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

Abstract.

Over a period of 17 months, variations in δ¹⁵N and δ¹⁸O 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 δ¹⁵N-NO₃ values (concentration weighted mean (cwm) 10.6‰) and δ¹⁸O-NO₃ values (cwm 5.1‰) were higher compared to the second tile drain (cwm δ¹⁵N: 7.2‰; cwm δ¹⁸O: 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 δ¹⁵N-NO₃ after fertilization with urea and a decrease in δ¹⁵N-NO₃ values associated with increased δ¹⁸O-NO₃ values when nitrate fertilizer was applied. Low δ¹⁸O 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.