Where nutrients are lost by water erosion, the eroded soil is impoverished and crop productivity is reduced, apart from the water contamination. Soluble N in the runoff is biologically available and contributes to eutrophication of the water and, depending on the form and content in the water, may be toxic to aquatic organisms. In order to evaluate the N content in the form of ammonium and nitrate in the runoff water, an experiment was conducted with simulated rainfall from April to November 2006, on an Inceptisol under conventional tillage. A combination of three factors was studied: crop type, sowing form and simulated rainfall test. The crops black oat ( Avena strigosa) [ Avena nuda] and common vetch ( Vicia sativa) were studied, as well as the sowing forms: mechanical, along rows in slope direction; broadcast by hand; and mechanized along contour lines; and five simulated rainfall tests, at a constant and planned intensity of 64 mm/h and rainfall duration of 1 h. The first four rainfall tests were applied during the crop cycles and the fifth on the crop residues maintained on the soil surface. Nitrogen application to oat as urea cover dressing, immediately before rainfall test 1, resulted in a higher N-NH 4+ content in the runoff water from oat than from vetch, exceeding the acceptable level for the aquatic community. The residues decomposition on the soil surface under rainfall, the content of soluble N-NO 3- in the runoff water was higher in the crops of contour sowing than of slope sowing. The contents of soluble N-NH 4+ and N-NO 3- in the runoff water decreased from the beginning to the end of the crop cycles, but increased in the rainfall test on vetch mulch, to higher values than in the treatment with oat residue.
