SUMMARYCrop residues on the soil surface of no-till systems can intensify ammonia volatilization from N fertilizers applied to cereal crops. This study assessed the magnitude of N losses through ammonia volatilization from urea applied to no-till winter (wheat) and summer crops (maize) on a Typic Hapludox in the south-central region of Paraná, southern Brazil. In addition, the potential of alternative N sources (urea with urease inhibitor, liquid fertilizer, ammonium nitrate and ammonium sulfate) and different urea managements (fertilizer applied in the morning or afternoon) were evaluated. Two experiments with maize and wheat were carried out for two years, arranged in a randomized block design with four replications. Nitrogen volatilization losses were assessed with a semi-open static collector until 21 days after fertilization. In winter, the losses were low (<5.5 % of applied N) for all N sources, which were not distinguishable, due to the low temperatures. In the summer, volatilization rates from urea were higher than in the winter, but did not exceed 15 % of applied N. The main factor decreasing N losses in the summer was the occurrence of rainfall in the first five days after fertilization. Urea with urease inhibitor, nitrate and ammonium sulfate were efficient to decrease ammonia volatilization in maize, whereas the application time (morning or afternoon) had no influence.Index terms: nitrogen fertilization, efficiency, yield. RESUMO: VOLATILIZAÇÃO DE AMÔNIA DE FERTILIZANTES NITROGENADOS APLICADOS NAS CULTURAS DO TRIGO E DO MILHO EM PLANTIO DIRETO, NO SUL DO BRASILOs resíduos culturais na superfície de solos em plantio direto (PD) podem intensificar a volatilização de amônia, quando da aplicação dos fertilizantes nitrogenados em sistemas de produção de cereais. Este estudo visou avaliar a magnitude das perdas de N por volatilização, quando da aplicação superficial de ureia nas culturas do trigo e do milho, em Latossolo Bruno em PD, no centro-sul do Paraná. Adicionalmente, avaliou-se o potencial de fontes alternativas de N (ureia com inibidor de urease, fertilizante líquido, nitrato de amônio e sulfato de amônio) e de variantes no manejo da ureia (fertilizante aplicado pela manhã ou à tarde) em reduzir as perdas de amônia por volatilização. Dois experimentos foram conduzidos, por dois anos, com as culturas do trigo (Triticum aestivum L.) e milho (Zea mays L.), seguindo um delineamento de blocos casualizados, com quatro repetições. As perdas de N por volatilização de amônia foram avaliadas utilizando-se um coletor semiaberto estático, no período de até 21 dias após a aplicação dos fertilizantes. No inverno, as perdas foram baixas para todas as fontes nitrogenadas (<5,5 % N aplicado) em razão das baixas temperaturas e independentes do regime de chuvas. No verão, as taxas de volatilização de amônia, quando da aplicação superficial de ureia, mantiveram-se inferiores a 15 % do N aplicado e foram dependentes da ocorrência de chuvas nos primeiros cinco dias após a aplicação do fertilizante. A utilização de ure...

nitrogen losses by ammonia (nH 3 ) volatilization can be reduced by appropriate irrigation management or by alternative n sources, replacing urea. the objective of this study was to evaluate the efficiency of irrigation management and N source combinations in decreasing NH 3 volatilization from an Argissolo Vermelho Distrófico típico cultivated for 28 years with black oat (Avena strigosa) and maize (Zea mays), under no-tillage in the region of depressão central, rio grande do sul, Brazil. the experiment was arranged in a randomized block design with split plots with three replications, where the main plots consisted of irrigation systems: no irrigation; irrigation immediately before and irrigation immediately after fertilization. the subplots were treated with different n sources: urea, urea with urease inhibitor and slow-release fertilizer, at an n rate of 180 kg ha -1 , broadcast over maize, plus a control treatment without n fertilization. ammonia volatilization was assessed using semi-open static collectors for 1, 2, 4, 6, and 10 days after n fertilization. In general, more than 90 % of total nH 3 -n losses occurred until three days after n fertilization, with peaks up to 15.4 kg ha -1 d -1. The irrigation was efficient to reduce NH 3 losses only when applied after n fertilization. However, reductions varied according to the n fertilizer, and were higher for urea (67 %) and slightly lower for urea with urease inhibitor (50 %) and slow-release fertilizer (40 %), compared with the mean of the treatments without irrigation and irrigation before fertilization. the use of urea with urease inhibitor instead of urea was only promising under volatilization-favorable conditions (no irrigation or irrigation before n fertilization). compared to urea, slow-release fertilizer did not reduce ammonia volatilization in any of the rainfed or irrigated treatments.

High nitrogen (N) losses by ammonia (NH 3 ) volatilization from urea can compromise nitrogen fertilization efficiency and corn yield. The purpose of this study was to assess the effects of irrigation management and the addition of the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) on NH 3 -N losses from urea and on corn yield. To this end, two experiments were carried out in the 2011/12 crop season on a sandy clay loam Acrisol in the Central Basin region of Rio Grande do Sul, southern Brazil. Experiment I consisted of two different N sources (urea and urea plus the inhibitor) at a rate of 200 kg ha -1 N and two corn sowing times [viz., an early time (Sept. 3, 2011) and an intermediate time (Oct. 3, 2011)]. Experiment II considered a combination of three irrigation management systems (viz., without irrigation on the fertilization day and on the next 7 days; 20 mm of water immediately before N fertilization; and 20 mm water after N fertilization) and two N sources (urea and urea plus the inhibitor) at 150 kg ha -1 N. A control treatment without topdressed N fertilization was also performed in parallel with the two experiments. The NH 3 -N losses from common urea increased with increasing soil moisture in Experiment I (25 % applied N), and with irrigation before N fertilization in Experiment II (27 % applied N). The inhibitor reduced NH 3 -N losses from urea by 46 to 80 %. Also, irrigation after fertilization reduced ammonia volatilization by 83 % on average, with little effect from the inhibitor. The effects of the inhibitor and post-fertilization irrigation on the mitigation of NH 3 losses by volatilization from urea were not additive; in addition, they led to no increase in corn yield.

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