Phosphorus, potassium and organic carbon concentrations in runoff water and sediments under different soil tillage systems during soybean growth

Runoff transports nutrients and impoverishes agricultural soils, causing off-site environment contamination. The objective of this study was to quantify P, K and organic carbon concentrations in runoff under different soil tillage systems. The experiment was carried out in the field, under simulated rainfall, during soybean growth, from April 2003 to May 2004, in the Santa Catarina highlands, Southern Brazil, on a typical Hapludox. The evaluated soil tillage treatments (which were replicated) were: (i) continuous bare soil (BS), (ii) conventional tillage on a desiccated residue (CT), (iii) no till on a burned residue, in a never-tilled soil (BNT), (iv) no till on a desiccated residue, in a never-tilled soil (DNTnt) and (v) no till on a desiccated residue, in soil tilled 4 years before (DNTt). Fodder radish (Raphanus sativus) residues were managed as indicated prior to planting soybean crop. As a result, no-till treatments have higher P, K and organic carbon concentrations in the superficial 0–0.025 m soil layer and in runoff sediments than CT. The highest enrichment ratios were of 0.90, 1.34 and 1.17 for P, K and organic carbon, and occurred in burned no till, bare soil and conventional treatments, respectively. P and K concentrations in runoff water reached in the first rainfall test 1.0 and 15.8 mg L−1 in the no-till treatments, and were higher than in the CT treatment, where these contents were only of 0.09 and 1.9 mg L−1, respectively. Crop residue burning increased P and K and reduced organic carbon levels in the topsoil layer compared to other non-burned treatments. In the runoff water, burning increased P and K concentrations, while in sediments, it presented higher P and organic carbon contents and reduced K levels, in relation to non-burned plots. Positive linear correlations were obtained between P and K in runoff water and P, K and organic carbon in sediments, and their respective levels in the 0–0.025 m soil layer. P and K concentrations in runoff water decreased exponentially in the no-till treatments, and linearly in CT and BS with the number of rains applied. P contents in sediments exponentially decreased, while K potentially increased with the size of the particles in the sediments transported in runoff. There was no relationship between organic carbon content in runoff sediments loss and sediment size.