Mosses collected decades ago and stored in herbaria are often used to assess historical nitrogen deposition. This method is effectively based on the assumption that tissue N concentration remains constant during storage. The present study raises serious doubt about the generality of that assumption. We measured tissue N and C concentrations as well as δ¹⁵N, δ¹³C, Pb and Mg in herbarium and present day samples of seven bryophyte species from six sites across Denmark. While an increase in nitrogen deposition during the last century is well-documented for the study site, we surprisingly found foliar N concentration to be higher in historical samples than in modern samples. Based on δ¹⁵N values and Pb concentration, we find nitrogen contamination of herbarium specimens during storage to be the most likely cause, possibly in combination with dilution though growth and/or decomposition during storage. We suggest ways to assess contamination and recommend caution to be taken when using herbarium specimens to assess historical pollution if exposure during storage cannot be ruled out.

中文翻译：

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植物标本室苔藓是历史氮沉积的可靠指标吗？☆

几十年前收集并存储在草皮中的苔藓通常用于评估历史氮沉积。该方法有效地基于以下假设：组织N浓度在存储过程中保持恒定。本研究对这一假设的普遍性提出了严重怀疑。我们测量组织N和C的浓度以及δ ¹⁵ N，δ ¹³从横跨丹麦六个站点C，铅和Mg 7苔藓植物物种的植物标本和本天的样品英寸 尽管上个世纪氮素沉积的增加在研究地点有充分的记录，但令人惊讶的是，我们发现历史样品中的叶面氮浓度比现代样品中的高。基于δ ¹⁵N值和Pb浓度，我们发现储藏期间标本室标本中的氮污染是最可能的原因，可能与储藏期间生长和/或分解引起的稀释相结合。如果不能排除贮藏期间的暴露，我们建议使用评估标本污染的方法，并建议在使用植物标本室标本评估历史污染时要谨慎行事。