Analyzing biases of nitrogen contents and δ ¹⁵N values arising from acidified marine sediments with different CaCO₃ concentrations

In some studies, the researchers pretreated and measured organic carbon, nitrogen and their isotopes (δ¹³C and δ¹⁵N) of marine sediment together, to save costs and resources of analysis. However, the procedure of acidification to remove inorganic carbon for analysing δ¹³C can affect the values of nitrogen and δ¹⁵N, and the biases vary a lot depending on the CaCO₃ contents of sediments. In this study, the biases of total nitrogen (TN) and δ¹⁵N values arising from acidified sediments were compared between the CaCO₃-poor (1%–16%) and CaCO₃- rich (20%–40%) samples. TN and δ¹⁵N values were altered during acid treatment (without centrifugation) that possibly led to N-containing compounds volatilization. For CaCO₃-poor samples, acidification led to a range of 0%–40% TN losses and 0‰–2‰ shift in δ¹⁵N values; and 10%–60% TN losses and 1‰–14‰ shift in δ¹⁵N values for CaCO₃-rich samples. The biases from most samples exceeded the precision of the instrument (0.002% for TN and 0.08‰ for δ¹⁵N), and high biases could mislead our judgment for the environmental implication of the data. Thus, avoiding co-analysis of organic carbon, nitrogen and their isotopes (δ¹³C and δ¹⁵N) in sediments, even for CaCO₃- poor marine sediments, is necessary.