Carbon Biogeochemistry of Forested Headwater Streams at Harvard Forest 2006-2007

Headwater streams make up greater than 80% of total channel length in the United States and play important roles in regulating nutrient, organic matter, and sediment fluxes from terrestrial to downstream ecosystems. Headwater streams are common features of many upland-forested watersheds in New England, yet are not explicitly factored into forest water, carbon, and nutrient budgets. Here we report on recent efforts to examine terrestrial and stream ecosystem linkages in carbon biogeochemistry in a hemlock-dominated watershed. A prototype stream biogeochemical system (SBS) was tested at the Harvard Forest LTER. The SBS allows a suite of stream water properties to be characterized in real-time over extended periods of time thus allowing questions to be asked at a wide range of time scales. The prototype SBS was field-tested in Bigelow Brook West on the Prospect Hill research tract at Harvard Forest. Bigelow Brook West was selected because the watershed’s hydrology, stream ecology, forest composition, land-use history, and carbon sequestration have been characterized, thus aiding the interpretation of SBS data and facilitating multi-disciplinary research. The prototype system monitors stream and air temperature, pCO2 , colored dissolved organic matter, total suspended sediments, PAR, water depth, pH, and dissolved O2. Independent estimates of organic and inorganic water chemistry were used to validate and interpret SBS data. Here we report on preliminary findings from the pilot SBS at Harvard Forest to highlight the usefulness of headwater stream chemistry data to a wide-diversity of ecosystem scientists. 1) Low pH constrains dissolved inorganic carbon solute fluxes to downstream ecosystems. 2) CO2 concentrations were always supersaturated in Bigelow Brook, averaging about 6x atmospheric values in summer, thus the stream is an important source of CO2 to the atmosphere. Variation in CO2 appears to be linked to multiple mechanisms including changes in stream temperature, soil CO2, and in-stream processes. 3) Diurnal variation in stream dissolved oxygen concentrations is linked to both in situ stream processes and watershed processes, such as evapotranspiration. 4) Colored dissolved organic matter (CDOM) concentrations were tightly linked to discharge with concentrations increasing with discharge. The surprising additive effect of increasing DOC concentration with discharge cause DOC fluxes to increase non-linearly with water fluxes in Bigelow Brook. 5) Low inorganic N and P concentrations suggest that organic N and P may account for a disproportionate fraction of N and P fluxes to downstream ecosystems. Preliminary SBS data have already altered our conceptual model regarding terrestrial and aquatic linkages in the Bigelow Brook watershed and have illuminated a fundamental lack of understanding of the mechanisms regulating diurnal and storm-driven stream water chemistry in hemlock-dominated watersheds.

源头溪流（headwater streams）占美国河道总长度的80%以上，在调控陆地生态系统向下游生态系统输送的养分、有机质与沉积物通量方面发挥着关键作用。源头溪流是新英格兰地区诸多山地森林流域的常见景观，但森林水、碳及养分收支核算并未明确将其纳入考量范畴。本研究针对一处铁杉主导型流域的碳生物地球化学过程，探讨陆地与溪流生态系统间的关联，现将近期相关研究进展汇报如下。我们在哈佛森林长期生态研究站（Harvard Forest LTER）对一套原型溪流生物地球化学系统（Stream Biogeochemical System, SBS）开展了测试。该系统可在长期监测中实时表征多项溪流水体属性，从而支持多时间尺度下的科学问题探究。原型SBS系统在哈佛森林展望山研究区的西大洛溪（Bigelow Brook West）完成了野外测试。选取西大洛溪作为测试点位，是因为该流域的水文特征、溪流生态、森林组成、土地利用历史以及碳封存情况均已有详实研究基础，这有助于对SBS监测数据进行解译，并推动多学科交叉研究的开展。该原型系统可监测溪流与空气温度、二氧化碳分压（pCO₂）、有色溶解有机质（colored dissolved organic matter, CDOM）、总悬浮沉积物、光合有效辐射（Photosynthetically Active Radiation, PAR）、水深、pH值以及溶解氧（dissolved O₂）。研究通过独立获取的水体有机与无机化学组分数据，对SBS监测数据进行验证与解译。本文汇报了哈佛森林原型SBS系统的初步研究结果，旨在彰显源头溪流化学数据对于众多生态系统科学研究者的应用价值。1. 偏低的pH值会限制溶解无机碳溶质向下游生态系统的输送通量。2. 西大洛溪的二氧化碳浓度始终处于过饱和状态，夏季平均浓度约为大气背景值的6倍，因此该溪流是大气二氧化碳的重要排放源。二氧化碳浓度的变化似乎与多种机制相关，包括溪流温度、土壤二氧化碳含量以及溪流内部过程的改变。3. 溪流溶解氧浓度的日变化与溪流原位过程及流域过程（如蒸散发）均存在关联。4. 有色溶解有机质（CDOM）浓度与径流密切相关，随径流量增加而升高。值得注意的是，西大洛溪中溶解有机碳（Dissolved Organic Carbon, DOC）浓度随径流量增加呈现累加效应，导致DOC输送通量随水体通量呈非线性增长。5. 水体中无机氮（N）与磷（P）浓度偏低，表明有机态氮和磷在向下游生态系统输送的氮磷总通量中占比可能远超预期。原型SBS系统的初步监测数据已更新了我们对于西大洛溪流域陆地-水生生态系统关联的认知模型，同时也揭示出当前我们对铁杉主导型流域内调控溪流化学日变化与暴雨驱动变化的机制仍存在根本性认知缺口。