Abstract
To simulate growth conditions experienced by microbiota at O2-limited interfaces of organic matter in
compost, an experimental system capable of maintaining dual limitations of oxygen and carbon for extended
periods, i.e., a pO2-auxostat, has been used. 15N tracer studies on thermophilic (53&C) decomposition processes
occurring in manure-straw aggregates showed the emission of dinitrogen gas from the reactor as a result of
simultaneous nitrification and denitrification at low pO2 values (0.1 to 2.0%, vol/vol). The N loss was confirmed
by nitrogen budget studies of the system. Depending on the imposed pO2, 0.6 to 1.4 mmol of N/day (i.e., 20 to
40% of input N) was emitted as N2. When the pO2 was raised, the rates of both nitrification and denitrification
increased instantaneously, indicating that ammonia oxidation was limited by oxygen. In auxostats permanently
running at pO2 > 2% (vol/vol), the free ammonium pool was almost completely oxidized and was converted to
nitrite plus nitrate and N2 gas. Labelling of the auxostat with [13C]carbonate was conducted to reveal whether
nitrification was of autotrophic or heterotrophic origin. Incorporation of 13CO2 into population-specific
cellular compounds was evaluated by profiling the saponifiable phospholipid fatty acids (FAs) by using
capillary gas chromatography and subsequently analyzing the 13C/12C ratios of the individual FAs, after their
combustion to CO2, by isotope ratio mass spectrometry. Apart from the observed label incorporation into FAs
originating from a microflora belonging to the genus Methylococcus (type X group), supporting nitrification of
a methylotrophic nature, this analysis also corroborated the absence of truly autotrophic nitrifying populations.
Nevertheless, the extent to which ammonia oxidation continued to exist in this thermophilic community suggested that a major energy gain could be associated with it.
