Interaction between biochar and nitrogen fertilizer improves nitrogen utilization efficiency, closely connected with rhizosphere microbes involved in nitrogen-cycling

Abstract

Biochar is an eco-friendly amendment for improving nitrogen (N) utilization efficiency in agricultural ecosystems. However, the responses of N-cycling functional profiles of rhizosphere microbes to the co-application of biochar and N fertilizer remain greatly elusive. In this study, we designed a rhizo-box experiment with two biochar levels (0 and 2 %) and four nitrogen fertilizer application rates (N1, 0 mg kg⁻¹; N2, 50 mg kg⁻¹; N3, 100 mg kg⁻¹; and N4, 200 mg kg⁻¹) to investigate the effects of biochar and N fertilizer co-application on plant growth, nitrogen utilization efficiency, and rhizosphere microbial nitrogen-cycling functions. The results showed that biochar addition significantly promoted root biomass by 138.7 %, 192.6 %, 24.8 %, and 19.9 % under the N1, N2, N3, and N4 treatments, respectively. Additionally, nitrogen utilization efficiency was improved by 247.1 % and 29.3 % with biochar application under the N2 and N4 treatments. Meanwhile, metagenomic results revealed that rhizosphere microbes exhibited enhanced functions related to glutamate degradation, nitrate reduction and denitrification under the co-application of biochar and N fertilizer, while N fixation functions were inhibited. The variations in microbial N-cycling functions were driven by changes in key taxa involved in N-cycling (e.g., Sphingomonas, Sinomonas, and Streptomyces), which were closely correlated with NH₄⁺-N and NO₃^--N concentrations. Therefore, our results indicate that the improvement in nitrogen utilization efficiency due to the interaction between biochar and nitrogen fertilizer can be attributed to root proliferation, increased nitrogen availability, and feedback effects from rhizosphere microbes involved in nitrogen cycling.