The use of wood ash as an additive in biochar production was shown to increase biochar yields and was suggested to improve the recycling of ash-derived nutrients. However, there is limited knowledge on the interaction of ash addition with pyrolysis conditions and their effects on biochar yield and properties. Here, we performed experimental pyrolysis in an auger reactor applying different ash concentrations, temperatures, and residence times. Biochar yield (dry and ash-free) increased by 26% and carbon-conversion efficiency of pyrolysis by 36% when 9 wt % ash was added to softwood. Up to this concentration, yield increase was correlated linearly, while higher ash concentrations did not further increase yield significantly. The ash-induced yield increase was lower at 400 °C than for 425–500 °C. Biochar’s content of polycyclic aromatic hydrocarbons was not affected by ash addition. The content of polychlorinated organic pollutants increased with ash amendment but remained well below applicable thresholds. The electron exchange capacity of biochars was increased while the micropore specific surface area and thermal stability decreased with ash addition. The elevated potassium content of ash-amended biochars promoted sunflower growth. Biochar derived from ash-amended biomass is safe to be applied in soil and may be a promising approach for enhanced nutrient recycling and carbon sequestration.

The use of biochar is an important tool to improve soil fertility, reduce the negative environmental impacts of agriculture, and build up terrestrial carbon sinks. However, crop yield increases by biochar amendment were not shown consistently for fertile soils under temperate climate. Recent studies show that biochar is more likely to increase crop yields when applied in combination with nutrients to prepare biochar-based fertilizers. Here, we focused on the root-zone amendment of biochar combined with mineral fertilizers in a greenhouse trial with white cabbage (Brassica oleracea convar. Capitata var. Alba) cultivated in a nutrient-rich silt loam soil originating from the temperate climate zone (Bavaria, Germany). Biochar was applied at a low dosage (1.3 t ha−1). The biochar was placed either as a concentrated hotspot below the seedling or it was mixed into the soil in the root zone representing a mixture of biochar and soil in the planting basin. The nitrogen fertilizer (ammonium nitrate or urea) was either applied on the soil surface or loaded onto the biochar representing a nitrogen-enhanced biochar. On average, a 12% yield increase in dry cabbage heads was achieved with biochar plus fertilizer compared to the fertilized control without biochar. Most consistent positive yield responses were observed with a hotspot root-zone application of nitrogen-enhanced biochar, showing a maximum 21% dry cabbage-head yield increase. Belowground biomass and root-architecture suggested a decrease in the fine root content in these treatments compared to treatments without biochar and with soil-mixed biochar. We conclude that the hotspot amendment of a nitrogen-enhanced biochar in the root zone can optimize the growth of white cabbage by providing a nutrient depot in close proximity to the plant, enabling efficient nutrient supply. The amendment of low doses in the root zone of annual crops could become an economically interesting application option for biochar in the temperate climate zone.