Quality of drained waters of the irrigated rice frames during the establishment of pre-germinated cultivation system

Abstract

Irrigated rice (Oriza sativa L.) pre-germinated production systems occupy approximately 100 thousand ha, annually cultivated in Rio Grande do Sul State, mainly in poorly drained areas, and with infestation of competing plants, especially red rice (Oriza sativa L.). This system constitutes the main technologic resource for rice crops produced on an ecological basis, by means of soil tillage and water management. Despite this, water outlets, specifically after seeding, have outstanding agronomic and environmental implications. Loss of nutrients and the physical and chemical quality of drained water have been questioned by public research and environmental institutions. This work evaluated the effects of different soil tillage systems and retention time of water in irrigated rice frames. The experiment was conducted in Viamão town, Rio Grande do Sul State. Two different soil tillage systems and four different times of frame drainage after seeding were investigated. Results showed that the main limiting attributes of water quality were turbidity, biochemical oxygen demand (BOD) and phosphorus (P) content, reaching 1800 TNU, 115 mg L^-1, and 1.6 mg L^-1, respectively. These attributes showed values higher than established environmental thresholds. Water quality was inappropriate to discharge directly into water courses, regardless of treatments.An alternative is the immediate placement in rice production areas, in more advanced development stages of plants. Pre-germinated rice crops require a system framework to avoid nutrient loss and consequent reduction of soil fertility.