Although it has been separately reported that biochar primes the decomposition of soil organic matter (SOM) or fresh organic matter, little is known about the simultaneous effects of biochar on SOM versus plant litter mineralization. We applied dual 13C/14C isotopic labels to partition soil CO2 efflux and C pools into three sources: SOM, litter and biochar. Biochar made by slow pyrolysis (400 °C) of 14C labeled residues of rice (Oryza sativa, C3) and maize (Zea mays, C4) litter were added separately or in combination to a silty Fluvisol with a C3 isotopic signature and incubated at 25 °C over a period of 6 months. Biochar decomposition was very slow, with a mean rate of 0.017% d−1. Approximately 63% of biochar-derived CO2 was produced in the first month. Mixing with litter reduced biochar mineralization by 14%. Addition of biochar alone to soil induced a cumulative positive priming effect (0.24 mg C g−1 soil) on SOM decomposition over 183 days, a much smaller effect than litter-induced priming (1.05 mg CO2-C g−1 soil). Compared to soils with only litter amended, biochar and litter added in combination decreased SOM mineralization by 19% while increasing litter mineralization by 6.9%, with no net changes in total CO2 release. Increased litter- but not SOM-derived C in microbial biomass in the presence of biochar suggested that biochar caused preferential microbial utilization of litter over SOM. Given that immobilization of mineral N in the soil-litter mixture was markedly enhanced following the addition of biochar, we proposed that the biochar-induced preferential microbial utilization of litter over SOM was due primarily to alterations in N cycling. In conclusion, the priming effects of litter on SOM are changed by the presence of biochar.
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