USDA LTAR Common Experiment measurement: Total nitrogen (TN) and total dissolved nitrogen (TDN) concentration v1

Total dissolved nitrogen (TDN) is composed of dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON). DON is the N-containing component of dissolved organic matter (DOM) and part of the biologically reactive N pool in aquatic ecosystems that can degrade water quality in N-sensitive waters. Evidence suggests that DON may be important in triggering harmful algal blooms, particularly the DON from synthetic urea fertilizers. TDN measurements therefore contribute to an estimate of the N most available to phytoplankton. DIN export from agroecosystems is reasonably well characterized, but the factors that regulate spatial and temporal patterns of DON are not as well understood. Although the export of DON into groundwater has received recognition for more than 100 years and the contribution of DON to total nitrogen (TN) is often significant, leaching losses of N from agricultural systems are often assumed to be dominated by DIN and uninfluenced by DON. The most widely recognized N digestion method is total Kjeldahl N (TKN), whereby organic N is converted to ammonia-N, followed by titrimetric or colorimetric detection. The recommended measurement of TDN at moderate to high concentrations (i.e., >0.5 mg N/L) uses the high-temperature combustion and chemilumniscent detection method.