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Surface-activated biochars not only represent a useful carbon sink, but can also act as useful filtering materials to extract plant nutrients (e.g. NH4+) from wastes (e.g. animal or municipal waste streams) and added thereafter to soils. Biochars produced by low-temperature pyrolysis of fibrous debarking waste from pine (PI) and eucalyptus (EU) were pre-treated with either diluted (L) or undiluted (S) alkaline tannery waste (L-PI, S-PI, L-EU, S-EU). Biochars produced from untreated feedstock were used as controls. Samples were characterised by FT-IR, solid-state CP MAS 13C NMR, XPS, SEM microphotographs, and BET specific surface area. Elemental composition, carbon recovery, yield, surface charge, and NH4+ sorption/desorption properties were also studied. Carbon recovery was lower in biochars prepared from L-EU and S-EU (43 and 42%, respectively) than in control EU (45%) but these biochars showed greater changes in their chemical characteristics than those made from L-PI and S-PI, which showed minimal decrease in recovered carbon. The specific surface area of the biochars decreased with treatments, although acidic surface groups increased. In subsequent sorption experiments, treated biochars retained more NH4+ from a 40 mg N/L waste stream (e.g. 61% retention in control EU and 83% in S-EU). Desorption was low, especially in treated biochars relative to untreated biochars (0.1–2% v. 14–27%). The results suggest that surface activated biochars can be obtained with negligible impairment to the carbon recovered.

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Greenhouse tomato (Lycopersicum esculentum Mill.) producers are urged to reduce their environmental footprint. Here, the suitability of biochar produced from tomato crop green waste as a substrate for soilless, hydroponic tomato production was evaluated. Substrates containing different combinations of biochar (BC) and pine (Pinus radiata D. Don) sawdust (SD) were produced (BC0-SD100, BC25-SD75, BC50-SD50, BC75-SD25, and BC100-SD0) and characterized. The effect of these substrates on tomato growth, yield, and fruit quality was studied. Most of the measured properties of substrates containing biochar were suited to use as a soilless substrate. The electrical conductivity (EC) of substrates containing biochar was initially high (>4.6 mS·cm−1), but was easily reduced to <0.5 mS·cm−1 by rinsing with water before use. The pH of substrates containing biochar was higher than is considered acceptable for tomato production (7.5–9.3) but did not significantly (P < 0.05) affect any plant growth, yield, and fruit quality indicators measured compared with those of plants grown in pine sawdust. The results support the concept of creating a closed loop system whereby biochar produced from tomato crop green waste is used as a substrate for soilless, hydroponic tomato production, providing a sustainable means to support the growth of high-value food crops.