| Humus is an important component of soil organic matter and is important for soil fertility and nutrient cycling.Global warming has led to a marked reduction in seasonal snow cover at high latitudes and altitudes,which may affect soil humus accumulation by altering the surface hydrothermal environment and decomposer activity in alpine forest organic matter turnover processes.However,it is still unclear how snow cover reduction affects soil humus accumulation and stability in the context of global warming.In this study,we investigated the effect of snow cover reduction on the accumulation of soil extractable humic substance,humic acid,and fulvic acid in a subalpine Abies faxoniana forest in western Sichuan Province for two consecutive years,using a field snow cover control experiment with three snow cover gradients:control(natural snow cover),snow cover reduction(50% snow cover removal)and snow cover removal(100% snow cover removal).The study investigated the effect of snow cover reduction on soil extractable humic substance,humic acid,fulvic acid,and spectroscopic characteristics of humic substances,and the dynamic correlation between snow cover reduction and soil humus by combining dynamic monitoring of environmental factors,microbial biomass,and enzyme activity.The main findings are as follows:(1)After 2 years of incubation,both snow cover reduction and snow cover removal significantly reduced winter soil surface temperatures and increased the frequency of soil freeze-thaw cycles.In the organic soil layer,snow cover removal significantly reduced soil water content,organic carbon,total nitrogen and total phosphorus.Soil water content,organic carbon,total nitrogen and total phosphorus were all higher in winter than in the growing season and were highest at 1.5 years of incubation.In the mineral soil layer,snow removal significantly increased soil p H,but reduced soil total phosphorus content.Soil organic carbon content was highest at 0.5years of incubation and total phosphorus content was highest at 1 year of incubation.In addition,snow cover treatment significantly affected soil carbon cycle phase enzyme activity.In the organic layer,snow removal significantly reducedβ-1,4-glucosidase activity but increased polyphenol oxidase activity;in the mineral layer soils,snow removal increased β-1,4-glucosidase activity and winter polyphenol oxidase activity but reduced growing season polyphenol oxidase activity.Both organic and mineral layer soils showed higher β-1,4-glucosidase and polyphenol oxidase activities in winter than in the growing season.(2)After 2 years of incubation,snow cover treatment significantly affected the soil extractable humus,humic acid and fulvic acid content.Compared to the control,snow cover removal significantly reduced the extractable humus,humic acid and fulvic acid content of the soil in the organic layer.In the mineral layer,however,snow removal only affected the humic acid content of the winter soils,which was mainly higher in the control than in the snow removal.The results of the net accumulation of extractable humus,humic acid and fulvic acid showed that all three substances accumulated in winter and degraded during the growing season.Secondly,the degree of soil humification was lower under the three snow cover treatments.Snow cover removal significantly reduced the degree of winter soil humification compared to the control.Based on the results of the spectroscopic characterisation of humus,it was found that the snow cover treatment had no significant effect on soil Δlog K values and A600/C values during the 2-year incubation,but significantly increased soil E4/E6 values in the organic layer and decreased soil E4/E6 values in the mineral layer.At the same time,it was found that the soil humus type under all three snow cover treatments was a younger Rp-type structure and the humus material in the soil was always in a more active state.(3)Correlation analysis showed that soil extractable humus,humic acid and fulvic acid content were significantly and positively correlated with soilβ-1,4-glucosidase activity,moisture,organic carbon,total nitrogen and total phosphorus content,and negatively correlated with temperature and number of freeze-thaw cycles.The degree of soil humification was positively and significantly correlated with microbial biomass nitrogen and negatively with polyphenol oxidase,cellulose,and number of freeze-thaw cycles.Random forest model analysis revealed that polyphenol oxidase was the key influencing factor for the extractable total humus and huminic acid content of the organic layer of the soil,with a relative weight of15.73% and 15.47%,respectively,while total nitrogen was the key influencing factor for the fulvic acid content,with a relative weight of 32.40%.In the mineral soil layer,soil water content was the key influencing factor for total extractable humus and fulvic acid content with relative weights of 19.23% and 20.97%,respectively,and peroxidase was the key influencing factor for huminic acid content with relative weights of 27.16%,respectively.In summary,the humification process in the soils of Abies faxoniana forests in the subalpine west of Sichuan is not a continuous accumulation of humic substances,but a dynamic process of continuous degradation and accumulation.Soil humic substances are mainly accumulated in winter and degraded during the growing season.The reduction of snow cover reduces the content of soil humus and affects the degree of soil humification,and this process is mainly influenced by the quality of the soil substrate,environmental factors,and enzyme activity.These results are informative for an in-depth understanding of the formation of soil humus in subalpine forests and the mechanisms of soil fertility maintenance in subalpine forests. |
PDF Full Text Request