Interaction between nitrogen fertilizer and biochar fertilization on crop yield and soil chemical quality in a temperate region

The North China Plain suffers excessive application of nitrogen (N) and soil degradation. Recently, biochar has been promoted as an agricultural soil amendment to ameliorate soil quality, increase crop yield and mitigate greenhouse gas emissions. However, most proofs on the positive effects of biochar addition have been based on small plots or short-term field studies located in tropical or subtropical regions with defective soils. A long-term field experiment was designed with five N levels and two biochar rates to observe the changes in crop (winter wheat and summer maize) growth and soil chemical quality. Notably, crop yield was strongly dependent upon N application, with both wheat and maize yields increasing with N application. Biochar addition increased crop yield but not significantly, although the increase in grain yield was 0.96 t/ha in a rotation. Correlation analysis revealed that the optimal root-layer soil mineral N (Nmin) for crop production was around N120, especially with biochar addition. The effect of fertilization on soil chemical quality was mainly reflected in the increase in soil organic carbon (SOC), and the highest value was obtained at N60 whether or not biochar was applied. Overall, biochar addition did not appear to promote wheat and maize growth or increase the yield on calcareous alluvial soils in temperate regions, but significantly enhanced SOC content, especially at N60, which may play an important role in sustainable agricultural production.