Changes in nitrogen isotopic compositions during composting of cattle feedlot manure: Effects of bedding material type

Temporal changes in δ15N 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 δ15N of total N in the manure increased sharply from +7.6‰ to +9.9‰ and from +11.4‰ to +14.3‰, 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 δ15N values (greater than +20‰) of NH4+ and NO3- in the co-composted manure. The δ15N of total N in RHM was substantially lower (by more than 3‰) than that in SDM, suggesting that the δ15N 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 15N isotope dilution.