Enhancing the root growth rate with biochar improves the green water utilization in Danish sandy soil

The agricultural sector is facing challenges due to population growth, changing dietary preferences, and the need for greenhouse gas mitigation and climate change adaptation. Biochar seems to be useful multifunctional material that can help mitigate climate change by the action of carbon sequestration and improve soil structure and water retention, which helps plant growth by ameliorating the rhizosphere. Other powerful tools are the Super Absorbent Polymers (SAPs), which can improve soil water-holding capacity and so, adaptation to drought events. West Denmark is dominated by sandy soil that is not able to retain much water and leach nutrients, which implicates the use of blue water by the farmers. One way to optimize the use of green water instead of blue water and nutrients is an expanded, deep, and efficient root system. To test the improvement of root growth, a semi-field experiment took place in Foulum (Denmark). For this purpose, pots with a hole in the bottom part, closed by a mesh, were used. The pots were filled with 3 different types of soil: sandy, clay, and pelletised clay soil. For each soil, a hole wase made and filled with different biochars: four willow-based biochar pyrolyzed at 300°C, 400°C, 500°C and 600°C with a residence time of 1.30h and one commercialised biochar. Prior to being utilized, the biochar underwent a thorough series of analyses to ensure its quality and efficacy. Additionally, it was subject to a neutralisation process, achieving a pH between 7 and 8.6, to lower its pH to levels that are commensurable with root growth and functioning. A part of the biochar was subsequentially pelletised with a binder, while the rest was used mixed with SAPs, both treatments were tested. To determine the root growth parameters for the green water achievement, the bottom of the pot was checked to see the roots and time that it took for them to grow to the bottom. The aim of this project was to investigate whether pelletized biochar and biochar combined with SAPs can improve root growth in Danish sandy soil and speed up crop access to groundwater. Results did not show significant differences regarding root growth as affected by biochar and biochar mixed with SAPS. In contrast, root growth was affected by soil type (α = 0.05), with a faster growing rate observed in clay soil (1.46 cm day-1) than the other soil types that shower average values of 1.12 cm day-1. The emergency rate recorded after 8 days of seeding followed results similar of root rate growth with no differences between biochar treatments, while some slight effects of the pore filling were found 12 days after seeding. It is likely that the effects of biochar were limited by the amount used or by biochar neutralisation, which may have negatively interacted with the soil. Further research on biochar neutralisation and its influence on root growth is further required to identify the most suited biochar properties to improve soil and crop performance in Danish soils.