Nitrogen transformation during co-composting of spent pig manure, sawdust litter and sludge under forced-aerated system

Forced-aerated piles were built to investigate the transformation of nitrogen during co-composting of spent pig litter and sludge at 2:1 wet volume ratio. The total N concentration of spent litter-sludge increased during the early stage of composting due to the concentration effect as a consequence of strong degradation of carbon compounds in the spent litter-sludge piles. After day 21, the total N concentration decreased but there was no significant loss in nitrogen. The total mass of N was maintained at around 30 kg during the composting process. The ammonium- and nitrite-oxidizing bacteria increased in numbers during the active decomposition process (from day 0 to day 21), then dropped gradually as composting proceeded. At this early stage, the NH₄⁺-N concentration declined and the (NO₃⁻+NO₂⁻)-N increased. However, the rapid decrease in NH₄⁺-N concentration during the first 14 days of composting did not correspond to a rapid increase in (NO₃⁻+NO₂⁻)-N, indicating that some inorganic N was immobilized to organic N, some NH₄⁺-N was volatilized, and some (NO₃⁻+NO₂⁻)-N was lost via denitrification. Denitrification only occurred during the early stage of composting and decreased significantly once the air was blown into the pile. Spatial variations in several physico-chemical and microbial parameters were found among four different locations of the forced-aerated piles, especially during the first 7 weeks of composting. The surface location had significantly lower temperature and population size of denitrifying bacteria than the top, middle and bottom locations, but higher concentration of total C and higher counts of ammonium-oxidizing bacteria were recorded in the surface location.