Comparison of total nitrogen data from direct and Kjeldahl-based approaches in integrated data sets

There are multiple protocols for determining total nitrogen (TN) in water, but most can be grouped into direct approaches (TN-d) that convert N forms to nitrogen-oxides (NO_x) and combined approaches (TN-c) that combine Kjeldahl N (organic N +NH₃) and nitrite+nitrate (NO₂+NO₃-N). TN concentrations from these two approaches are routinely treated as equal in studies that use data derived from multiple sources (i.e., integrated data sets) despite the distinct chemistries of the two methods. We used two integrated data sets to determine if TN-c and TN-d results were interchangeable. Accuracy, determined as the difference between reported concentrations and the most probable value (MPV) of reference samples, was high and similar in magnitude (within 3.5–4.5% of the MPV) for both methods, although the bias was significantly smaller at low concentrations for TN-d. Detection limits and data flagged as below detection suggested greater sensitivity for TN-d for one data set, while patterns from the other data set were ambiguous. TN-c results were more variable (less precise) by many measures, although TN-d data included a small fraction of notably inaccurate results. Precision of TN-c was further compromised by propagated error, which may not be acknowledged or detectable in integrated data sets unless complete metadata are available and inspected. Finally, concurrent measures of TN-c and TN-d in lake samples were extremely similar. Overall, TN-d tended to be slightly more accurate and precise, but similarities in accuracy and the near 1 : 1 relationship for concurrent TN-d and TN-c measurements support careful use of data interchangeably in analyses of heterogeneous, integrated data sets.