Soil type regulates carbon and nitrogen stoichiometry and mineralization following biochar or nitrogen addition

Most studies on the effects of biochar and fertilizer on soil carbon (C) and nitrogen (N) mineralization, and microbial C and N content, are restricted to a single soil type, limiting our understanding of the interactions between these factors and microbial functions. To address this paucity in knowledge, we undertook a 3-year experiment using four contrasting soils to assess the role of peanut shell biochar and fertilizer on C and N mineralization, microbial C and N, and N stoichiometry. Across all four soils, biochar significantly (P < 0.05) increased soil carbon mineralization (C_min) and nitrogen mineralization (N_min) over three years compared to fertilizer and the control. Biochar also increased total C (C_soil) across the four soils in year 1, with the Fluvisol recording greater total C in year 2 and Phaeozem having greater total C in year 3. Biochar resulted in a higher microbial biomass C (C_mic), total N (N_soil) and microbial biomass N (N_mic); the degree of change was closely related to C_soil and N_soil. There was a positive correlation between C_mic:N_mic and C_soil:N_soil; while C_soil and C_mic increased following amendment with biochar, which reduced the soil C and N stoichiometric imbalance (N_imb) caused by the increase in the C to N ratio. However, fertilizer exacerbated the imbalance of soil C and N stoichiometry. Fertilizer also reduced the C_soil:N_soil and C_mic:N_mic ratios. Soil pH had a positive correlation with C_soil, C_mic, N_mic, C_min, N_min, C_soil:N_soil, C_mic:N_mic, and biochar increases this correlation. The soil pH was negatively correlated with C_imb:N_imb and N_soil. Fertilizer was positively correlated C_imb:N_imb and N_soil. In contrast, fertilizer N application lowered microbial biomass C:N. We conclude that biochar reduces the imbalance of soil C and N stoichiometry, whereas fertilizer increased this imbalance. Biochar had a greater impact on C and N in soils with a lower pH.