The present study was designed to evaluate the nutrient release from decomposing water hyacinths [Eichhornia crassipes (Mart.) Solms.] in agricultural drainage water. In a laboratory incubation study, nutrient release from the decomposing water hyacinths was measured under aerobic and anaerobic conditions at varying levels of water hyacinth additions. In a field study, impact of water hyacinth die‐off on drainage canal water quality was evaluated after a herbicide application.In the laboratory study, under both aerobic and anaerobic systems about 21–45% of the added N was lost resulting in low accumulation of inorganic N. At all levels of water hyacinth additions, soluble P release into the water was higher under anaerobic conditions than aerobic conditions. At the end of the 100‐day decomposition, chemical oxygen demand (COD) of the water hyacinth treated water was reduced by 42–73% under aerobic conditions, and by 16 to 57% under anaerobic conditions and biological oxygen demand (BOD) of the water was reduced by about 67–85% under aerobic conditions and 20–53% under anaerobic conditions. Electrical conductivity (EC) and K, Mg, and Ca concentration of the water were not influenced by the oxygen status (aerobic or anaerobic conditions) of the water for any level of water hyacinth addition.Under field conditions, water hyacinth die off as a result of a herbicide application resulted in an increased concentration of soluble N and P in the drainage canal water.

/ Potential use of reservoirs and flooded fields stocked with aquatic plants for reduction of the nutrient levels of organic soil drainage water was evaluated. The treatment systems include 1) a large single reservoir (R1) stocked with waterhyacinth ( Eichhornia crassipes), elodea ( Egeria densa), and cattails ( Typha sp.) in series; 2) three small reservoirs in series with waterhyacinth (R2), elodea (R3), and cattails (R4), grown in independent reservoirs; 3) a control reservoir (R5) with no cultivated plants; 4) a large single flooded field planted to cattails; 5) three small flooded fields in a series planted to cattails; and 6) a flooded field with no cultivated plants. Drainage water was pumped daily (6 hours a day, and 6 days a week) into these systems for a period of 27 months at predetermined constant flow rates. Water samples were collected at the inlet and outlet of each treatment system and analyzed for N and P forms.The series of reservoirs stocked with aquatic plants functioned effectively in the removal of N and P from agricultural drainage water, compared to a single large reservoir. Allowing the water to flow th rough the reservoir stocked with waterhyacinth plants with a residence time of 3.6 days was adequate to remove about 50% of the incoming inorganic N. Allowing the water to flow through a series of two small reservoirs, R2 and R3, with a residence time of 7.3 days was necessary to remove about 60% of the incoming ortho-P. Flooded fields were effective in the removal of inorganic N, but showed poor efficiency in the removal of ortho-P.

Flooded organic soil as a treatment system for nitrate‐nitrogen (NO3‐N) removal from agricultural drainage water was evaluated under controlled conditions. Nitrate‐N reduction rates in the flood water (agricultural drainage water) and in the underlying organic soil column were measured as a function of NO3‐N concentration (10, 25, and 50 µg NO3‐N/ml), energy source, and temperature (8, 18, and 28°C). Labeled NO3‐N was used to differentiate between denitrification and NO3‐N reduction to ammonium‐N (NH4‐N) or organic N. Nitrate‐N reduction rates in the soil column incubated at 28°C with no excess flood water were best described by first‐order kinetics with an average rate constant (k) of 0.751 day−1. Flood water NO3‐N removal rates were also described by first‐order kinetics. Nitrate removal rates from low oxygen‐demand flood water (expressed as first‐order rate constants) with an underlying soil column were 0.038, 0.750 day−1 for an incubation temperature of 8, 18, and 28°C, respec‐ 28°C, respectively. In soil columns incubated with high oxygen‐demand flood water, most of the added NO3‐N disappeared before it reached the underlying soil column, with k values of 0.172, 0.292, and 0.790 day−1 for an incubation temperature of 8, 18, and 28°C, respectively. In all treatments, denitrification was the dominant process in the removal of flood water NO3‐N. At low temperature (8°C), a greater portion of 15NO3‐N was reduced to 15NH4‐N and organic N. Most of the immobilized 15NO3‐N appeared as soluble organic N. At 8°C, flood water NO3‐N diffused deeper into the soil column as compared to the flooded soil columns incubated at 28°C. The results obtained in this study indicate that flooded organic soil can function as an effective sink for reducing NO3‐N levels of agricultural drainage waters.

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