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 (Cmin) and nitrogen mineralization (Nmin) over three years compared to fertilizer and the control. Biochar also increased total C (Csoil) 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 (Cmic), total N (Nsoil) and microbial biomass N (Nmic); the degree of change was closely related to Csoil and Nsoil. There was a positive correlation between Cmic:Nmic and Csoil:Nsoil; while Csoil and Cmic increased following amendment with biochar, which reduced the soil C and N stoichiometric imbalance (Nimb) 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 Csoil:Nsoil and Cmic:Nmic ratios. Soil pH had a positive correlation with Csoil, Cmic, Nmic, Cmin, Nmin, Csoil:Nsoil, Cmic:Nmic, and biochar increases this correlation. The soil pH was negatively correlated with Cimb:Nimb and Nsoil. Fertilizer was positively correlated Cimb:Nimb and Nsoil. 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.