Effects of Nitrogen Application Rate Reduction Combined with Biochar Amendment on Apple Yield and Quality, Photosynthetic Carbon Allocation, and Soil Carbon Retention

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.