夏季南极普里兹湾结合态氮同位素组成及其与硝酸盐生物利用的关系

The relationship between nitrogen isotopic composition of particulate organic matter and biological utilization of nitrate in surface waters of Antarctic sea area can provide invaluable insights into the potential use of sedimentary record to reconstruct the history of oceanic new production and sequestration of atmospheric CO2. However, such understanding is greatly limited by the lack of samplings and the obvious contradiction between predicted and field data. In this study, we will focus on Prydz Bay and the adjacent waters during austral summer, and a combination of natural isotope abundance measurement, flow cytometric sorting and isotope tracer assay (15NO3-, 15NH4+, 15N2) will be applied. Several key hypotheses, including the isotopic heterogeneity of particulate organic nitrogen pool, ammonium utilization by phytoplankton, biological N2 fixation and nitrification, will be evaluated to obtain an accurate relationship between δ15N of fixed nitrogen (PON and nitrate) and degree of nitrate utilization in the Antarctic sea area. The nitrogen isotopic fractionation factor during nitrate utilization will be derived at biological taxon level. This study in the cutting-edge field will reveal the dominating factor(s) on controlling the isotopic variability of fixed nitrogen pools in the Antarctic sea areas, and also help better understand the role of Southern Ocean under global climate change.

确立南极海域颗粒有机物氮同位素组成与硝酸盐生物利用程度的关系，是评估沉积物氮同位素组成是否可作为重建南极海域新生产力及吸收大气CO2能力历史变化的关键，已有的少量数据表明，颗粒有机物和硝酸盐氮同位素组成的变化与理论预测结果之间存在明显的矛盾。本项目将以夏季普里兹湾这一理想研究区域为对象，结合天然氮氧同位素测定、流式细胞仪分选、氮同位素示踪（15NO3-、15NH4+、15N2）等技术，评估颗粒有机物储库氮同位素组成不均一性、铵盐吸收、生物固氮作用、硝化作用对氮同位素组成与硝酸盐生物利用程度关系的可能影响，最终建立南极海域颗粒有机物及硝酸盐氮同位素组成与硝酸盐生物利用程度的准确关系。从生物类群水平上确定硝酸盐吸收的氮同位素分馏因子，揭示结合态氮储库同位素组成变化的主控因素，为评估南大洋在全球气候变化中所起的作用奠定科学依据。

揭示南极海域硝酸盐生物利用程度的特征受控机制，是评估沉积物氮同位素组成是否可作为重建南极海域新生产力及吸收大气CO2能力历史变化的关键。本项目基于氮稳定同位素（15N）示踪，探讨了南极海域硝酸盐吸收特征及相关同位素效应。研究揭示夏季南极海域上层水体氮吸收结构的物理-生物耦合，即融冰注入淡水的过程在调控南大洋高纬海区水体氮吸收动力学方面扮演着关键角色，其可能机制是融冰过程通过向上层水柱注入淡水，强化了水体层化，在改变水体物理场的同时，也在很大程度上影响了南大洋水体氮循环，进而影响生物泵效率及大气CO2浓度。在南极海域，基于流式细胞仪分选颗粒物用于不同生物类群（如细菌、浮游植物、碎屑等）的同位素异质性分析手段仍需进一步展开研究。发现南大洋硝酸盐氮氧同位素数据主体落在1:1线上，氮氧同位素分馏程度相近。在生物吸收改造之外，证明物理海洋学过程对于同位素信号的大尺度分布具有重要作用。总体上，本研究揭示了南极海域硝酸盐吸收过程的同位素约束，为判断全球变化大背景下海区碳氮循环的可能响应提供了重要参考，也为将今论古解读沉积物记录提供了依据。