Nitrogen cycling in Piedmont vegetated filter zones. I. Surface soil processes

Surface runoff is a major transport mechanism for particulate-bound and dissolved N species from agricultural fields. One means of reducing nutrient loading in surface waters is the use of vegetative filter zones. The objective of this study was to evaluate the effectiveness of two forested filter zones (FFZ) for removing N from runoff in the Piedmont region of North Carolina. We used a spreading device to ensure dispersed flow in the FFZ. In addition to measuring inputs and outputs from each FFZ, we characterized the N cycle in the surface 30 cm of the soil profile to determine the fate of different N species retained in the FFZ. N loading increased as water passed through FFZ1: NO3-N increased by 1.6 kg ha-1 yr-1, organic-N increased by 13.4 kg ha-1 yr-1 and NH4-N decreased by 0.2 kg ha-1 yr-1. The second FFZ was more effective with net retention of 0.2 kg ha-1 yr-1 for NO3-N, 0.5 kg ha-1 yr-1 for organic-N and 0.2 kg ha-1 yr-1 for NH4-N. The FFZ were ineffective during the winter and spring when water filled pore space exceeded 35% in FFZ1 and 25% in FFZ2, and infiltration was low. Infiltration was the key factor controlling N pollutant removal from surface runoff. Therefore, the clayey soils of the Piedmont may not be as effective as the sandy coastal plain soils studied by other authors. Results from the analysis of the N cycle suggest that both uptake by the vegetation and leaching to deeper soil layers were the dominant fates of inorganic-N