ASSESSMENT OF PLANT-AVAILABLE NUTRIENTS IN ORGANIC-PRODUCTS USING AN AIRLIFT BIOREACTOR

Waste products can be diverted from landfill and used as fertilizer wh en the nutrient availability in these products can be reliably determi ned. The aim of this study was to determine whether aerobic incubation in an internal-loop airlift bioreactor is a reliable method for asses sing plant-available N and P in organic waste products. Published resu lts of pot experiments with ryegrass were used as a reference for avai lable N and P. The incubation in the airlift reactor was also compared with conventional incubation in polyethylene bags. Samples of eight d ifferent organic products were incubated for 20 d in airlift reactors maintained at pH 5. The plant-available N fraction was determined by m easuring inorganic concentrations in the reactor solution, and the pla nt-available P fraction was determined by extraction of P from the sol ution using iron oxide-coated filter paper. The available N fraction i n the organic products as determined in the airlift reactor was closel y related (R-2 = 0.73) with the N uptake by ryegrass (Lolium perenne L .). For P, there was no relationship between the P extracted in the ai rlift reactor and the P uptake by ryegrass. The incubation in the airl ift reactors to assess plant-available N in organic waste products was more rapid than the pot experiment with ryegrass and was more reliabl e than the conventional incubation in polyethylene bags. We recommend that plant-available N and P be determined in different airlift reacto rs, because of possible pH-reIated P dissolution and because the iron oxide-coated filters contain NH4+ which may hamper determination of N.