成熟大树根际激发效应及其影响机制——以杉木、马尾松为例

Soil organic carbon (SOC) pool determines the functioning of terrestrial ecosystems and regulates the release and sequestration of major greenhouse gases at the global scale. Plant roots can strongly accelerate or suppress SOC mineralization - a phenomenon known as the rhizosphere priming effect (RPE). The RPE has emerged as a chief driving force for changing global SOC pool in response to global environmental change. Laboratory studies have indicated that the RPE can enhance the rate of SOC decomposition up to four-fold, which is as significant as the effects of soil temperature and moisture. Furthermore, it has been reported that the magnitude of the RPE for tree seedlings grown under greenhouse conditions is significantly larger than herbaceous species. However, the magnitude and controls of RPE of mature trees on SOC dynamics in forest ecosystem under realistic field conditions remain largely unknown. In this study, we will quantify the RPE of two mature trees (Cunninghamia lanceolata and Pinus massoniana) under field conditions in a subtropical forest. We will also explore the relationships between RPE and other key variables such as root exudation rates and extracellular enzyme activities by using a (13) C natural abundance method. The results are expected to provide new insights about mechanisms regulating the RPE on SOC decomposition. These mechanisms may improve the performance of the terrestrial ecosystems models. The findings will also inform management of SOC stability in forest ecosystem.

土壤有机碳库是维持陆地生态系统功能稳定、缓解全球气候变化及其反馈过程的调节者。根际激发效应（rhizosphere priming effect, RPE），由根的活动导致土壤有机碳分解速率的变化，是影响土壤有机碳动态变化重要的驱动力。室内实验表明，RPE最高可使土壤有机碳分解速率加快近4倍，其作用与温度、水分相当，且树木幼苗RPE显著大于作物和草本。然而，目前仍不清楚野外条件下成熟大树RPE的变幅多大？与主控因子的关系如何？这严重限制了我们对森林生态系统根际激发效应的深入认识。基于上述科学问题，本项目采用天然(13)C示踪方法，选取亚热带人工林典型树种杉木和马尾松的成熟大树为研究对象，拟探讨成熟大树RPE的方向与强度，并解析其与根系分泌有机碳速率、胞外酶活性等因子间的关系。结果将为土壤有机碳过程模型的优化提供核心参数，为森林生态系统土壤有机碳的科学管理提供新思路。

森林土壤有机质是维持陆地生态系统功能稳定、缓解全球气候变化及其反馈过程的重要调节者。根际激发效应（rhizosphere priming effect, RPE），由根的活动导致土壤有机质分解速率的变化，是影响土壤有机质动态变化重要的驱动力。室内实验表明，RPE最高可使土壤有机碳分解速率加快近4倍，其作用与温度、水分相当。而且树种产生的激发效应最强，农作物和草本次之。然而，目前仍不清楚野外条件下成熟大树RPE的变幅多大？微生物调控机制是什么？这对深刻理解与准确预测大尺度上根际激发效应对生态系统固碳和气候反馈作用提出了巨大的挑战。我们以亚热带常绿阔叶林主要优势种为研究对象，采用天然碳（13C）和氮（15N）同位素示踪手段及密闭动态碱液CO2吸收技术，考察了原位条件下成熟大树根际激发效应对土壤碳氮矿化速率影响的季节及年际动态变化，测定了分泌物速率，微生物胞外酶活性及群落结构。我们发现土壤碳氮矿化根际激发效应的方向和幅度及变化趋势差异很大，植物种的影响非常显著。土壤碳分解的激发效应表现为正激发，有明显的季节动态变化，土壤氮矿化的激发效应表现为负激发。随着培养时间的进行，成熟大树持续增强土壤碳分解，土壤氮矿化并不持续抑制。相比外生菌根，内生菌根引起相对更高的土壤碳分解的激发效应，这主要通过调控真菌、革兰氏阳性菌的生长近而影响微生物胞外酶活性的化学计量比来实现。根系还可以通过引起的微生物周转速率的加快提高根际激发效应的强度近而影响土壤碳的丢失。这些结果表明成熟大树根系及其共生真菌能够增强土壤碳分解。未来应该更加关注根系所引起的微生物群落结构变化，以及相伴随的碳利用效率和周转的变化对土壤碳库长时间尺度的影响。

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