Technical Note: Comparison between a direct and the standard, indirect method for dissolved organic nitrogen determination in freshwater environments with high dissolved inorganic nitrogen concentrations

摘要

Research on dissolved organic nitrogen (DON) in aquatic systems with highdissolved inorganic nitrogen (DIN, the sum of NO₃–, NO₂– andNH₄+) concentrations is often hampered by high uncertaintiesregarding the determined DON concentration. The reason is that DON isdetermined indirectly as the difference between total dissolved nitrogen(TDN) and DIN. In this standard approach to determine DON concentrations,even small relative measurement errors of the DIN and TDN concentrationspropagate into high absolute errors of DON concentrations at high DIN : TDNratios. To improve the DON measurement accuracy at high DIN : TDN ratios,we investigated the DON measurement accuracy of this standard approach according tothe DIN : TDN ratio and compared it to the direct measurementof DON by size-exclusion chromatography (SEC) for freshwater systems. Forthis, we used standard compounds and natural samples with and without DINenrichment. We show that for the standard approach, large errors of thedetermined DON concentrations at DIN : TDN ratios >0.6 occur for bothstandard compounds and natural samples. In contrast, measurements of DON bySEC always gave low errors at high DIN : TDN ratios due to the successfulseparation of DON from DIN. For SEC, DON recovery rates were 91–108% forfive pure standard compounds and 89–103% for two standard compounds,enriched with DIN. Moreover, SEC resulted in 93–108% recovery rates forDON concentrations of natural samples at a DIN : TDN ratio of 0.8 and thetechnique was successfully applied to a range of samples from waste watertreatment plants to forest and agricultural streams. With 2.5 h of measurementtime per sample, SEC is slower, but more accurate than the standard approachfor determination of DON concentrations in freshwaters with DIN : TDNratios >0.6. To sum up, the direct DON measurement by SEC enables betterunderstanding of the nitrogen cycle of urban and agricultural freshwatersystems.