Effects Of Snow Removal On Soil Nitrogen Dynamics In Subalpine Coniferous Forest In Western Sichuan

Subalpine of western Sichuan located in the eastern edge of Tibetan Plateau which is sensitive to climate change and has profound biological diversity.Warming induced winter snow reduction caused profoundly effect on this region.Up to now,amounts of researches have been conducted in high latitude tundra and boreal forest ecosystems,however,the studies on the characteristics of snow distribution in subalpine of western Sichuan is still not sufficient.In comparison with polar tundra and boreal forest,seasonal snow cover in Tibetan Plateau eastern edge mountain forest has shorter snow period,earlier melting period,less snow amount and significant annual fluctuation.As a result,effects of snow reduction will cause a different effect on soil nitrogen dynamics in this region.In view of these,12 pairs wood shelters were constructed in the middle October in2015.Wood shelters defensed snow accumulation effective during the whole winter.Soil N pools and organic nitrogen mineralization characteristics in early snow cover?ESC?,deep snow cover?DSC?,and snow cover melting?SCM?periods were conducted.Results will be conducted below:1.Seasonal snow in this region has obvious dynamic characteristic.In the winter,the changetrend of soil temperature was consistent with air temperature,however,a slightly delaywas observed in soil temperature.Winter seasonal snow has strong adiabatic capacity andlow thermal conductivity which is conducive to the stability of soil temperature and retardthe heat flowing between air and soil temperature.Effects of air temperature fluctuation tosoil temperature and freezing conditions decreased by increasing of soil depth.During thewhole winter,in snow removal plots,the average soil temperature in 0 cm,5 cm,10 cm,20 cm depth was-1.16?,-0.02?,0.21?and 0.36?,respectively.However,in control plots,the same soil depth average temperature was-0.04?,0.32?,0.54?and 0.52?,respectively.In addition,snow removal lowered the minimum temperature in 0 cm,5 cm,10 cm,20 cm depth 7.33,2.10,1.17 and 0.75?,respectively.Snow removal caused significant impact on winter freezing soil condition,snow removal make0,5,10 and 20 cm depth soil freezing time shorten 19,16,11 and 2 d respectively,but make the freezing and thawing cycles increase 39,12,6 and 1 times respectively.2.There are obvious dynamic characteristics of the soil active nitrogen in different snowconditions,but there are significant differences due to different nitrogen forms.Soil ammonium nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?and dissolved organic nitrogen?DON?maximum all appeared in the SCM period.Snow removal was beneficial to the accumulation of soil active nitrogen.During the whole winter,snow removal increased the content of NH₄⁺-N,NO₃^--N,DON by 38.6%,23.5%and 57.3%respectively.The average content of NH₄⁺-N in the winter was 35.92 mg/kg,in addition,the average content of NO₃^--N was 70.88 mg/kg,indicating that NO₃^--N was the major content of active nitrogen pools in this region.3.Effects of snow removal on soil nitrogen mineralization rate had a significant dynamiccharacteristics during the sampling periods and a significant difference was observed in the SCM period.Frequent freeze-thaw cycles contributed to the decomposition of surface litters and plant roots.Maximum nitrification and mineralization rates appeared at DSC period.Soil nitrification rate increased by 22.8%,17.2%and 83.4%,respectively,during the ESC,DSC and SCM period in snow removal plots.In the winter,soil nitrogen mineralization process was mainly nitrification,and the change trend of soil net nitrification rate was basically consistent with the net nitrogen mineralization rate.4.Snow removal caused a significant effect on soil microbial community structure but theeffect value changed in different microbial communities.The content of soil microbial biomass carbon?MBC?in ESC and DSC increased by 35.49%and 39.42%in snow removal plots.However,soil microbial biomass nitrogen?MBN?was slow to respond to the change of snow conditions.The content of MBN decreased at first and continuously increased by the end of winter which got the minimum in DSC period.MBC:MBN has a similar trend to MBC,that is,snow removal induced a significant difference in ESC and DSC period.5.Snow removal induced an inhibitory effect on soil nitrogen conversion enzyme activity?Urease activity?URA?,nitrate reductase activity?NARA?and Nitrite reductase activity?NIRA??.In DSC period,snow removal caused a significantly reduced in URA,while,NARA displayed a significant decline in DSC and SCM period.Similar to NARA,snow removal also significantly reduced NIRA in DSC period.To sum up,snow removal reduced soil temperature of the coniferous forest in the subalpine spruce forest and leaded to more frequent freeze-thaw cycles in the winter.Snow removal induced a faster surface litter decomposition rate and higher plant roots mortality and also caused significant effects on soil aggregate structure and soil microbial community composition.As a result,soil nitrogen mineralization and denitrification rate were regulated through these processes.In addition,subalpine of western Sichuan had high soil nitrogen content and strong microbial community activity in winter,and nitrification contributed all most of soil nitrogen mineralization process.Snow condition changed induced by the climate change in the future will regulate the soil nitrogen transformation process through influencing the soil chemical properties,thermal and moisture condition,microbial community structure and invertase activities during winter.