Isotopically enriched ammonium shows high nitrogen transformation in the pile top zone of dairy manure compost

Nitrogen isotope ratios (δ15N) of NH4+ in dairy manure compost piles with and without bulking agent (10 % w∕w) were compared to understand the effects of the use of bulking agent on nitrogen conversion during manure composting. The δ15N–NH4+ values in each of three pile zones (top, side and core) were also compared. At the end of the process, piles with bulking agent showed significantly higher δ15N values (17.7 ± 1.3 ‰) than piles without bulking agent (11.8 ± 0.9 ‰), reflecting the significantly higher nitrogen conversion and NH3 loss in the former. The samples from the top zone, especially in the piles with bulking agent, showed very high NH4+ concentrations with significantly high 15N (δ15N: 12.7–29.8 ‰) values, indicating that extremely high nitrogen conversion, nitrification–denitrification activity of the microbes and NH3 volatilization occurred in this zone.