Response of nitrogen distribution and roots of forage maize to biochar application at different depths

Application of nitrogen fertilizer is the main way to increase yield in forage production. However, excessive nitrogen application can increase nutrient leaching and cause environmental problems. In recent years, biochar has been used as a new soil amendment to slow and control the the release of nitrogen fertilizer which has achieved some initial effect. However, the application methods for biochar need to be optimized. The effects of biochar application at different depths on soil nitrogen and plant nitrogen distribution require further analysis. In this study, no biochar was used as the control, and 0－20, 20－40, and 40－60 cm deep treatments were set up to conduct field pot experiments. The results showed that compared with other treatments, surface application of biochar (0－20 cm) was more conducive to the increase of biomass, which could effectively increase the total biomass by 19.3%. The fruit biomass was 24.6% higher than that of the control treatment (P < 0.05). In terms of nitrogen accumulation, the nitrogen content of the aboveground component of maize (Zea mays) increased by 29.9%. The nitrogen content of the fruit and leaf sheath was significantly higher than that of the control treatment (P < 0.05), which was 29.7% and 42.3% higher than that of the control treatment, respectively. The nitrogen accumulation was consistent with the increase in biomass. In terms of soil nitrogen fixation, the soil nitrate nitrogen content at 0－20 cm in depth was higher than that of the other treatments. Meanwhile, a moderate increase in the application depth to 20－40 cm was more conducive to soil ammonium nitrogen fixation. All the biochar application depth treatments could promote root growth. The total root length at application at 20－40 cm in depth was significantly longer than that of the control treatment (P < 0.05). The total root surface area at an application depth of 0－20 cm was 49.1% higher than that of the control treatment. The total root volume was 215.1% higher than that of the control treatment (P < 0.05). Therefore, a biochar application depth of 0－20 cm could significantly improve root growth. This lead to an increase in nitrogen accumulation in the aboveground component, improving soil nitrate nitrogen retention capacity, and increasing forage maize yield. The results have provided a scientific basis for nutrient management of forage maize and strategic use of biochar.