Soil Nitrogen Dynamics In Alpine/Subalpine Coniferous Forests Of Western Sichuan:Elevation And Snow Effects

Alpine/subalpine forest community is located in the alpine valleys, Soil nitrogen dynamics might be different along an elevational gradient, western Sichuan; Second, alpine/subalpine forest is a sensitive area of global climate change, the future climate change caused by the seasonal snow changes might have a profound impact on soil nitrogen transformation process. In view of this, this study involves two related test: (1) selection of similar exposure and slope direction as the research sample plot (Al: 3600 m; A2:3600 m; A3:3000 m), the dynamic sampling (the non-growing season, the early-growing season, the mid-growing season, the late-growing season), the determination of labile nitrogen pools and nitrogen transformation rate;(2) according to the survey of the canopy and the monitoring of snow, Soil nitrogen dynamics respond to the changes of snow depth were estimated under three depths of snow pack (deep snow pack, middle snow pack and shallow snow pack). The main results were as follows:1) Obvious seasonal dynamics were found in soil labile nitrogen pools (ammonium, nitrate, microbial biomass nitrogen and dissolved organic nitrogen), but the seasonality varied with the measured nitrogen pools. The concentrations of soil nitrate (8.38-89.60 mg kg-1) were significantly higher than those of ammonium (0.44～8.43 mg kg-1) in the four sampling dates (non-growing season, the early-growing season, the mid-growing season and the late-growing season). Except for nitrate, the contents of ammonium, microbial biomass nitrogen and dissolved organic nitrogen varied significantly with elevations and the altitude effects on those pools were dependent on seasons.2) There were obvious dynamics in the rates of soil net nitrogen ammonification, nitrification, mineralization and fixation. The rates of net nitrogen nitrification, mineralization and fixation were higher in the non-growing season than in other three seasons but net ammonification rate was very low and negative. Soil net nitrogen mineralization rate was the lowest in the-early growing season, but the highest in the mid-growing season. Soil net ammonification rates varied significantly with elevations (P<0.05). During the experimental period, soil nitrification contributed mostly to soil nitrogen mineralization and had a similar seasonality with soil nitrogen mineralization. The rate of soil net nitrogen mineralization at A2 was lower than those of other two elevations but no significant difference was observed. Soil net nitrogen fixation rate was low in each season (-0.017-0.092 mg kg-1 d-1), and was relatively higher in the non-growing season than in other seasons.3) Average air temperature of sample area is 2.60℃ in winter. As the air temperature seasonal dynamics, the dynamic change of soil temperature also appear similar, but the soil temperature change slightly lag air temperature change. Soil temperature increased with the increase in depth of snow pack. Daily soil temperature of SS snow pack was most volatile, Daily soil temperature of MS snow pack was second, but Daily soil temperature of DS snow pack was relatively stable. The winter average soil temperature was: DS(-0.57℃)>MS(-0.65℃)>SS(-0.82℃). Soil Labile nitrogen pools had obvious temporal dynamics. The concentrations of each nitrogen pool were the highest in the snow melting period. Through the winter, the soil ammonium nitrogen content increased 6.82～10.35 times, soil nitrate content increased by 79.17%～137.87%, soil labile organic nitrogen and microbial biomass nitrogen content increased with the process of snow forming, stable and melting. The changes in depth of snow pack did not affect soil nitrogen components.4) Soil net nitrification rate was significantly higher than soil net ammonification rate, and soil net nitrogen mineralization rate was mainly determined by nitrification during the winter. Soil net ammonification, nitrification and mineralization rates were inversely proportional to the depth of snow pack and significant differences in net nitrogen mineralization rate were found among snow packs. Soil ammonium, nitrate and dissolved organic nitrogen leaching fluxes was 0.07,0.58,0.42 g m-2, accounting for 6.88%,53.62% and 39.50% of soil nitrogen leaching fluxes in winter respectively. Soil nitrogen leaching fluxes were higher in MS snow pack than that in DS snow pack and SS snow pack, but no significant differences were observed among snow packs.In summary, soil nitrification is the major process of net soil nitrogen mineralization and soil nitrogen mineralization were not affected by elevational gradient. Winter-induced high nitrate may be favorable for vegetation growth in the early spring. Moreover, a stronger net nitrogen fixation rate during the wintertime implied a protective mechanism of forest ecosystems on soil nitrogen warming-induced decrease in snow pack might reduce soil temperature and increase freeze-thaw cycles in this region in future, and then might increase soil nitrogen mineralization rate over the winter and nitrogen leaching during the early spring. Meanwhile, Thus, changes in snow cover might alter seasonal patterns of soil labile nitrogen pools, nitrogen transformation and nitrogen leaching pattern in the alpine/subalpine forests of Western Sichuan.