Exploring Suitable Biochar Application Rates with Compost to Improve Upland Field Environment

A field experiment was carried out to investigate crop productivity, emissions of carbon dioxide (CO₂) and nitrous oxide (N₂O), and soil quality of an upland field treated with compost and varying rates of biochar (BC) derived from soybean stalks during crop growing periods in a corn and Chinese cabbage rotation system. Compost was supplemented with BC derived from soybean stalks at varying rates of 5, 10, 15, and 20 t ha⁻¹ (BC5, BC10, BC15, and BC20, respectively); the control (BC0) area was untreated. Our results reveal that crop productivity and emissions of CO₂ and N₂O varied significantly with the biochar application rate. Moreover, irrespective of the biochar application rate, crop productivity was improved after BC application as compared to the control treatment area, by 11.2–29.3% (average 17.0 ± 8.3%) for corn cultivation and 10.3–39.7% (average 27.8 ± 12.7%) for Chinese cabbage cultivation. Peak emissions of CO₂ and N₂O were mainly observed in the early period of crop cultivation, whereas low CO₂ and N₂O emissions were determined during the fallow period. Compared to the control area, significant differences were obtained for CO₂ emissions produced by the different biochar application rates for both crops. During the two cropping periods, the overall N₂O emission was significantly decreased with BC5, BC10, BC15, and BC20 applications as compared to the control, ranging from 11.1 to 13.6%, 8.7 to 15.4%, 23.1 to 26.0%, and 15.0 to 19.6%, respectively (average 16.9% decrease in the corn crop period and 16.3% in the Chinese cabbage crop period). Soil quality results after the final crop harvest show that bulk density, soil organic carbon (SOC), pH, and cation exchange capacity (CEC) were significantly improved by biochar application, as compared to the control. Taken together, our results indicate that compost application supplemented with biochar is potentially an appropriate strategy for achieving high crop productivity and improving soil quality in upland field conditions. In conclusion, appropriate application of biochar with compost has the concomitant advantages of enriching soil quality for long-term sustainable agriculture and reducing the use of inorganic fertilizers.