杉木细根周转与根际激发对CO2浓度升高和氮沉降的响应

Rhizosphere interactions, such as the crucial components of fine foot turnover and rhizosphere priming effect (RPE), play a central role in terrestrial ecosystem functions, which, at the global scale, may control as much as 50% of the total CO2 released from terrestrial ecosystems, and regulate virtually all aspects of carbon, nutrient and biogeochemical cycling. It is now well accepted that plant-root mediated processes play an important role in regulating SOM dynamics, and understanding of these functions is needed to accurately describe critical processes like fine root turnover and RPE at realized levels. However, limited observations of root dynamics and RPE make it difficult to quantify both the degree to which roots influence SOM dynamics and the mechanisms responsible for such effects. Hence, it is critical to quantify correlationship between fine root turnover with RPE and their relationship with microbial C metabolism in order to develop robust projections of ecosystem C storage and potential forest C cycle-climate feedbacks. In this study, Chinese fir seedlings grown in OTC, effect of elevated CO(2) and nitrogen deposition on fine root turnover will be evaluated by using the root window technique and soil coring methods. Using a natural (13)C tracer method to separately measure SOM-derived CO(2) from root-derived CO(2), this proposed project aims to connect the level of rhizosphere-dependent SOM decomposition with RPE and fine root turnover of the whole plant-soil system, and to mechanistically link the RPE to soil microbial turnover and community composition as well as rhizosphere microbial C metabolism. The results from these innovative experiments will lead to significant advances in our understanding the mechanisms underlying the impact of elevated CO(2) and N deposition on fine root turnover and RPE. By generating empirical functions, the results also will be useful for reducing uncertainty in estimates of root dynamics in response to climate change and soil factors, and for understanding the importance of complex studies on respective regulatory mechanisms in changing climate and sustainable production in Chinese fir forests, thereby impacting other areas of science.

根际过程在陆地生态系统碳、养分和生物地球化学循环中发挥核心作用，约调控一半的全球陆地生态系统CO2释放量。细根周转和根际激发效应是根际过程、土壤有机质动态和生态系统生产力的关键组分和驱动力，对气候变化因子反应敏感。为了更准确模拟地下碳过程以及预测根系对全球变化的响应，亟需探明细根周转和根际激发效应对气候变化的差异响应机制及其与主控因子间的函数关系。针对该科学问题，本项目拟选取我国亚热带的杉木人工林为研究对象，在OTC平台模拟CO2浓度升高和氮沉降下，借助根窗法监测细根周转、天然13C示踪法研究根际激发效应对CO2浓度升高和氮沉降的差异响应模式与机理，并建立细根周转率和根际激发效应与微生物周转等主要调控因子之间的函数关系，结果将为森林生态系统地下碳氮循环模型提供参数，为未来气候变化背景下准确预测全球气候变化与土壤碳循环间的反馈关系和我国杉木人工林地下碳动态提供新的思路和内核数据支撑。

细根周转和根际激发效应（rhizosphere priming effect, RPE）是根际过程、土壤有机质动态和生态系统生产力的关键组分和驱动力，对气候变化因子反应敏感。项目组通过系列试验，获得了许多新结果、新进展。首先，在RPE方面，发现模拟CO2浓度升高和氮沉降能显著影响RPE。与对照相比，CO2浓度升高显著降低了杉木根际激发效应，变幅为-148%～-44%；施氮和二者交互则显著提高了激发强度，变幅分别为69%~268%、-28%~168%，可从CO2浓度升高和氮沉降能够改变植物对地上部分与地下部分的碳分配、土壤呼吸速率以及土壤微生物和胞外酶活性方面加以解释。其次，在细根动态方面，探明了杉木细根动态特征，更新了杉木寿命研究的认知。通过连续四年根窗法原位观测杉木细根周转过程，发现杉木细根生长量和死亡量存在明显的季节和年际动态，进一步分析杉木细根寿命超过900天；杉木1 级根在全年都处于生长的状态，其中1 级根在夏季新生出的数目最多且生长速度最大。第三，在RPE与主控因子关系方面，建立了RPE与根系属性及其主控因子之间的量化关系。RPE与吸收根生物量分配和比根长以及微生物生物量的碳和周转存在显著的正相关关系，揭示了植物根际激发效应强度受气候、土壤和植物种类（功能属性）影响的内在机制。最后，系统阐明了野外条件下RPE的变异规律，为该领域研究从室内走向野外提供了关键数据支撑。发现微生物氮矿化是引起树种正的激发效应的主要原因，而树种与微生物间氮竞争的加剧是造成高种植密度条件下激发效应降低的机制之一；首次量化了野外根际激发效应的纬度分异规律；基于植物物候和多因子交互作用，提出新机制：当土壤微生物氮矿化大于氮固持时，根际碳投入促进正的根际激发效应，反之，则产生负的根际激发效应。项目组成员已发表SCI收录论文6篇，培养1 名博士生和2 名硕士生。