EVALUATION OF NITROGEN AVAILABILITY IN IRRADIATED SEWAGE-SLUDGE, SLUDGE COMPOST AND MANURE COMPOST

A field experiment was conducted during 2 yr to determine plant availa bility of organic N from organic wastes, and effects of gamma irradiat ion on organic N availability in sewage sludge. The wastes investigate d were: digested, dewatered sewage sludge (DSS), irradiated sewage slu dge (DISS), irradiated, composted sewage sludge (DICSS), and composted livestock manure (CLM). The annual application rates were: 10, 20, 30 , and 40 Mg solids ha(-1). Fertilizer N was added to the control, to w hich no waste was applied, as well as to the waste applications to ens ure approximately equal amounts of available N (110 kg N ha(-1)) for a ll treatments. Lettuce (Lactuca indica L.), petunias (Petunia x hybrid a Vilm.), and beans (Phaseolus vulgaris L.) were grown in 1990 and two cuts of lettuce were harvested in 1991. Crop yields and plant N conce ntrations were measured. Assuming that crop N harvested/available N ap plied would be approximately equal for the control and the waste treat ments, the N from organic fraction of the wastes, which is as availabl e as that in fertilizer, was estimated. With petunia In 1990 and the c ombination of first and second cut of lettuce in 1991, the percentage ranged from 11.2 to 29.7 in nonirradiated sludge, 10.1 to 14.0 in irra diated sludge, 10.5 to 32.1 in sludge compost and 10.0 to 19.7 in manu re compost. Most often, the highest values were obtained with the lowe st application rates. Yields of petunia and N concentrations in second cut lettuce in 1991 were lower with irradiated sludge than with nonir radiated sludge suggest that the availability of organic N in digested sludge may have been reduced after irradiation. Irradiation of sludge appears to have released NH4+-N. The availability of organic N, howev er, appears to have been reduced by irradiation by greater amount than the increase in NH4+-N.