Longitudinal dynamics of nitrogen and phosphorus forms along agricultural drainage ditches: Evidence for nitrate source mixing and possible transformation

Diffuse nitrogen (N) and phosphorus (P) pollution from agriculture is a leading cause of freshwater eutrophication. Agricultural drainage ditches can modify the concentrations and forms of nutrients transported from fields to receiving waters. We characterized springtime concentration patterns of N and P forms and dissolved organic carbon (DOC) in four agricultural drainage ditches differing in site characteristics, vegetation, and nutrient management. Dual nitrate isotopes (δ 15 N–NO 3 − and δ 18 O–NO 3 − ) were measured along one ditch to evaluate longitudinal nitrate isotopic patterns in relation to potential source mixing and possible transformation. DOC was strongly and positively correlated with dissolved organic nitrogen (DON) and orthophosphate–P (PO 4 –P), moderately and negatively correlated with NO 3 –N, and weakly correlated with ammonium–N (NH 4 –N). In the two ditches draining fields that received poultry litter during the study, DOC, DON, and PO 4 –P increased following litter application and subsequent rainfall, consistent with field-derived inputs. On May 17, DOC, DON, and PO 4 –P decreased from the field-edge to the outlet, with the largest DON and PO 4 –P concentration declines in two ditches occurring between the field-edge and the first in-ditch sampling location. NO 3 –N behaved differently, including a 6.8-fold increase from 0.46 mg L −1 at the field-edge to 3.12 mg L −1 at the outlet of the isotope-monitored ditch. In that ditch, upper-reach isotopic patterns were consistent with changing nitrate-source contributions and mixing, whereas the lower reach showed a positive relationship between δ 18 O–NO 3 − and δ 15 N–NO 3 − (slope = 0.907, R 2 = 0.857), consistent with possible isotope-fractionating nitrate transformation. These results indicate that agricultural ditches are not uniform nutrient conduits: organic N, P, and nitrate follow distinct longitudinal patterns, and increases in nitrate concentration coincide with isotopic patterns consistent with possible fractionating transformations within the same ditch. Consequently, longitudinal nutrient patterns may reflect the combined influence of field inputs, hydrologic source mixing, and in-ditch processing.