Combined biochar and nitrogen fertilization at appropriate rates could balance the leaching and availability of soil inorganic nitrogen

Biochar has been proposed to ameliorate soil properties and plant growth. However, it remains unclear how the interaction between biochar and nitrogen (N) fertilizer impacts soil inorganic nitrogen (SIN) leaching and availability in dryland systems. Therefore, a two-year field experiment was carried out on the Loess Plateau in northern China to study the effects of biochar combined with N fertilizer on the leaching and availability of SIN. Biochar applied at 0, 20 and 40 t ha−1 (B0, B1 and B2, respectively) interacted with three N fertilization levels (0, 120 and 240 kg N ha−1; N0, N1 and N2, respectively). Winter wheat (Triticum aestivum L.) was cultivated in a winter wheat-summer fallow cropping system. We measured wheat aboveground biomass and residual SIN in the soil profile (0–60 cm at 10 cm intervals) using standard extraction methods (2 M KCL, shaking at 25 °C for 1 h). Additionally, to ascertain whether field-aged biochar captured SIN and to determine residual SIN availability, we also used a modified extraction method (2 M KCL, shaking at 60 °C for 2 h) and ion exchange membranes (IEMs) to extract SIN from plow layer soil (0–20 cm). Our results indicated that biochar application alone in the absence of N fertilization had no significant effect on wheat biomass or residual SIN in the soil profile. However, compared with the application of N fertilizer alone, the application of biochar at 20 t ha-1 combined with N fertilizer not only increased wheat biomass by 12.2–13.8% but also significantly decreased residual NO3−-N in the subsoil by 13.2–74.7%. Nevertheless, long-term N fertilization at 240 kg N ha-1 led to large amounts of residual NO3−-N without a significant increase in crop biomass, which inevitably increased the risk of leaching during the fallow period. Although the application of biochar at 40 t ha-1 combined with N fertilizer more effectively decreased residual SIN in the subsoil, this approach was impractical because it decreased wheat biomass. Furthermore, the difference between NO3−-N extracted via the modified method and via the standard method increased with biochar application under each N level. Thus, field-aged biochar absorbed a certain amount of NO3−-N, thereby sequestering N in the soil after two years of N fertilization. Hence, biochar could reduce the residual NO3−-N available for leaching during the fallow period. However, notably, overuse of biochar could reduce the amount of NO3−-N available not only for leaching but also for crops. Ultimately, the application of biochar at 20 t ha-1 combined with N fertilization at 120 kg N ha-1 is a promising dual-win strategy for improving N availability while concurrently mitigating SIN leaching.