Variations in the δ15N and δ18O values of nitrate in drainage water of two fertilized fields in Mecklenburg-Vorpommern (Germany)

Over a period of 17 months, variations in δN-15 and δO-18 values in drainage water nitrate from two collector-drain outlets were investigated. Both fields were conventionally farmed. One field was 2.5 ha (outlet 1) and the other 15 ha (outlet 2) in size; each received similar N amounts per hectare during fertilizer application. In tile drain 1, the δN-15-NO3 values ( concentration weighted mean (cwm) 10.6%) and δ 18O-NO3 values (cwm 5.1%) were higher compared to the second tile drain (cwm δN-15: 7.2%; cwm δO-18: 2.9%). Changes in isotope values of drainage water nitrate could be observed in tile drain 1 after fertilizer application. These changes included an increase in δN-15-NO3 after fertilization with urea and a decrease in δN-15-NO3 values associated with increased δO-18-NO3 values when nitrate fertilizer was applied. Low δO-18 values in drainage water nitrate of both tiles indicate that nitrification took place for most of the sampling period. Denitrification was only observed in water of tile 2 for a period of six months during summer and autumn..