WRRCTR No.141 Irrigation of Californiagrass with Domestic Sewage Effluent: Water and Nitrogen Budget and Crop Productivity

Abstract

Californiagrass (paragrass) irrigated with effluent from secondarily treated domestic sewage showed excellent response as a means for disposal of large amounts of water, effective removal of nitrogen, and high production of excellent fodder. This grass, already well established in Hawaii, is used for pasture and fodder, endures flooding, and because of its allelopathic habit forms dense, easily maintained monostands. The water, nitrogen, and biomass budgets of the grass over a l7-mo period, from April 1979 through August 1980, were measured in eight large percolate-style lysimeters filled with the Lahaina series soil (Tropeptic Haplustox). The experiment was conducted on the grounds of the Mililani Wastewater Treatment Plant in central Oahu, Hawaii. Under irrigation rates which reached as great as 98 mm/day 5 days a week, consumptive use of water by the grass averaged 4 mm/day and was linearly correlated with biomass production. The monthly effluent nitrogen content ranged from 17 to 59 mg/l with an average level of 34 mg/l. With effluent nitrogen application rates which ranged from 475 to 2 600 kg/ha/yr, an average 69% was harvested in the grass, 3% percolated, nearly 28% was denitrified, while the soil nitrogen status remained unchanged or decreased slightly. Even with the highest effluent irrigation rates, nitrate nitrogen levels in the percolate remained less than the 10 mg/l recommended maximum for potable water. Crop productivity was linear with applied nitrogen. Dry weight averaged 150 tons/ha/yr, with a maximum short-term productivity equivalent to 193 tons/ha/yr. The calculated crude protein content with the highest nitrogen application rates was 13%, while the caloric value was 4 000 kcal/kg, and no nitrate nitrogen levels in the forage exceeded 0.1%.