Hubbard Brook Nitrogen Oligotrophication (HBNO): In-situ Nitrogen Mineralization and Nitrification, 2021-2023

The goal of this project is to test the overarching hypothesis that positive feedback mechanisms involving changes in seasonal cycles that diminish N availability to plants such that plant N demand is not met by soil N availability in northern forests. Specifically, we hypothesize that increasing N demand by plants (induced by increasing temperatures, longer growing seasons, and other environmental changes) leads to greater N resorption by trees in autumn, increased C:N in litter, and greater net immobilization of N by soil microbes in the following spring. However, the timing of snowmelt and soil freezing in spring may further affect net mineralization and N availability for plants. These hypotheses are being tested with a combination of observational, experimental, and modeling approaches at Hubbard Brook Experimental Forest in New Hampshire: 1) measurements at 14 previously established sites along an elevation/aspect climate gradient; 2) litter and snow manipulation experiments at six sites along the climate gradient to create variation in soil climate conditions and microbial N immobilization during spring. We leveraged 14 sites previously established along an elevation and aspect-driven climate gradient at Hubbard Brook as a “natural climate experiment" to test our hypothesis that a positive feedback between N cycling during fall senescence and spring contributes to declining N availability in northern forests. This elevation gradient encompasses variation in mean annual air temperature of ~2.5 °C that is similar to the change projected to occur with climate change over the next 50–100 years in the northeastern U.S. There is relatively little variation in soils along the gradient. We are utilizing three sites at higher elevation (~550-660 m, north facing) and three sites at lower elevation (~375-500 m, south facing) for the litter and snow manipulation experiments to maximize the differences in temperature among the 14 sites.
Litterbox manipulation:
The objective of the litterfall manipulation experiment is to determine whether increases in autumn litter C:N ratios contribute to greater N immobilization by microbes and reductions in net mineralization and plant N uptake in spring, and ultimately, N oligotrophication in northern forest ecosystems. We applied early (low C:N litter that is lost from from hardwood foliage in the first two weeks of autumn) and late (high C:N litter that falls in the last two weeks of autumn) season litter in October 2022 that was collected in fall 2021 at rates equal to standing mass of litter (300 g m2). We also applied native litter that was collected from the forest floor of each intensive site to represent background levels of C:N in litter samples. This litter was applied to one litterbox at each of the six intensive sites. Following application of litter, we installed deer netting around and on top of each of the litterboxes to eliminate litter loss from wind. Soil samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and anlalysed for Nitrogen mineralization and nitrification, as described in the methods section.
Snow manipulation:
The objective of the snow manipulation experiment is to determine whether the timing of spring snowmelt, length of the spring, and soil freezing in spring affect microbial N immobilization, hydrologic losses, net mineralization, and plant N uptake. The snow manipulation treatment was conducted in the spring of 2022 and 2023. We manually halved (Removal treatment) or doubled (Addition treatment) snow water equivalent (SWE) in experimental plots in March of 2022 and 2023 to accelerate or delay by an average of one week, respectively, the onset of spring snowmelt. Soil Samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and analysed for Nitrogen mineralization and nitrification, as described in the methods section.
These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.

本项目的核心目标为验证下述核心假说：北部森林中，涉及季节周期变化的正反馈机制会降低植物可获取的氮有效性 (N availability)，使得植物氮需求无法通过土壤氮有效性 (soil N availability) 得到满足。具体而言，我们提出的假说是：由气温升高、生长季延长及其他环境变化诱导的植物氮需求增加，会导致树木在秋季的氮回收 (N resorption) 量提升，凋落物碳氮比 (C:N ratio) 升高，并在次年春季促使土壤微生物对氮的净固持 (net immobilization of N) 作用增强。然而春季融雪与土壤冻结的时间节点，可能会进一步影响净矿化 (net mineralization) 过程与植物可获取的氮有效性。

本研究在美国新罕布什尔州的哈伯布鲁克实验林 (Hubbard Brook Experimental Forest)，结合观测、实验与建模三种手段对上述假说进行验证：1）沿海拔/坡向气候梯度，对14处已设立的固定样地开展监测；2）沿气候梯度在6处样地开展凋落物与降雪操控实验，以调控春季土壤气候条件与微生物氮固持过程。我们依托哈伯布鲁克实验林中沿海拔与坡向构建的气候梯度上的14处固定样地，将其作为“天然气候实验”，用以验证我们的假说：秋季衰老期与春季的氮循环之间的正反馈，会加剧北部森林的氮有效性下降。该海拔梯度的年平均气温差异约为2.5℃，这一差值与美国东北部未来50至100年因气候变化预计出现的气温变化幅度相当，且梯度沿线的土壤性质相对均一。为最大化14处样地间的气温差异，我们选取了3处高海拔（约550-660米，北向坡）样地与3处低海拔（约375-500米，南向坡）样地，用于开展凋落物与降雪操控实验。

凋落物箱操控实验：
本凋落物输入操控实验的目标为明确：秋季凋落物碳氮比 (C:N ratio) 升高是否会提升微生物的氮固持量，降低春季的净矿化速率与植物氮吸收量，并最终导致北部森林生态系统的氮贫营养化 (N oligotrophication)。我们于2022年10月施加了2021年秋季采集的两类凋落物：早季凋落物（阔叶树在秋季前两周脱落的低碳氮比凋落物）与晚季凋落物（秋季最后两周脱落的高碳氮比凋落物），施加量等同于样地现存凋落物量（300克/平方米）。同时我们还施加了从6处核心研究样地的林地表层采集的本地凋落物，以代表凋落物样品的碳氮比背景水平。每处核心研究样地的一个凋落物箱均施加了上述凋落物。施加凋落物后，我们在每个凋落物箱的周围与顶部安装了防鹿网，以避免凋落物因风力发生流失。我们分别于2021年11月、2022年4月、2022年5月、2022年6月、2022年11月、2023年4月、2023年5月、2023年6月采集这些样地的土壤样品，并按照方法章节所述的流程，对样品的氮矿化与硝化作用进行分析。

降雪操控实验：
本降雪操控实验的目标为明确：春季融雪时间、春季时长与春季土壤冻结过程，是否会对微生物氮固持、水文损失、净矿化过程与植物氮吸收产生影响。该降雪操控处理于2022年与2023年春季开展。我们于2022年3月与2023年3月，对实验样地的雪水当量 (Snow Water Equivalent, SWE) 分别进行减半（移除处理）与加倍（添加处理），以分别将春季融雪起始时间平均提前与推迟一周。我们分别于2021年11月、2022年4月、2022年5月、2022年6月、2022年11月、2023年4月、2023年5月、2023年6月采集这些样地的土壤样品，并按照方法章节所述的流程，对样品的氮矿化与硝化作用进行分析。

本数据集隶属于哈伯布鲁克生态系统研究 (Hubbard Brook Ecosystem Study, HBES) 项目。HBES是在哈伯布鲁克实验林开展的协同研究项目，该实验林由美国农业部林务局 (USDA Forest Service) 北方研究站 (Northern Research Station) 运营维护。