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).本研究采用¹⁵N₂同位素示踪技术（¹⁵N₂ isotopic tracer technique）测定固氮速率。采样工作依托近乎月度开展的夏威夷海洋时间序列（Hawaii Ocean Time-series）科考航次进行。采集6个离散深度（5、25、45、75、100及125 m）的全海水样品，将其分装至经酸洗处理的4.3 L聚碳酸酯瓶（polycarbonate bottles）中。首先将¹⁵N₂气体溶解于海水，随后取100 mL该¹⁵N₂富集海水，加入至4.3 L聚碳酸酯采样瓶内。采用膜进样质谱仪（membrane inlet mass spectrometer）测定¹⁵N₂富集海水储备液的原子百分富集度。添加¹⁵N₂示踪剂的培养瓶被挂载至自由漂流阵列中，并在样品采集时对应的离散深度处进行培养。该阵列于黎明前布放，样品在原位光照与温度条件下培养24小时。回收阵列后，将每个培养瓶内的全部样品体积过滤至预灼烧的玻璃微纤维滤膜（Whatman 25 mm GF/F）上，随后将滤膜放置于培养皿内的预灼烧铝箔片上，于-20℃条件下冷冻保存。将滤膜置于60℃下干燥24小时，经压片处理后，使用元素分析仪-同位素比值质谱仪（Carlo-Erba EA NC2500 联用 ThermoFinnigan Delta S）分析每张滤膜上的总氮质量及其同位素特征。