Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O grande volume de Ãgua utilizado na agricultura compromete a sua disponibilidade para o consumo humano. Diante desse problema, Ã necessÃrio avaliar a utilizaÃÃo de Ãguas residuais na produÃÃo agrÃcola, como uma opÃÃo para a conservaÃÃo dos recursos hÃdricos. Ãguas residuais oriundas de campos de petrÃleo podem ser uma boa opÃÃo para irrigaÃÃo, especialmente em regiÃes de pouca e alta variabilidade de chuvas como o Nordeste brasileiro. No entanto, Ã necessÃrio avaliar os seus efeitos no solo, como acÃmulo de nutrientes e decomposiÃÃo da matÃria orgÃnica, bem como no aspecto nutricional e no crescimento das plantas. Os objetivos deste trabalho foram: i) Avaliar as mudanÃas ocorridas nos sais solÃveis e nas fraÃÃes da matÃria orgÃnica do solo e no crescimento de plantas de girassol (Helianthus annuus L.) cv. BRS 321, cultivadas em solo de textura arenosa irrigado com Ãgua residual oriunda da extraÃÃo de petrÃleo tratada por filtragem e osmose reversa; ii) Avaliar as alteraÃÃes nutricionais do girassol (Helianthus annuus) BRS 321 irrigado com Ãgua residual da extraÃÃo de petrÃleo e os efeitos sobre a produÃÃo de biomassa e de aquÃnios; e iii) Avaliar a decomposiÃÃo dos restos culturais de girassol (Helianthus annuus) cv. BRS 321 em Ãreas irrigadas com Ãgua residual da extraÃÃo de petrÃleo. O girassol foi cultivado durante trÃs perÃodos sucessivos em Ãreas irrigadas com Ãgua residual da extraÃÃo de petrÃleo tratada por filtragem (APF) e por osmose reversa (APO) ou Ãgua subterrÃnea do aquÃfero AÃu (ACA). Ao fim de cada ciclo foram determinados os valores de pH, condutividade elÃtrica (CE), a razÃo de adsorÃÃo de sÃdio (RAS) e os teores de Na, Cl, Mg, Ca, K, HCO3-, Ag, As, B, Ba, Cd, Co, Cr3+, Hg, Ni, Pb, Zn, alÃm de C nas fraÃÃes hÃmicas. Em raÃzes, parte aÃrea e aquÃnios determinaram-se os teores de N, P, K, Ca, Mg, S e Na e avaliaram-se o crescimento e o acÃmulo de biomassa. AlÃm disso, avaliou-se a decomposiÃÃo dos resÃduos da parte aÃrea das plantas de girassol. Houve alteraÃÃes nos atributos quÃmicos do solo, em destaque o acrÃscimo de sais, que foi maior nas Ãreas irrigadas com APF, e elevaÃÃo do pH, independente do tipo de Ãgua utilizado. A elevaÃÃo da salinidade e alcalinidade do solo reduziu o crescimento e o acÃmulo de biomassa das plantas. AlÃm disso, a irrigaÃÃo proporcionou alteraÃÃes nutricionais das plantas, que refletiram no crescimento e na produÃÃo de aquÃnios do girassol e na decomposiÃÃo dos resÃduos da cultura, com efeitos positivos ou negativos, conforme o tratamento da Ãgua residual. Concluiu-se que a Ãgua residual da extraÃÃo de petrÃleo utilizada na irrigaÃÃo afeta os atributos quÃmicos do solo, bem como a composiÃÃo nutricional e o desenvolvimento das plantas, principalmente quando irrigadas com a APF. AlÃm disso, a irrigaÃÃo com Ãgua residual alterou a mineralizaÃÃo dos resÃduos vegetais do girassol, mas nÃo a de decomposiÃÃo do carbono orgÃnico nas fraÃÃes hÃmicas. / The large volume of water used in agriculture compromises its availability for human consumption. Faced with this problem, it is necessary to evaluate the use of wastewater in agricultural production as an option for the conservation of water resources. Wastewater from oil fields can be a good choice for irrigation, especially in regions of low or high rainfall variability, as in the Brazilian Northeast. However, it is necessary to evaluate its effects on the soil, such as the accumulation of nutrients and decomposition of organic matter, as well as the nutritional aspect and plant growth. The aims of this study were: i) to assess the changes that take place in the soluble salts and soil organic matter fractions, and in the growth of plants of the BRS 321 cultivar of the sunflower (Helianthus annuus L.) cultivated in soil of a sandy texture and irrigated with wastewater obtained from oil extraction and treated by filtration and reverse osmosis; ii) to assess the nutritional changes in the BRS 321cultivar of the sunflower (Helianthus annuus) irrigated with wastewater from oil extraction, and the effects on biomass and achene production; and iii) to evaluate the decomposition of plant residue from the BRS 321 cultivar of the sunflower (Helianthus annuus) in areas irrigated with wastewater from oil extraction. The sunflower was grown for three successive periods in areas irrigated with wastewater from oil extraction treated by filtering (APF) and reverse osmosis (APO), or with groundwater from the AÃu aquifer (ACA). At the end of each cycle, pH values were determined, together with electrical conductivity (EC), the sodium adsorption ratio (SAR) and the levels of Na, Cl, Mg, Ca, K, HCO3-, Ag, As, B, Ba, Cd, Co, Cr3+, Hg, Ni, Pb, Zn, and of C in the humic fractions. The concentrations of N, P, K, Ca, Mg, S and Na were determined in the roots, shoots and achenes, along with growth and the accumulation of biomass. In addition, the decomposition of shoot residue from the sunflower plants was evaluated. Changes were seen in the chemical properties of the soil, highlighting an increase in salts, which was greater in the areas irrigated with APF; a rise was also seen in pH irrespective of the type of water used. The rise in soil salinity and alkalinity reduced the growth and accumulation of plant biomass. Moreover, irrigation resulted in nutritional changes in the plants, which were reflected in the growth and production of achenes, and in the decomposition of the plant residue, having positive or negative effects according to the treatment of the wastewater. It was concluded that wastewater from oil extraction used in irrigation affects the chemical properties of the soil as well as the nutritional composition and development of the plants, especially when irrigated with APF. Furthermore, irrigation with wastewater changed the mineralisation of the sunflower plant residue, but not the decomposition of the organic carbon in the humic fractions.