Leaching of N added to soils as ammonium or as other forms that transform into it depends partially on nitrification because ammonium is retained by the soil negative charges, while nitrate remains completely in the soil solution. Since the decrease of the soil pH at fertilized sites can negatively affect nitrification, our study aimed to evaluate the effect of adding an acidifying phosphate together with nitrogen fertilizers on nitrogen leaching in an acid soil. The experiment was carried out in 2003, in an Alfisol with clay and organic matter contents of 760 and 40 g kg-1, respectively and pH (H2O) of 4.8. Treatments consisted of three nitrogen sources (urea, ammonium sulfate and calcium nitrate) at rates of 150 mg kg-1 of N plus one control treatment without N in factorial combination with triple superphosphate (TSP) at a rate of 751 mg kg-1. Treated experimental soil units were packed into PVC leaching columns and percolated with 300 mL of distilled water every seven days, during 19 weeks, totalizing an amount equivalent to 720 mm of rain. TSP addition anticipated ammonium leaching due to displacement of ammonium from the soil negative charges by added calcium. Nitrate leaching from the soil treated with calcium nitrate occurred in the first percolations regardless of TSP addition. In soils treated with the other two N fertilizers, nitrate leaching was intensified only after the seventh percolation, but it was anticipated to the fifth percolation by TSP addition due to earlier nitrification caused by displacement of exchangeable ammonium to the soil solution. TSP addition doubled nitrate leaching in the urea treated soil, while leaching decreased by about 20 % when TSP was mixed with ammonium sulfate or calcium nitrate. In the absence of TSP, 70 % of the N added via urea remained in the soil after the end of the percolations, but in the presence of TSP all applied N was lost. Overall, TSP combined with ammonium or amide-containing nitrogen fertilizers anticipates, rather than delay N leaching, possibly as a result of calcium addition.
percolation; denitrification; nitrogen fertilizers; urea; ammonium sulfate; calcium nitrate
