Ammonium and nitrate ions from soil organic N mineralization are the predominant N mineral forms available to plants. The ion ratio and distribution along the soil profile show daily fluctuations and oscillate seasonally. NH4+ and NO3- soil content were evaluated in two adjacent plots, one of them farmed continuously with barley, soybean and barley in a no-tillage system for more than 10 years, with pH 6.0 and fertilized with P and K. A second plot was maintained undisturbed, with native vegetation (mostly grasses), with pH 4.8 and low soil fertility. The study was carried out in a Typical Red Dystrophic Argisol at an experimental station of Universidade Federal do Rio Grande do Sul, Eldorado do Sul - RS, in 2002 and 2003. Seasonal soil samples were taken on average every ten days at the farmed plot (5/31/2002 until 10/23/2003) and fallow plot (6/13/2002 until 10/17/2002). Short-term samples (5/19/2003 until 09/29/2003) were taken every one to three days. In both cases, soil samples were taken from the top 20 cm soil layer. No nitrogen fertilization was applied in either plot during the whole period. Seasonal trends showed increasing mineral N (NH4+ and NO3-) from spring to summer, and decreasing from autumn to winter, in 2002 and 2003. N mineralization followed approximately the average temperature oscillation pattern for the sampled period. Mineral N in the short-term samples was not constant from one day to the next. The variation was related to rainfall occurrence, which was responsible for a sharp decrease in mineral N content. Nitrate and ammonium contents in the undisturbed plot were similar to those observed in the plot growing barley. The 1:1 ratio of NH4+ and NO3- observed both in the farmed plot and the undisturbed plot indicates that there is a balance between both ionic forms, independent of soil pH and fertility level.
nitrogen; non-tillage system; temperature; precipitation; pH
