Assessment of plant available nutrients in organic products using an airlift bioreactor
Velthof, G.L.; Van Beusichem, M.L.; Raijmakers, W.M.F.; Janssen, B.H.
Journal of Environmental Quality 27(5): 1261-1267
ISSN/ISBN: 0047-2425 DOI: 10.2134/jeq1998.00472425002700050035x
Waste products can be diverted from landfill and used as fertilizer when the nutrient availability in these products can be reliably determined. The aim of this study was to determine whether aerobic incubation in an internal-loop airlift bioreactor is a reliable method for assessing plant-available N and P in organic waste products. Published results of pot experiments with ryegrass were used as a reference for available N and P. The incubation in the airlift reactor was also compared with conventional incubation in polyethylene bags. Samples of eight different organic products were incubated for 20 d in airlift reactors maintained at pH 5. The plant-available N fraction was determined by measuring inorganic concentrations in the reactor solution, and the plant-available P fraction was determined by extraction of P from the solution using iron oxide-coated filter paper. The available N fraction in the organic products as determined in the airlift reactor was closely related (R2 = 0.73) with the N uptake by ryegrass (Lolium perenne L.). For P, there was no relationship between the P extracted in the airlift reactor and the P uptake by ryegrass. The incubation in the airlift reactors to assess plant-available N in organic waste products was more rapid than the pot experiment with ryegrass and was more reliable than the conventional incubation in polyethylene bags. We recommend that plant-available N and P be determined in different airlift reactors, because of possible pH-related P dissolution and because the iron oxide-coated filters contain NH4+ which may hamper determination of N.