Effects Of Seasonal Snow Cover On Soil Biochemical Characteristics Of Alpine Forest In Western Sichuan

Seasonal snow cover is a crucial ecological factor that controls winter ecological processes in cold ecosystems.However,due to climate warming,the seasonal snow cover in the northern hemisphere has significantly decreased,leading to significant impacts on the seasonal snow cover traits(e.g.snow cover depth,snow cover cycles,and coverage area)of cold ecosystems.Subalpine forest ecosystems in western Sichuan,located on the eastern edge of the Tibetan Plateau,are ecologically sensitive areas that respond to climate change.The reduction of winter snow cover may significantly impact soil microbial biochemical properties and the processes of carbon and nitrogen mineralization by altering the soil microenvironment.However,the mechanisms and effects of these changes have not received adequate attention to date.Therefore,field snow cover manipulation devices were set up in the subalpine Abies faxoniana coniferous forest in western Sichuan,and three treatments were designed: ambient snow cover(control),snow cover removal by 50%(snow reduction treatment),and snow cover removal by 100%(snow removal treatment).The in-situ soil core cultivation method was employed to study the effects of seasonal snow variations on soil micro-environment,microbial activity,microbial biomass,microbial community composition and diversity,and soil carbon and nitrogen mineralization rate in the soil organic and mineral layer of Abies faxoniana forest during key periods of winter(snow formation,coverage,and melt periods)and growth season(early-,middle-,and late growth season)from 2018 to 2020.The main results are as follows:(1)The highest depth of winter snow cover was measured at 39 ± 4 cm.In the snow reduction and snow removal treatments,the depth decreased by 53.63% and 100% compared to the control,respectively.The snow cover treatments(snow reduction and snow removal)caused more severe fluctuations in soil temperature.The mean temperature of the soil organic layer decreased by 0.41°C and 0.24°C,and that of the soil mineral layer decreased by 0.58°C and 0.47°C,respectively,due to the snow reduction and snow removal.The effect of snow reduction on soil temperature in winter was the most significant,whereas snow removal resulted in lower soil temperature in the growth season.As a result of the snow reduction and removal,the frequency of soil freeze-thaw cycles increased by 17.67 and15.33 times,and soil water content decreased by 1.34% and 13.13%,respectively.However,the snow cover treatments did not significantly affect soil p H.(2)The effect of snow cover treatments on soil β-glucosidase activity was only significant,but its interaction with the sampling season had a significant impact on soil β-glucosidase,cellulase,urease,and nitrate reductase activities,with significant differences observed between various soil layers.Snow reduction and snow removal decreased β-glucosidase activity by 5.23%～11.61% and urease activity by 8.35%～10.77%,but increased nitrate reductase activity in the soil organic layer by 7.58%～11.80% and decreased it in the soil mineral layer by 3.56%～9.88%,respectively.In winter,snow reduction and snow removal inhibited the activities of β-glucosidase,cellulase,and nitrate reductase,but promoted urease activity in the soil organic layer during the snow formation period.In the late growth season,snow reduction and snow removal inhibited urease activity and significantly promoted nitrate reductase activity.Although the snow cover treatments had minimal impact on soil microbial biomass carbon content,they significantly increased microbial biomass nitrogen content by 39.18%～79.18%.The snow formation and melt periods were when microbial biomass nitrogen increased the most.(3)In comparison to the control,snow reduction and removal significantly increased the Chao and ACE abundance indices for fungi and bacteria,with increases of 3.13～6.41%and 6.41%～2.54% for fungi,and 2.12～5.95% and 2.00%～5.92% for bacteria.Winter was primarily when there were significant microbial abundance changes.Snow reduction increased the abundance of Basidiomycota fungi and Acidobacteria bacteria at the phylum level while decreasing the abundance of Mortierellomycota fungi and Actinobacteria bacteria.At the genus level,snow reduction led to an increase in the Boletaceae fungi and a decrease in the Mortierella,Acidothermus and Acidimicrobiia bacteria.The fungal and bacterial communities were not signigicantly changed by the snow removal treatment,though.(4)The snow cover treatments had a significant impact on the soil carbon mineralization rate,but snow reduction and snow removal had opposite effects on the soil nitrogen mineralization rate.Snow reduction and snow removal reduced the soil carbon mineralization rate by 11.39% ～ 26.14% compared to the control.However,snow reduction lowered the soil nitrogen mineralization rate by 122.50%,snow removal increased it by178.39%.Additionally,the concentrations of soil organic matter,total nitrogen,dissolved organic carbon,and dissolved organic nitrogen were significantly impacted by the snow cover treatments.Soil organic matter concentration increased by 30.72% ～ 121.6% as a result of snow reduction and snow removal,and dissolved organic nitrogen concentration increased by 42.54% ～ 132.09%.Snow reduction and snow removal led to a 32.85% increase in the concentration of dissolved organic carbon in the soil organic layer and an 11.42%decrease in the soil mineral layer.Snow reduction and snow removal caused a 63.06%increase in the total nitrogen concentration in the soil organic layer,and both treatments led to an equivalent increase and decrease of 9.68% in the soil mineral layer.(5)The partial least square structural equation modeling(PLS-SEM)showed that,the snow reduction and snow removal suppressed the soil carbon mineralization rate through soil cooling,decreased enzyme activity,and soil organic matter accumulation.Simultaneously,soil nitrogen mineralization rates were lowered by soil cooling and decreased water content driven on by snow reduction and snow removal.The soil nitrogen mineralization rate is decreased by lowered soil enzyme activity and organic matter accumulation due to snow reduction.Inhibitory effects of snow removal on denitrifying enzymes and promoting effects on total nitrogen decrease the soil nitrogen mineralization rate in the soil organic layer,and promoting effects of snow removal on denitrifying enzymes and inhibiting effects on total nitrogen increase the soil nitrogen mineralization rate in the soil mineral layer.In conclusion,the soil micro-environment variations driven by seasonal snow reduction and removal can affect soil carbon and nitrogen mineralization by regulating soil temperature and water content dynamics to alter microbial activity(β-glucosidase,cellulase,denitrifying enzyme activity and microbial biomass nitrogen),microbial community richness(Chao and ACE indices for fungi and bacteria),soil organic matter turnover.In comparison to soil with an ambient snow cover,snow reduction diminised the of soil carbon mineralization rate.However,the effect on the soil nitrogen mineralization rate depends on the snow cover depth.From the reduction of snow cover to the complete removal of snow cover,the soil nitrogen mineralization rate will change from decreasing to increasing.These results offer reference data to shed insight on how subalpine forest communities are responding to and adapting to climate change.