| There is widespread phenomenon that significant different thickness and duration snow cover in the alpine forests, as affected by gap, block, manifold, and micro-terrain. Soil nitrogen transformation are regulated by soil biotic and abiotic factors (example water and heat conditions and freeze-thaw cycles, etc.) in the alpine forest. Insulating effect of snow, soil freeze-thaw cycle and soil freezing intensity are closely related to snow thickness or density. Therefore, the snow cover would change the microenvironment of soil nitrogen cycle, might have a profound impact on soil nitrogen transformation process, but always no attention. In view of this, we selected three depths of nature snow pack (deep snow pack, middle snow pack and shallow snow pack) to simulate snow cover change under climate change of the future in the subalpine fir forest at an altitude of 3000 m, western Sichuan during 2012 to 2013 and 2013 to 2014 winter. And we also combined with the method of field position monitoring and indoor analysis in order to understand the response characteristics of soil nitrogen transformation to varied snow thickness. The main results were as follows:(1) As the seasonal dynamics of air temperature appeared similar to the dynamic change of soil temperature 5 cm depth within two winter, were the first decreased and then increased significantly. Daily soil temperature of shallow snow packs (SS) was most volatile. Average soil temperature of 5 cm depth during 2012 to 2013 and 2013 to 2014 winter were:DS (-0.57 C)> MS (-0.65 C)> SS (-0.82 C), DS (-0.83 C)> MS (-0.87 C)> SS (-1.00 C). Also, the average soil moisture content of two year winter was:SS (44.56%)> MS (39.57%)> DS (38.49%). The soil moisture content of shallow snow packs(SS) was higher significant than deep snow packs(DS) during snow formed and melt period, but no significant difference among three snow packs during snow stable period.(2) Obvious seasonal dynamics were found in soil labile nitrogen pools (ammonium, nitrate and microbial biomass nitrogen) during the winter. Soil NH4+-N, NO3--N, MBN content arrived highest level in the snow melt period (SMP). The content of NO3--N in the region of winter soil (16.69～71.13 mg/kg) was significantly higher than the content of NH4+-N (0.25～6.04 mg/kg). The average soil MBN content of two winter was:DS (11.64 mg/kg)>MS (mg/kg 11.10)>SS (9.45 mg/kg). It was suggested that snow packs have no significant effect on soil NH4+-N and MBN content, while the soil NO3--N content were significantly affected by snow packs. Overall, soil NH4+-N and NO3--N content of SS or MS snow pack was higher than DS snow patches (SSP period excluded). Soil NH4+-N and NO3--N content of the 2013 to 2014 winter was higher than the 2012 to 2013 winter.(3) Soil net nitrification rate(0.09-0.34 mg kg-1 D-1) of three snow packs was higher than soil net ammonification rate(0.01-0.03 mg kg-1 D-1), and soil net nitrogen mineralization rate(0.14～0.36 mg kg-1 D-1) was mainly determined by nitrification during two winter. Soil nitrogen transformation rates of 2013 to 2014 winter were declined than 2012 to 2013 winter, and soil net nitrogen mineralization rates of DS, MS, SS snow pack respectively declined by 56.34%,64.87%,21.89% in comparison with that in 2012 to 2013 winter. The effects of snow pack on soil nitrogen transformation was related to critical period of snow cover depth and its processes. To a certain extent, soil net ammonification rate, net nitrification and net nitrogen mineralization rates were increased with snow depths reducing, but no significant difference. Average soil net nitrogen mineralization rate of snow packs in two winter was:MS(0.25 mg kg-1 D-1)>DS(0.19 mg kg-1 D-1)> SS(0.13 mgkg-1 D-1).(4) The three nitrogen-related soil enzyme activities changed significantly during snow cover critical period two winter, and arrived the highest level in the snow melt period. In general, the three nitrogen-related soil enzyme activities under shallow snowpack were 0.8 to 3.9 times of those under deep snowpack during two winter. The effects of snow pack on soil nitrogen-related soil enzyme activities was related to critical period of snow cover and enzyme type. Overall, in the beginning and thawing periods of seasonal snow pack, the activities of urease, nitrate and nitrite reductase enzyme under shallow snowpack significantly increased in both two winter. Although the activities of the studied enzymes were observed to be higher than those under deep-and moderate snowpacks in deep winter, no significant difference was found under the three snow packs. Meanwhile, the effects of snowpack on the activities of the measured enzymes were closed related with soil temperature and moisture content.(5) From snow formed period (SFP) to snow stable period (SSP), the number of soil AOB and AOA in three snow packs reduced significantly with decreasing temperature. Although soil AOB and AOA gene abundance was significantly affected by snow packs, and those genes abundance responded differently to snow packs. In terms of the soil AOB of DS or MS snow pack was higher than that in SS snow pack, while the number of AOA is manifested as SS or MS snow DS snow pack.In conclusion, it was found that higher soil nitrogen levels and ammonification, nitrification, immobilization, denitrification process during the winter in the subalpine spurce forest of Western Sichuan, and obviously influenced by different depth of snow cover. Soil nitrogen transformation process were closely related to snow depth and snow critical period. Visibly, warming-induced in the future snow decreased might have affect soil temperature, moisture, freeze-thaw cycles, soil microorganisms and enemy activity, and then might affect soil nitrogen transformation process in the subalpine forest of western Sichuan. |
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