Nitrogen and Chloride Movement in Small Upland Piedmont Watersheds: II. Nitrogen and Chloride Transport in Runoff

Selective management practices were used on two upland Southern Piedmont watersheds to assess their influence on overland transport of Cl, soluble-N, and total Kjeldahl-N (TKN). Ammonia-N and TKN were measured in both runoff water and sediment phases and related to these transport modes. Chloride (112 kg Cl/ha) served as a tracer anion because of its inert biological activity. Optimum rates of N-fertilizer (∼140 kg N/ha) were applied in recommended split-applications for corn growth. Annual NH₄-N and TKN losses were 35 to 40% less when associated with terraces and double cropping of corn (Zea mays L.) and winter rye (Secale cereale L.) than from a watershed without conservation practices. This difference was related primarily to differences in sediment yield from the two watersheds. No real annual differences were detected for Cl and NO₃-N losses between watersheds. Total seasonal nutrient losses were strongly affected by the quantity of sediment transported during highly erosive periods of May, June, and July. Chloride and TKN concentrations in runoff were more constant among cropping seasons; therefore, individual storm yields were best related to runoff water volume. Nitrate-N and NH₄-N concentrations in runoff decreased rapidly during corn canopy development causing the quantity of these N species to be best related to sediment transport. An inadequate number of runoff storms occurred during these short periods of high soil chemical concentrations, derived from Cl- and N-fertilizer sources, in the surface depth (0 to 8 cm) to significantly develop a relation to runoff concentrations. Runoff water and sediment variation accounted for 82 to 93% of nutrient loss variation on the nonconservation watershed. The same variables accounted for as little as 61% of the nutrient variation on the conservation watershed. Runoff losses of plant nutrients may be reduced by shifting most of their application to periods of rapid plant canopy development and periods of less intense rainfall.