Effect of co-composted biochars on nitrogen availability to spinach

Co-composted biochar is supposed to be an effective sorbent for nitrate and thus might reduce nitrate leaching from agricultural soils. However, the underlying mechanism as well as the effect on nitrate availability is still under discussion. In the current study, a pot trial with baby leaf spinach was conducted to evaluate the influence of two co-composted biochars on nitrogen availability. Two wood-based biochars were co-composted with grass clippings and woody scrap material from landscaping activities. Additionally, a compost without biochar was prepared. Composts were mixed to an arable soil at rates of 30, 60 and 120 t ha^‑1 and the amount of nitrate was supplemented to 100 kg N ha^‑1. Afterwards, mixtures were filled in balcony boxes, moistened and placed in a greenhouse for 40 days. As control, the soil without compost was treated in the same way. Further, for the soil without compost and for the soils with the highest compost application rate, additional treatments without nitrogen fertilization were prepared. After the 40 days of storage, half of these balcony boxes were also fertilized with nitrate at 100 kg N ha^‑1. Then baby leaf spinach, pre-cultivated in peat press pots, was planted in all balcony boxes. Irrespective of compost application rate and in case of the highest application rate and nitrogen fertilization date, no negative effect of co-composted biochars on N uptake was observed. In addition, the nitrogen balance of treatments with co-composted biochars was comparable to the soil without compost. However, for the treatments with biochar-free compost an increasing balance shortfall with increasing amounts of compost was observed. This might be due to nitrogen immobilization of the woody residues. In view of the comparable N uptake by plants for compost treatments with and without biochar as well as significant amounts of easily extractable nitrate in all soils at the end of the experiment, we assume that the co-composted biochars used in this experiment had no significant effect on nitrogen availability to the plants.