Disponibilidade de nitrogênio no solo e produtividade de arroz irrigado sob fracionamentos da adubação nitrogenada

Abstract

In the irrigated rice crop, nitrogen is required during almost its entire cycle, being recommended the fractionation of the nitrogen fertilization at the beginning of the tillering and at the beginning of the reproductive phase to maximize its production potential. However, the nitrogen dynamics in the soil is considerably complex, due to the multiplicity of chemical and biological reactions in which it is involved, determining its availability to the plants. The nitrogen fertilization fractionation in different phenological stages can promote changes in the nitrogen availability in the soil and in the soil solution, influencing the nutrient uptake by the plant and promoting increase on the crop grain yield. In this context, two experiments were developed with the objective: to determine the rate of absorption of nitrogen supplied to irrigated rice plants, the availability of nitrogen in the soil, plants and in the soil solution, in function of the nitrogen fertilization fractionation, as well as its effects on crop grain yield (Chapter I); to evaluate grain yield and productive stability of medium and early cycle irrigated rice cultivars in six environments in the state of Rio Grande do Sul, Brazil, when submitted to fractionation of nitrogen fertilization (Chapter II). The experiment I was conducted in the field in the counties of Cachoeira do Sul, Cachoeirinha and Santa Maria in the 2016/17 growing season, and the treatments were composed by nitrogen fertilization fractionation in different phenological stages (Control: 50% V3 + 50% R0; 67% V3 + 33% R0; 60% V3 + 20% V6 + 20% R0 and 100% V3) in the irrigated rice cultivar IRGA 424 RI. Experiment II was conducted under field conditions at Cachoeira do Sul, Cachoeirinha, Camaquã, Santa Maria, Santa Vitória do Palmar and Uruguaiana in the 2016/17 and 2017/18 growing season, the treatments were the same nitrogen fertilization fractionation tested in experiment I, however, also testing them in the cultivar BRS PAMPA. It was observed that higher the amount of nitrogen applied in the V3 stage, greater its availability in the soil and in the soil solution during the vegetative phase, providing a higher accumulation of dry matter, nitrogen content and accumulation in the shoots and roots in the early stages of the development of the culture. At the end of the evaluation period, it wasn’t observed influence of nitrogen fertilization fractionation on nitrogen accumulation, yield components and grain yield of irrigated rice crop (Chapter I). The proportion of the fractionation 60% V3 + 20% V6 + 20% R0 provided an increase on the grain yield, however, it is associated to a greater productive instability in relation to the tested environments. The fractionation 100% V3 obtained a reduction of the grain yield when compared to the accomplishment of its greater parceling, nevertheless, among the nitrogen fertilization fractionation, it presented the greatest productive stability. The fractionation 50% V3 + 50% R0 provides a reduction in grain yield when compared to the fractionation 67% V3 + 33% R0, furthermore it’s associated to a greater production instability.