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

摘要

Intensive agricultural farming practices have the potential to cause high levels of nitrate—nitrogen (NO_3~-J-N) to be released from tile drainage systems. A better understanding of the temporal dynamics of NOJ-N loading, δ~(15)N and δ~(18)O from standard drainage systems is needed, in order to improve our understanding of NOJ-N transport and transformation processes; particularly, with regards to the imperfectly drained agricultural soils found within Atlantic Canada. Three conventional subsurface drainage plots (48×8 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 NOJ-N concentrations and isotopic signatures of δ~(15)N and δ~(18)O for NO_3~--N. Maximum NO_3~--N loads were observed in the winter and fall, when both discharge and concentration of the NO_3~--N were highest. Mean isotope values in NO_3~ ranged from 3.1 to 8.5%, for δ~(15) N and -3.2 to 17.7‰ for δ~(18)O. Results suggest that NOJ-N from the drainage water was derived from organic sources (i.e. manure and soil organic matter) and that loss via deni-trification does not impart an identifiable signature upon the NO_3~--N pool. The dual isotope approach examined here provides insight into N source and transformation processes which may be contributing to the NO_3~-N found within the drainage water.