Examination of nitrate concentration, loading and isotope dynamics in subsurface drainage under standard agricultural cropping in Atlantic Canada

Intensive agricultural farming practices have the potential to cause high levels of nitrate–nitrogen (NO3−–N) to be released from tile drainage systems. A better understanding of the temporal dynamics of NO3−–N loading, δ15N and δ18O from standard drainage systems is needed, in order to improve our understanding of NO3−–N transport and transformation processes; particularly, with regards to the imperfectly drained agricultural soils found within Atlantic Canada. Three conventional subsurface drainage plots (48 × 48 m) placed at a 0.80 m soil depth were monitored over a seven month period on sandy loam soil in Onslow, Nova Scotia. Each plot received similar applications of both organic and inorganic fertilizer. Water samples were obtained and analyzed for NO3−–N concentrations and isotopic signatures of δ15N and δ18O for NO3−–N. Maximum NO3−–N loads were observed in the winter and fall, when both discharge and concentration of the NO3−–N were highest. Mean isotope values in NO3− ranged from 3.1 to 8.5‰ for δ15N and −3.2 to 17.7‰ for δ18O. Results suggest that NO3−–N from the drainage water was derived from organic sources (i.e. manure and soil organic matter) and that loss via denitrification does not impart an identifiable signature upon the NO3−–N pool. The dual isotope approach examined here provides insight into N source and transformation processes which may be contributing to the NO3−–N found within the drainage water.

► The dual isotope approach identified transformation processes contributing to the nitrate found. ► Nitrate was derived from organic sources (i.e. manure and soil organic matter). ► Denitrification did not impart an identifiable signature upon the nitrate pool. ► Mineralization was continuous through all seasons.