Biochar for agricultural water management

Agricultural runoff and wastewater can contain considerable nutrient loads released to the environment without precaution and lack of treatment. Capturing and reusing these nutrients in agriculture can have significant economic and environmental potential.
In this work, various types of biochar produced from sidestreams were assessed for nutrient uptake. Biochar has been shown to increase both soil water holding capacity and available water capacity, but it also has the potential to recover nutrients from runoff. However, the biochar’s efficiency depends on its properties, soil texture, the raw material(s) and processing conditions. Char activation increases the char’s specific surface and active sites and will likely improve the nutrient adsorptive efficiency.
This work assessed commercially available biochars’ capacity to adsorb nutrients from a solution and their potential to act as a substrate for retaining nutrients from runoff water. The biochar materials were pyrolysis products of various organic materials, wood and agricultural sidestreams. In addition zeolite – a recognised adsorption material was included for the sake of comparison
The results indicate that most biochar has some capacity to retain NH4-N. The capacity to retain NO3-N again varied between not considerable to ca 0.34 mg/g NO3-N (Mg(OH)2 cherry biochar). Thermal activation of biochar increases the number of adsorption sites and thus also improves the adsorption efficiency.
Commercial biochars did not show any significant affinity towards phosphate, but only after being coated with Mg(OH)2. Instead, the natural phosphorous content of biochar seems to leach into a solution easily. However, phosphorous leaching did not have any noticeable fertilising effect, neither that of NO3-N nor NH4 -N.
In conclusion, Mg(OH)2-coated biochar performed better than all other materials to retain NO3-N (0.34 mg/g or 74% retention rate) and PO4 (0.25 mg/g or 98% retention rate). Zeolite retained better NH4-N (78%) compared to all other materials, followed by coated cherry biochar (48% retention rate) and coated biochar (34% retention rate).
Biochar is a circular product which has been successfully applied as a soil amendment, thereby improving the structure and water-retaining capacity of various soils. Based on the results obtained here, tested substrates’ role in contributing to the nutrient balance varies between nutrients and materials applied.