Temporal changes in delta N-15 of cattle feedlot manure during its composting with either rice hull (RHM) or sawdust (SDM) as bedding materials were investigated. Regardless of the bedding material used, the delta N-15 of total N in the manure increased sharply from +7.6 parts per thousand to +9.9 parts per thousand and from +11.4 parts per thousand to +14.3 parts per thousand, respectively, in RHM or SDM, within 10 days from the commencement of composting. Such increases could be attributed primarily to N loss via NH3 volatilization and denitrification based on the very high delta N-15 values (greater than +20 parts per thousand) of NH4+ and NO3- in the co-composted manure. The delta N-15 of total N in RHM was substantially lower (by more than 3 parts per thousand) than that in SDM, suggesting that the delta N-15 of the composted manure was affected not only by N loss but also by the type of bedding material used. Specifically, the higher N concentration in the rice hull than in the saw dust could lead to a greater N-15 isotope dilution. (C) 2007 Elsevier Ltd. All rights reserved.