The Missing Nitrogen: The Role of Dissolved Organic Nitrogen(DON) in the Nitrogen Cycle of Surface Waters

地表水氮循环的几项研究都指出了溶解有机氮（DON）在微生物氮转化中的重要性。在示踪剂实验中，溶解无机氮（DIN）的消失率往往超过DIN在颗粒中的同化率。“缺失”15N是这类实验的常见现象，并且在所有形式的无机氮（硝酸盐、铵等）中都有报道。缺失的氮和缺失的15N证明了在光区氮循环中存在另一个不稳定区：溶解有机氮（DON）。有证据表明，缺失的15N、缺失的氮与Suzuki et al.（1985）的新DON都是相关的。本研究将通过使用15N示踪技术追踪DIN通过食物网的途径，量化DON在光区氮循环中的作用。采用粒度分馏、时间过程和常规实验，对DON池的浓度、产率和同位素富集进行了研究。这些实验是针对南加州湾的低营养地表水和产量更高的蒙特利湾提出的，但这种现象具有广泛的普遍性，我们期望这些结果将远远超出这些地理位置。

Several lines of research on the nitrogen cycle of surface waters point to the importance of dissolved organic nitrogen (DON) in microbial nitrogen transformations. The disappearance rate of dissolved inorganic nitrogen (DIN) often exceeds the assimilation rate of DIN into particles in tracer experiments. The "missing" 15N is a common phenomenon of such experiments and has been reported for all forms of inorganic nitrogen (nitrate, ammonium, etc). The missing nitrogen and the missing 15N are evidence for the existence of another labile compartment in the nitrogen cycle of the euphotic zone: dissolved organic nitrogen (DON). The evidence suggests that the missing 15N, the missing nitrogen and the new DON of Suzuki et al. (1985) are all related. This study will quantify the role of DON in nitrogen cycling in the euphotic zone by tracing the pathways of DIN through the food web using 15N tracer techniques. Using size fractionation, time course and conventional experiments, this study will investigate the concentration, production rate and isotopic enrichment of the DON pool. The experiments are proposed for the low nutrient surface waters of the Southern California Bight and for the more productive Monterey Bay, but the phenomenon is of wide generality and we expect that the results will be useful far beyond these geographical locations.