Land application of sugar beet by-products: Effects on nitrogen mineralization and crop yields

Land application of food processing wastes has become an acceptable practice because of the nutrient value of the wastes and potential cost savings in their disposal. Spoiled beets and pulp are among the main by-products generated by the sugar beet (Beta vulgaris L.) processing industry. Farmers commonly land apply these by-products at rates >224 Mg ha^-1 on a fresh weight basis. However, information on nutrient release in soils treated with these by-products and their subsequent impacts on crop yield is lacking. Field studies were conducted to determine the effects of sugar beet by-product application on N release and crop yields over two growing seasons. Treatments in the first year were two rates (224 and 448 Mg ha^-1 fresh weight) of pulp and spoiled beets and a nonfertilized control. In the second year after by-product application, the control treatment was fertilized with N fertilizer and an additional treatment was added as a nonfertilized control in buffer areas. Wheat (Triticum aestivum L.) was grown in the year of by-product application and sugar beet in the subsequent year. By-product treatments caused a significant reduction in wheat grain yield compared with the control. This was due to a decline in N availability as a result of immobilization. Based on microplots receiving 15N labeled beets, wheat took up <1% of spoiled beet-N (approximately 4.7 kg ha^-1) during the year of by-product application. In the second cropping year, sugar beet root yields were significantly higher in the fertilized control and by-product treatments than the nonfertilized control. The lack of significant difference in sugar beet yield between the fertilized control and by-product treatments was likely due to the greater availability of N in the second year. Labeled ¹⁵N data also showed that the sugar beet crop recovered a 17% of sugar beet-N, an equivalent of 86 kg N ha^-1, during the second cropping year. There was no difference in sugar beet root yield, N uptake, or soil N mineralization during the sugar beet cropping season between the pulp and the spoiled beet treatments at comparable rates of application.