Nitrogen input alleviates the priming effects of biochar addition on soil organic carbon decomposition

The combination of biochar and nitrogen (N) addition has been proposed as a potential strategy to sustain crop productivity and mitigate climate change by increasing soil fertility, sequestering carbon (C), and reducing soil greenhouse gas emissions. However, our current knowledge about how biochar and N additions interactively alter mineralization of native soil organic C (SOC), which is referred to priming effects (PEs), is largely limited. To address this uncertainty, C₃ biochar (pyrolyzing rice straw at 300, 550, and 800 °C) and its combination with N fertilizer (urea) were incubated in a C₄-derived soils at 25 °C. All these 3 types of biochar with different addition rates caused positive priming of native soil organic matter decomposition (up to +58.4%). The maximum negative priming effects (up to −25.4%) occurred in soil treated with 1% of N-bound biochar pyrolyzed at 300 °C. In addition, a negative correlation was found between the priming intensity and soil inorganic N content across all treatments. The decrease in biochar-induced PEs was related with a shift in microbial community composition and reduction in microbial biomass determined by chloroform-fumigation. Such a reduction, however, was not confirmed by PLFA analysis. These findings advance our understanding on the microbial mechanisms mediating net soil C balance with the adequate biochar use for blending traditional mineral fertilizers.