Microbiology of composting eucalypt bark

The main aims of this study were to examine the optimal conditions for composting and factors influencing microbiological changes during the Composting of Eucalyptus bark in the production of a plant growth medium. A bench-scale composter was designed to provide strict control over air composition, moisture content, temperature and mixing. The composter consisted of six 4-L capacity gas-tight units of PVC plastic, each of which was provided with a mixing paddle coupled to a common drive. A natural temperature rise was simulated by having the units immersed in a water bath, with the temperature increased at rates consistent with those observed in large-scale compost heaps. This provided a companitively inexpensive versatile system, with rates of CO 2 and CH4 production and 02 consumption automatically monitored by gas chromatography. Levels of volatilized NH3 and nitrogen oxides (NO.) were manually monitored from acid traps. The reproducibility of the system was as good as the best reported. Optimal conditions for the bench-scale composting of eucalypt bark were considered to be a temperature of 55 0 , on aeration rate of at least 20 mL min -1and an initial C:N ratio of 25-30, depending on the availability of nitrogen. Nitrogen, in decreasing order of availability, was provided in the form of urea, isobutylidene diurea (IBDU), fish wastes or sewage cake. Both respiratory activity and nitrogen loss data were considered to be of value in determining the economic as well as the microbiological optimal C:N ratio of bark compost. No further amendment other than water (giving an initial moisture content of 1147. d.w. basis) was found to be necessary. Ammonification and NH3 volatilization occurred during the first sixteen days of composting while volatilization of NO was substantial during times of undesirable nitrogen availability. Delaying ammonification in the urea amended composts (by either the addition of quinone or urea's replacement with IBDU) increased ammonia volatilization. Net nitrification followed peak net ammonification, but nitrate appeared to be produced largely chemically rather than biologically. Volatilization of NO. was greatest from compost prepared using sewage cake. Up to five peaks of CO 2 output were observed over a 30 day run, three occurring during the transition to thermophilic conditions and one or two peaks occurring during a plateau temperature of 55 0 . The predominant flora comprised Bacillus spp. during the mesophilic and early thermophilic phases (B.brevis and B.sphaericus followed by B.circulans and B.brevis then B.circulans, B.sphaericus or B.stearothersophilus). Bacillus spp. continued to predominate throughout the composting of sewage-bark and most of the fish-bark composts. However, actinomycetes (Streptomyces spp. and Thernomonospora spp.) and coryneforms predominated at latter stages of urea-bark composts. Strictly anaerobic bacteria appeared to be unimportant during the composting of bark. The predominant flora isolated during the mesophilic phase were not inhibited by compost components of any age, while members of the climax flora were inhibited by fresh compost components. Cellulase activity was not correlated with peaks in CO2 output, but showed a slow increase or decrease, depending on the initial C:N ratio, over 30 days composting. However, lipase activity correlated with the peak in CO 2 output at about day sixteen in a fish-bark compost. The identification of thermophilic Bacillus spp. was aided by a study of their esterase mobilities and the use of numerical taxonomy. Phenolic compounds present after 30 days composting were phytotoxic. However, levels of residual ammonium could largely account for the phytotoxicity exhibited by water extracts from most of these composts. Eucalypt bark composts had a higher density than pine-bark composts or peat moss, but were as good as or better than the latter materials with regard to their water characteristics and particle-size distribution.