Doctoral Dissertation Research: Inter- and Intra-Storm Variability of Dissolved Organic Carbon and Nitrogen Fluxes from a Mid-Latitude Broadleaved Deciduous Forest Canopy

随着燃料、生物质燃烧、化肥使用和农业作业中人类排放的增加以及热带森林的采伐，中纬度落叶林在全球碳(C)和氮(N)下沉方面变得越来越重要。许多研究对森林生态系统中碳和氮的储存进行了研究，但很少有研究表征来自冠层来源的溶解有机碳(DOC)和溶解有机氮(DON)的生态水文动态。尽管风暴事件是DOC和DON的动员和输送的关键时期，但目前还没有研究全面地研究中纬度落叶林中所有冠层来源的DOC和DON通量的风暴内变异性。关于冠层来源DOC和DON在气象条件和集水区特征方面的总体去向和迁移的量化数据很少。为了填补这些数据空白，本博士论文研究项目将在风暴内尺度上对两个冠层形态不同的共同优势树种的穿透雨、树干径流和凋落物渗滤液进行采样，并表征气象条件和集水条件对DOC和DON的影响。还将对样品进行碱阳离子分析，以用于终端成员混合分析(EMMA)。EMMA将被用来评估冠层通量、可变源面积(VSA)和径流之间的水文关系。以前的EMMA评估了穿透雨和凋落物渗滤液对径流化学和VSA的影响，但忽略了树干径流。许多阔叶落叶乔木的树干径流优先进入地下侧向流。因此，某些物种的树干径流可能对径流和/或VSA有贡献。这项研究将是第一个在末端混合分析中评估树干径流作为末端成员的研究，也是第一个在风暴内尺度上监测单个流域所有冠层来源的DOC和DON通量的研究。结果将被用来审查大量采样实践和现有流域模型所做的假设，这些模型通常忽略了风暴内变异性和树干径流对森林流域水文和水质的影响。如果风暴内变化或树干径流对森林集水区地下通量、VSA和/或径流的化学或水文学有重大贡献，那么使用这些假设的广泛使用的土木工程和环境研究和设计模型将需要修正。如果风暴内的变化性显著影响森林集水区的生物地球化学过程，也可能需要纠正目前的批量抽样做法。然而，对中纬度落叶林中DOC和DON在离散风暴事件期间的系统内循环动力学的研究将为自然地理学和水文学中寻求更好地理解森林中水和溶质循环的相关理论提供信息。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

Mid-latitude, deciduous forests have become increasingly important global carbon (C) and nitrogen (N) sinks as human-related emissions from fuels, biomass burning, fertilizer use, and agricultural operations grow and tropical forests are harvested. Many studies have examined C and N storage within forested ecosystems, yet few have characterized the ecohydrological dynamics of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) derived from canopy sources. No known study has holistically examined the intra-storm variability of all canopy-derived DOC and DON fluxes for mid-latitude deciduous forests, although storm events being a critical period for the mobilization and transport of DOC and DON. Data quantifying the overall fate and transport of DOC and DON from canopy sources with respect to meteorological conditions and catchment characteristics are scant. To fill these data gaps, this doctoral dissertation research project will sample throughfall, stemflow, and litter leachate for two co-dominant tree species of contrasting canopy morphologies at the intra-storm scale and characterize the influence of meteorological conditions and catchment conditions on DOC and DON. Samples also will be analyzed for base cations for use in end-member mixing analyses (EMMA). EMMA will be used to evaluate hydrologic relationships between canopy-derived fluxes, variable source areas (VSA), and streamflow. Previous EMMAs have evaluated the influence of throughfall and litter leachate on streamflow chemistry and VSAs but neglected stemflow. The stemflow of many broadleaved deciduous trees gains preferential access to subsurface lateral flow. Hence, it is likely that the stemflow of certain species may contribute to streamflow and/or VSAs. This study will be the first to evaluate stemflow as an end-member in end-member mixing analysis and the first to monitor all canopy-derived DOC and DON fluxes for a single catchment at the intrastorm scale.Results will be used to review bulk sampling practices and assumptions made by current watershed models that generally disregard the influence of intrastorm variability and stemflow on the hydrology and water quality of forested catchments. If intrastorm variability or stemflow significantly contribute to the chemistry or hydrology of subsurface flux, VSAs, and/or streamflow for forested catchments, then widely-used models for civil engineering and environmental research and design that employ these assumptions will need to be corrected. Current bulk sampling practices also may need to be corrected if intrastorm variability significantly influences biogeochemical processes in forested catchments. Investigation of the intrasystem cycling dynamics of DOC and DON within mid-latitude deciduous forests during discrete storm events nonetheless will inform relevant theory in physical geography and hydrology that seeks to better understand the cycling of water and solutes in forests. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.