Xue Hao

To investigate the combined effects of a reduced nitrogen application rate and biochar amendment on fruit yield and quality, photosynthetic carbon allocation, and soil carbon retention in apple orchards, a two-year field experiment was conducted using Malus domestica Borkh. cv. ‘Red Fuji’/M9T337. Six treatments were established: three nitrogen levels (conventional nitrogen [TN], 20% nitrogen reduction [TN80%], 40% nitrogen reduction [TN60%]) with and without biochar amendment (+B). The TN80%+B treatment led to the highest anthocyanin, soluble solid, and soluble sugar values and sugar/acid ratio. The yield and economic benefits of TN80%+B were not significantly different from those of TN, while its agronomic efficiency was the highest among all treatments. Moderate nitrogen reduction combined with biochar increased ¹³C accumulation in fruits and roots, enhanced the ¹³C distribution rate in fruits (41% in TN80%+B), and promoted the transfer of photosynthetic carbon from aboveground parts to roots and soil. Soil organic carbon, microbial biomass carbon, particulate organic carbon, mineral-associated organic carbon, and their corresponding ¹³C-labeled carbon fractions all increased following application of nitrogen reduction combined with biochar, peaking under the TN80%+B treatment. Soil CO₂ emissions decreased with declining nitrogen inputs, with the lowest cumulative emissions observed under TN60%+B (1937.31 kg C ha⁻¹). Biochar also suppressed cellobiohydrolase activity and cbh abundance while enhancing α-glucosidase, β-glucosidase, and carbon-fixation gene abundances (cbbL, accA, and aclA). Overall, TN80%+B provides the most favorable balance among outcomes, delivering concurrent benefits of improved fruit quality, stable yield, enhanced soil carbon retention, and reduced CO₂ emissions.