Nitrogen export from Coastal Plain field ditches

Abstract:

Mitigating the adverse impact of nitrogen (N) fertilizer applications depends on an understanding of transport mechanisms and flow pathways. The objective of this study was to quantify N export from seven ditches on the Maryland Eastern Shore. Ditches were monitored between June 2005 and May 2006, including flow and sample analyses for storms and base flow. Mean total N and NO₃-N concentrations were 10.6 and 6.0 mg L⁻¹ (10.6 and 6.0 ppm) for ditch 8, which were 2 times the total N and NO₃-N concentrations for any other ditch. Greater mean concentrations in ditch 8 translated to 43.5 kg ha⁻¹ (38.8 lb ac⁻¹) total N loss and 24.9 kg ha⁻¹ (22.2 lb ac⁻¹) NO₃-N loss, which were not consistent with losses observed for any of the other ditches. The elevated losses in ditch 8 coincided with the presence of a manure storage shed located in this drainage basin. The two ditches (7 and 8) nearest the manure storage shed had the greatest increase in organic N loss as a function of drainage outflow, increasing 0.062 kg ha⁻¹ (1.56 lb ac⁻¹) per mm (in) drainage outflow compared to 0.017 kg ha⁻¹ (0.45 lb ac⁻¹) per mm (in) outflow for the other five ditches. Ditches 2 and 3 had the greatest outflow of water (640 mm [25.2 in]), contributing to greater NO₃-N loads—a consequence of greater groundwater drainage. Implementing management strategies that mitigate N losses from agricultural fields should be considered in the context of ditch hydrology and drainage basin features.