温度对川西亚高山3种森林土壤氮矿化的影响

Effects of temperature on soil nitrogen mineralization in three subalpine forests of western Sichuan,China

川西亚高山森林群落土壤氮循环对全球气候变化非常敏感。采用室内培养法,研究川西3个森林群落(天然针叶林、云杉人工林和桦木次生林)土壤有机层和矿质土壤层无机氮含量在两个培养温度(20℃和10℃)下4周内动态变化。结果表明:培养4周后,在20℃培养条件下天然针叶林、云杉人工林和桦木次生林硝态氮含量比在10℃培养条件下分别高出104.32%、52.11%和25.57%;而铵态氮含量仅高出10.18%、24.06%和44.82%。有机层土壤氨化速率、硝化速率和净氮矿化速率大多表现为20℃显著高于10℃;相反,温度对矿质土壤层氮转化速率影响大多不显著。此外,天然林土壤净氨化速率、硝化速率和净氮矿化速率均高于桦木次生林和云杉人工林。实验期间,3个森林群落土壤净硝化速率20℃比10℃高79.03%—128.89%,而净氨化速率仅高37.81%—63.33%。综上所述,温度变化对川西亚高山森林土壤氮矿化具有显著影响,而温度效应因森林类型、土壤层次和氮形态而不同。与矿质土壤层相比,土壤有机层氮矿化对温度变化更为敏感。

Temperature is a key factor that regulates almost all biochemical processes of terrestrial ecosystems, such as soil respiration, soil net N mineralization and soil enzyme activity. Warmer air temperatures would likely result in warmer soil temperatures which could, in turn, largely affect the biogeochemical processes of soils in these ecosystems. Soil nitrogen mineralization plays a key role in plants growth, net primary production （NPP）, soil nitrogen availability and its responses to climatic warming are very important in estimating and predicting forest ecosystems carbon budgets and nutrients biogeochemical cycling in terrestrial ecosystems. In comparison with soil nitrogen dynamics from different forest ecosystems in one climatic region, the effects of forest management were clearly evident on the soil fertility and belowground ecosystems functions and structures. The subalpine and alpine forest ecosystems in the eastern Tibetan Plateau located at the transitive zone from Qinghai-Tibet Plateau to Siehuan basin could be very sensitive to global climate change with important consequences for the global C and N balance. The magnitude of warming on the Tibetan Plateau is projected to be large relative to many other regions and the soils on the Tibetan Plateau contain large amounts of soil organic matter. Thus, soil nutrient turnover processes of alpine forest soils in this region could be more pronounced than in other ecosystems under future warmer conditions. Last century, natural coniferous forests in southwestern China were deforested and reforested with dragon spruce plantation and birch secondary forest. Forest conversation may induce great changes in soil biochemical properties, and further affect the responses of forest soils to projected global warming. To gain in-depth knowledge on the effect of experimental warming on soil nitrogen transformation processes, a laboratory incubation experiment was conducted in organic and mineral soil layers of three contrasting subalpine forest ecosystems （ natural coniferous forest, dragon spruce plantation and birch secondary forest） under two temperature treatments （20 ℃ and 10 ℃ ） for 4 weeks. Soil nitrate were 104.32%, 52.11% and 25.57% higher at 20 ℃ than 10 ℃, respectively, in the natural coniferous forest, dragon spruce plantation and birch secondary forest. However, ammonium content only increased by 10.18%, 24.06% and 44.82%, respectively, in the natural coniferous forest, dragon spruce plantation and birch secondary forest. The net soil ammonification, nitrification and nitrogen mineralization rates were significantly higher at 20℃ than 10 ℃ in the organic soil layer. Conversely, the effects of temperature on nitrogen transformation rates in the mineral layer were not significant different. Additionally, the soil nitrogen transformation rates in the natural forest were higher compared to the dragon spruce plantation and secondary birch forest and the temperature effect on nitrification rate was higher relative to the ammonification rate. During the incubation period, the soil nitrification rate was 79.03%-128.89% higher at 20 ℃ than 10 ℃, whereas the temperature only increased the ammonification rate by 37.81%-63.33%. The results of this study suggest that warming has a considerable impact on nitrogen mineralization in the subalpine forests of western Sichuan ; in addition, warming effect is dependent on forest type, soil layer and nitrogen species. Soil mineralization is more sensitive to warming in organic than mineral layer.