增温对青藏高原高寒草甸生态系统固碳通量影响的模拟研究

Simulation of effects of warming on carbon budget in alpine meadow ecosystem on the Tibetan Plateau

低温被广泛认为是高寒草甸生态系统首要限制性因子,因此增温可能会在某种程度上促进初级生产力,但是也可能由于土壤水分、N素营养状况的改变形成新胁迫而抑制生产力提高。此外,生态系统呼吸由于增温而提高的幅度也可能高于初级生产力提高的幅度,造成总碳库平衡的改变。利用青藏高原海北高寒草甸实测数据对生态系统过程模型Biome-BGC(V.4.2)进行了参数化,并利用研究区实测土壤水分(0-40 cm)和其它观测数据对模型进行了检验,证明模型模拟结果较为可靠。模型使用2005-2008年的海北气象站实测气象数据包括气温、降水等作为驱动数据,模拟了增温1.2-1.7℃下青藏高原海北定位站高寒草甸生态系统碳通量的变化,并整合分析增温试验平台上已发表的试验,与模拟结果进行对比,探讨增温对海北高寒草甸生态系统碳收支的可能影响。结果表明:2005-2008年青藏高原高寒草甸生态系统为弱的碳汇,短期增温导致系统净碳固定增加。增温直接影响系统碳通量,也通过土壤水分和土壤矿化氮变化间接影响碳通量,相比土壤水分和氮素,增温对影响碳通量变化过程中的效应更大;研究也揭示,在增温条件下,植物对土壤矿化氮的吸收量小于有机质分解产生的土壤矿化氮量,土壤矿化氮含量增加。

Low temperature is widely regarded as the most important limiting factor in the alpine meadow ecosystem. The alpine meadow ecosystem represents a huge carbon pool that is witnessing rapid increases in air temperature. While warming may alleviate the lowtemperature limitation to primary production, warming may decrease soil moisture by increasing the evaporation level and may stimulate ecosystem respiration. In addition, N may become a limiting factor if primary production enhances. This study focuses on how warming affects the H： O and N cycles, and how changes in these cycles affects carbon balance. First, the BiomeBGC model （ V. 4.2） is parameterized and validated using real time data from the Haibei Alpine Meadow Ecosystem Research Station, Chinese Academy of Sciences and then this model is used to assess the effects of warming. In the model increasing air temperature was set at 1.2-1.7℃ based on the previous results of the freeair temperature enhancement （FATE） experiment at the station. The simulated results showed warming increasesevapotranspiration and decreased soil moisture in the growing seasons, while slightly increasing soil moisture in non-growing season. Warming slightly but significantly increases the litter＇ s decomposition rate, and increases the annual concentration of inorganic N in soil by 7.9%. Warming promoteed both gross primary production （ GPP ） and heterotrophic respiration （R,）. The increases of SLAI （specific leaf area index） and assimilation rates leaded to increased GPP. The GPP increased by up to 34.3% every year. Because of the enhancement of microbial activity and increased litter input, the warming increased average annual RH by 17.2%. RH increased by 24.9% during the non-growing seasons, whereas it only increases by 12.3% due to decreasing water content in soils during the growing seasons. The GPP increase surpassed RH during the growing seasons. Warming increased annual mean NEE （net absorption） by 29.6%. The results suggest that in the future warming scenario, the alpine meadow ecosystem may remain a weak carbon sink. Warming plays the most important role in the change of carbon flux. Carbon flux is also influenced by water loss caused by warming, and the role of water loss in some processes was prominent. The balance between the effects of warming on litter decomposition and on plant absorbing resulted in increase of soil inorganic N. N availability and H20 are not limiting factors on plant growth in the short term, but this may change in the longterm; this should be studied further in the future. Comparisons with newly published simulation results related to the Tibetan Plateau are also discussed.