Phosphorus losses in water and sediments in runoff of the water erosion in oat and vetch crops seed in contour and downhill

In contour cultivations, the ability of the plants to retain sediments suspended in flowing water and the marks in the soil caused by preparation, promotes the deposition of these sediments in the furrows. The objective of this work was to quantify the level of extractable P in the erosion sediments and in the surface of the soil, the level of soluble P in the runoff water and the level of extractable P in the different sized sediments transported in the runoff water, and the P total losses by water erosion. The work was carried out under simulated rainfall between April and November 2006 in an INCEPTISOL soil, under conventional cultivation. The combination of three factors was studied; type of crop: oats (Avena strigosa) and vetch (Vicia sativa), method of seedling (downhill and in contour) and a simulated rainfall test (five tests with constant rainfall intensity of 64 mm h−1 for a duration of 1 h). The first four rainfall tests were made on the crops during their cycle, and the fifth one on the crop residues, which were maintained on the soil surface. The levels of P in the soil and in the runoff sediments were positively correlated; these levels in soil diminish from 60.7 mg kg−1 at the beginning of the crop cycle until 33.6 mg kg−1 at the end of the cycle, and increase to 61.5 mg kg−1 when measured in the crop residues; while, the P levels in runoff sediments diminishing from 27.5 mg kg−1 at the beginning of the crop cycle until 18.0 mg kg−1 at the end of the cycle, and increase to 30.8 mg kg−1 when measured in the crop residues. Seedling in contour produced sediments with a greater level of P (24.6 mg kg−1) than seedling downhill (21.4 mg kg−1); however, the P total losses were smaller in the contour (37.1 g ha−1) than downhill (51.4 g ha−1). The P level in the runoff sediments increased with the reduction in particle size; within one size class, the P level was greater in contour seedling than downhill. The level of soluble P in the runoff water was inversely proportional to the concentration of sediments in this water.