Time series measurements of nitrogen fixation in the subtropical North Pacific (extended through 2019)

Rates of N₂ fixation were measured using the ¹⁵N₂ isotopic tracer technique. Sampling occurred during near-monthly Hawaii Ocean Time-series cruises. Whole seawater samples from six discrete depths (5, 25, 45, 75, 100, and 125 m) were subsampled into acid-washed 4.3 L polycarbonate bottles. The ¹⁵N₂ gas was first dissolved into seawater and 100 mL of the resulting ¹⁵N₂-enriched water was added to 4.3 L polycarbonate sampling bottles. The resulting atom % enrichment of stocks of ¹⁵N₂-enriched seawater was measured using a membrane inlet mass spectrometer. Incubation bottles amended with the ¹⁵N₂ tracer were attached to a free-drifting array and incubated at the discrete depths from which samples had been collected. The array was deployed before dawn and samples were incubated at in situ light and temperature for 24 h. After recovery of the array, the entire volume from each bottle was filtered onto a pre-combusted glass microfiber filter (Whatman 25 mm GF/F) and filters were placed onto pre-combusted pieces of foil in Petri dishes and stored frozen at -20°C. Filters were dried for 24 h at 60°C, pelleted, and the total mass of N and its isotopic signature on each filter were analyzed on an elemental analyzer-isotope ratio mass spectrometer (Carlo-Erba EA NC2500 coupled with ThermoFinnigan Delta S).本数据集采用¹⁵N₂同位素示踪技术测定固氮速率。采样工作依托夏威夷海洋时间序列（Hawaii Ocean Time-series, HOT）的近月度航次开展。采集6个离散深度（5、25、45、75、100和125米）的全海水样品，将其分样至经酸清洗的4.3升聚碳酸酯瓶中。先将¹⁵N₂气体溶解于天然海水，随后取100毫升该¹⁵N₂富集海水加入至4.3升聚碳酸酯采样瓶内。采用膜进样质谱仪测定¹⁵N₂富集海水储备液的原子百分比富集度。加入¹⁵N₂示踪剂的培养瓶被固定于自由漂流阵列，并在样品采集的对应离散深度开展原位培养。该阵列于黎明前部署，培养全程维持原位光照与温度条件，持续时长为24小时。回收阵列后，将每个培养瓶内的全部水样过滤至预灼烧的Whatman 25 mm GF/F型玻璃微纤维滤膜上，随后将滤膜置于培养皿内的预灼烧铝箔片上，于-20℃冷冻保存。将滤膜置于60℃下干燥24小时后制成样品颗粒，随后使用元素分析仪-同位素比值质谱联用仪（Carlo-Erba EA NC2500与ThermoFinnigan Delta S联用）测定每张滤膜上的总氮含量及其同位素组成特征。