems such as "self-fertilization".Soil fauna can influence the accumulation and turnover of soil humus by directly(e.g.,feeding,fragmentation,mixing etc.)and indirectly(e.g.,regulating the number,structure,and activity of soil microorganisms etc.)ways.Elevation gradients can characterize both differences in hydrothermal environmental conditions and vegetation change patterns,and are considered to be the natural experimental designs for understanding environment and ecosystem processes.Therefore,seven typical ecosystems at different elevation gradients along the Minjiang River basin were taken as research objects,including the central subtropical forest(453 m),the northern subtropical forest(945 m),the dry valley(1563 m),the dry valley-mountain forest interlacing zone(2158m),the subalpine forest(3028 m),the alpine forest(3593 m),and the alpine meadow(3989m).Two common plant litter were selected from ecosystems at each altitude,and the litter bag method(3 mm mesh size allowed for soil animals to enter,0.04 mm mesh size basically excluded soil fauna)was used to study the influence of soil fauna on the humification process of dominant species on the altitude gradient,and the seasonal dynamics of soil fauna and humus content in each ecosystem were monitored,in order to understand the characteristics of soil humus and the key role of soil fauna in typical ecosystems in the upper reaches of the Minjiang River.The main results are as follows:(1)Throughout the experimental period,a total of 11,462 and 2,211 soil fauna were captured in the litter leaves and soil,respectively.The seasonal dynamics of individual density,group density and diversity index were significantly higher in autumn than in other seasons.Altitude significantly influenced individual density,group density and diversity index of soil fauna.Compared with other altitudes,the density of soil fauna was lower at1563 m and 3989 m,and higher at 453 m,945 m and 3028 m.Based on soil fauna feeding analysis,it was found that predatory and phytophagous soil fauna accounted for 87% of the total captures in the litterbag.The number of Herbivorous(1027 individuals)and Detritivores(1414 individuals)soil fauna in the litter bags of 3028 m elevation was highest,while the number of Herbivorous(256 individuals),Detritivores(196 individuals)and predatory(105 individuals)soil fauna in the soil was highest at 3593 m elevation.(2)During the whole experimental period,the total extractable humus substance(5.65 mg)and huminic acid accumulation(5.01 mg)of litter leaves were the highest at the elevation of 3989 m,the accumulation of fulvic acid(5.28 mg)in litter at the elevation of 945 m was the highest,the litter humification degree was highest(2.87%)at the elevation of1563 m.The total extractable humus substances,humic acid and fulvic acid were degraded in the non-growing season in the first year of decomposition,and were enriched during the growing season.The degree of litter humification peaks at the end of the growing season in the first year of decomposition.The average contribution of soil fauna to total extractable humic substance content,fulvic acid content and litter humification were greatest at 3989 m elevation with 74.10%,71.98% and 52.98%,respectively,and lower at 453 m elevation with 10.12%,8.80% and 12.47%,respectively.The effect of soil fauna to huminic acid on winter was lower than that of fulvic acid at all altitudes except for 3989 m,and on the contrary during the growing season.(3)After one year of decomposition,litter alkyl carbon content was highest at 1563 m(26.97%)and 945 m(26.56%)and lower at 3989 m(22.15%),and litter aryl carbon(14.48%)and oxyaryl carbon(5.32%)were highest at 3028 m elevation.Compared with other seasons,the content of alkyl and oxyalkyl carbon of litter was higher in summer,while the content of aryl carbon,oxygenated aryl carbon,carbonyl carbon and carboxylic carbon were lower than other times.The increase of soil fauna Shannon-Wiener diversity in leaf litter decreased the content of aryl,oxygenated aryl and carboxyl carbon.Conversely,the oxyalkyl carbon content is increased.Herbivorous soil fauna significantly promotes the formation of alkyl carbons in litter leaves.(4)Total extractable humic substance,humic acid and fulvic acid in the soil organic layer and mineral soil layer were the highest at elevation of 3028 m(13.45-112.14 g/Kg),and the lowest in the soil organic layer and mineral soil layer at altitudes of 945 m(2.94-32.36g/Kg)and 1563 m(1.18-13.99 g/Kg),respectively.Total extractable humic substance and fulvic acid content were lowest in March.In contrast,the ratio of humic acid to fulvic acid was highest at 1563 m and reached a maximum in March.Soil functional carbon did not vary significantly along the elevation gradient and was mainly influenced by seasonal changes.In August,soil alkyl and oxyalkyl carbons were significantly higher at 2158 m elevation than at other elevations.In April,soil alkyl,carbonylic and carboxy carbons were higher at 3028 m elevation than at other elevations.Soil functional carbon at all altitudes behaved as: oxyalkyl carbon > oxygenated aryl carbon > carboxylic carbon >alkyl carbon > aryl carbon > carbonyl carbon.(5)Redundancy Analysis showed that soil fauna,environmental factors and litter quality contributed 35.7%-58.5% of the variance explained to litter humus components and humification degree.Moreover,the contribution of soil fauna to the humification degree of leaf litter was negatively correlated with the soil fauna Simpson dominance index and moisture content.The variation expanlation degree of soil fauna,soil physicochemical properties and the existing stock of leaf litter on soil humus composition and humification degree was 33.8%-84.9%,and the relationship between soil humification degree and soil p H was relatively high.The path analysis showed that altitude influenced the effect of soil fauna on litter humification mainly by regulating the environmental factors,soil fauna functional groups and litter substrate quality.The humification degree of soil is comprehensively regulated by the physicochemical properties of soil,the density of soil fauna and the Shannon-Wiener diversity index on the altitude gradient.In summary,the soil fauna community structure in the Minjiang River basin was significant differences in elevation gradients,which significantly effcet litter humification process.Under the influence of moisture,the dry valley at 1563 m elevation and the alpine meadow at 3989 m elevation had lower individual density and group diversity of soil fauna,but the soil fauna contributions more to litter humification.The contribution of soil fauna to litter humification is regulated by a combination of environmental factors(e.g.,temperature and moisture)and substrate quality(e.g.,nitrogen and phosphorus concentrations).These results are helpful to further understand the formation mechanism of soil humus under typical ecosystems in the Minjiang River Basin,and provide a certain scientific basis for soil organic matter accumulation in regional ecosystems. |
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