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Irrigation of Forages with Rendering Plant Wastewater: Forage Yield and Nitrogen Dynamics¹

ABSTRACT

Wastewater from an animal by-product rendering plant containing N, P, and biochemical oxygen demand (BOD) at greater than 500, 60, and 500 mg L^–1, respectively, was applied to irrigated forages to study the suitability of forage species and the fate of applied nutrients. Applications of 10 and 20 cm yr^–1 supplied the nutrients at rates in excess of crop requirements. The treatments were compared with irrigation water and with water supplemented with either fertilizer N and P or C as sugar at rates similar to those in the wastewater. Reed canarygrass (Phalaris arundinacea L.) yields were doubled by the application of 10 cm yr^–1 of wastewater as compared with irrigation water treatments, but alfalfa (Medicago sativa L.) yields were not significantly affected. Doubling the wastewater application rate or adding comparable rates of N and P fertilizer did not further increase yields. The wastewater-irrigated alfalfa contained NO₃^–-N levels of 1500 to 1600 mg kg^–1 plant material, while reed canarygrass contained about 3000 to 3400. Both these levels would be considered potentially unsafe for livestock feed. There was an accumulation of soil NO₃^–-N levels of up to 30 to 40 mg kg^–1 soil throughout the surface 120 cm of soil after 5 yr of irrigation with 10 cm yr^–1 of wastewater and two to three times this level with 20 cm yr^–1 of wastewater. Higher levels of NO₃^–-N were observed in the soil when N was applied as fertilizer. Soil NH₄⁺-N levels were not greatly affected. A nitrogen balance over 6 yr at the high application rate suggested plant uptake, soil NO₃^–-N, and losses accounted for 30, 25, and 45% of wastewater N and 29, 39, and 32% of fertilizer N. A 1-yr ¹⁵N balance after 3 yr of irrigation indicated uptake, soil NO₃^–-N, and unaccounted-for N was about 10, 19, and 71% for wastewater and 15, 33, and 52% for fertilizer N. The greater losses of wastewater N compared with fertilizer N were attributed to enhanced denitrification due to oxidizable C in the wastewater. Attempts to simulate this effect with C as sugar were not effective.

Key Words: effluent • alfalfa • reed canarygrass • phosphorus • carbon • salt • sodium adsorption ratio • nitrogen-15 balance

¹ Contribution from the Research Station, Agriculture Canada, Lethbridge, Alberta, Canada T1J 4B1.

² Research scientist, Research Station, Agriculture Canada, Lethbridge, Alberta, Canada T1J 4B1, and research microbiologist, Allied Corporation, Syracuse Research Laboratory, Solvay, NY 13209.

Received for publication April 2, 1984.