Does biochar mixed with digestate affect crop yield, nitrogen use efficiency, and nitrate leaching?

Despite extensive research into the agronomic and agroecological effects of biochar on a global scale, several key questions remain unanswered. Further research is required to fully understand the impact of biochar on crop yield, nitrogen (N) turnover, and N use efficiency on soils with high nutrient contents and soil fertility. Additionally, there is a requirement for a more comprehensive analysis of the combination of biochar with organic and mineral fertilizers. Furthermore, most experiments have been conducted under controlled laboratory conditions or in short-term field trials, which limits the scope of insights gained. To close this research gap, a multi-year field experiment on a practical farm was conducted in southern Germany. On the stationary plots, biochar (0, 500, and 1000 kg ha⁻¹ yr⁻¹ biochar carbon (C)), digestate (170 kg ha⁻¹ yr⁻¹ total N), and mineral N fertilizer (30–100 kg ha⁻¹ yr⁻¹) were applied to the crop rotation silage maize-winter wheat-winter barley. In the experiment, the effects on crop yield, N use efficiency, nitrate dynamic, and nitrate leaching were investigated. High dry matter yields of silage maize (max. 24.2 t ha⁻¹), grain yields of winter wheat (max. 10.5 t ha⁻¹), and winter barley (max. 8.1 t ha⁻¹) were observed with organic-mineral fertilization. The sole application of biochar had no significant effect on crop yield and N uptake compared to the control treatment. When digestate was mixed with biochar without additional mineral N fertilization, there was a noticeable but not statistically significant reduction in N uptake and N use efficiency. N fertilization increased the nitrate stocks in the layer 1–3 m, significantly. However, biochar had no significant effect. Even in the treatments with intensive N fertilization, the nitrate concentration in the leachate was 23.0 mg l⁻¹ and thus remained under the limit for drinking water. The results of the field experiment show that the addition of medium quantities of biochar, either pure or in combination with digestate, at a site with high soil fertility, had no significant effect on crop yield, N use efficiency, or nitrate leaching.