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Nitrogen isotope ratios (<i>δ</i>¹⁵N) of NH₄⁺ in dairy manure compost piles with and without bulking agent (10 % <i>w</i>∕<i>w</i>) were compared to understand the effects of the use of bulking agent on nitrogen conversion during manure composting. The <i>δ</i>¹⁵N–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 <i>δ</i>¹⁵N 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 (<i>δ</i>¹⁵N: 12.7–29.8 ‰) values, indicating that extremely high nitrogen conversion, nitrification–denitrification activity of the microbes and NH₃ volatilization occurred in this zone.