Biochar as a plant stimulator and peat replacer: the importance of screening biochar for different applications from a fast screening to pot experiments and a strawberry trial

Abstract

Feeding the world's population and sustainably doing this will be some of the future challenges of agriculture. Agricultural practices put pressure on our environment in multiple ways. To start, tillage reduces soil fertility by increasing soil erosion and decreasing the soil's organic matter content (SOM), water holding capacity (WHC), infiltration rates, and biological activities, leading to soil degradation. Furthermore, tillage increases the soil bulk density, and alters the pH and nutrient availability, further decreasing soil fertility. In addition to tillage, pesticide and fertiliser usage by agriculture negatively impact soil health as they can acidify soils, making toxic metals more bioavailable and affecting the microbiome. The latter can fixate nitrogen, mineralise essential nutrients, produce plant hormones and stimulate the plant's immune system and, therefore play an important role in soil health and plant growth. Pesticides and fertilisers not only impact soil health, but via runoffs, they can impact the health of water bodies and form a threat to aquatic life as well as impacting land life by accumulation of toxic substances in organisms and eutrophication. Lastly, agriculture also uses soilless cultivation to provide the population with food. Soilless cultivation often uses peat as substrate and peat excavation significantly contributes to the agricultural CO2 emissions. Biochar can be a part of a more sustainable agriculture as it can be used to capture CO2. Furthermore, it can increase soil health by reducing the bulk density, improving the WHC and SOC, decreasing the availability of toxic metals by increasing the cation exchange capacity (CEC) and providing habitats for microorganisms. These changes in soil physicochemical and biological properties can improve plant growth. Moreover, biochar can decrease the amount of fertiliser and pesticides needed for plant cultivation as it can increase the amount of phosphorus (P), nitrogen (N), potassium (K), … as well as improve disease resistance. Despite the positive effects reported on plant growth and soil health, also neutral and negative effects on plant growth and soil health are reported. These differences between results come from the high diversity of biochars used. This diversity is introduced by the different biomasses used for biochar production, going from nutrient-poor biomasses like wood to nutrient-rich biomasses like manure. Additionally, also pyrolysis settings influence the physicochemical parameters of the resulting biochar. Despite the high amount of literature on biochar, few studies investigate the effect of different biochar types on plant growth. Therefore, the first aim of this study was to screen multiple biochars for their plant growth promotion and verify this result with a pot experiment (chapter 3). Therefore, eleven different biochars were screened with Arabidopsis thaliana, and four were used to further validate the obtained results in a pot experiment with Medicago sativa. Results clearly show that the concentration used significantly influences the growth outcome of A. thaliana. Furthermore, the screening confirmed that biochar type greatly influences the plant growth outcome. These differences can come from differences in the physicochemical properties like the WHC, porosity, ash content, … . Depending on the biomass used and the pyrolysis settings, biochars can contain potentially toxic elements (PTEs) including polyaromatic hydrocarbons (PAHs). To protect our environment from the introduction of PTEs originating from biochar, the Ithaka Institute, through the European Biochar Certificate (EBC) proposed maximal thresholds for several elements. Not all biochars fulfil these requirements and can, therefore not be used as such. Consequently, the second aim of this study was to investigate the effect of co-pyrolysis of chicken manure with tree bark on biochar toxicity and its potential to stimulate plant growth (chapter 4). Accordingly, before pyrolysis, tree bark and chicken manure were mixed in different concentrations (0, 25, 50, 75 and 100 wt%). Results showed a decrease in the PTE concentrations when treebark was added to the chicken manure, creating EBC-compliant biochars whereas 100% manure biochar or tree bark biochar created non-compliant biochars. Therefore, it could be concluded that copyrolysis can help creating EBC-compliant biochars. Furthermore, the co-pyrolysis biochars in comparison with the reference showed a plant growth potential without increasing plant stress, whereas an increase in the stress parameters or reduced growth was found in the biochars from 100% tree bark or manure. These results relate to the P and Na/K concentrations in the growing medium after biochar addition. Expanding agricultural activities to new unexplored fertile soils would put pressure on our environment and is therefore not a preferred option. However, expanding our agricultural activities to soils that are currently unavailable due to contamination could be a good option to expand our agricultural activities without pressuring the environment. Biochar has been reported to improve plant growth not only under control conditions (plants grown on agricultural soil without metal pollution) but also under metal-induced stress conditions. It is reported to decrease metal-induced stress and improve plant growth when grown on metal-contaminated soils. Nevertheless, like under control conditions, both positive as well as neutral and negative effects of biochar on plants grown on metal-contaminated soil were found. Therefore, the third aim of this study was to investigate the impact of four different biochars on M. sativa growth and stress grown on a metal-polluted marginal soil (chapter 5). Results demonstrate that insect frass-derived biochar and manure-derived biochar have great potential to improve plant growth when grown on a metal-polluted marginal soil. M. sativa grown with insect frass-derived biochar or manure-derived biochar showed a decreased total antioxidant capacity without an increase in lipid peroxidation in comparison with the reference, indicating that the plants need less defence and have more energy for growth. Additionally, in comparison with the reference plants, an increase in the pigment concentration of these plants was found. Also, spent peat-derived biochar showed potential to improve plant growth in plants grown on a metal-polluted marginal soil. Still, compared to insect frass-derived biochar and manure-derived biochar, the positive effects were lower, whereas no effect of pine bark-derived biochar was found in this study. In addition to expanding agricultural practices to suboptimal soils, expanding our horticultural practices and more specific soilless cultivation could be an option. However, the use of peat in soilless cultivation is unsustainable. Therefore, the last aim of this study was to investigate the effect of peat replacement with local alternatives on strawberry yield, growth, and disease susceptibility. Furthermore, the impact of biochar addition to peat-reduced and peat-free substrates was investigated (chapter 6). Hence, a strawberry trial was performed in 2021 and 2022, and strawberry growth and health and the rhizosphere microbiome were determined. Results showed potential for peat replacement with local alternatives as no adverse effects of peat-free substrates were found on strawberry health and growth compared to peat-reduced substrates. Nevertheless, in 2022 an increased root rot severity was found in strawberry plants grown on peat-free substrates compared to plants grown on peat-reduced substrates. However, this decrease disappeared after biochar addition. Furthermore, in 2021 an increase in the aboveground fresh and dry weight of strawberry plants was found when biochar was added to the substrate. The rhizosphere microbial community did not show large differences when biochar was added to the soil but did differ between the peat-free and peat-reduced substrate. To conclude, this study showed great potential of biochar as a step towards sustainable agriculture since biochar can promote plant growth during control conditions and when grown on polluted soils. Our study identified that manure-derived biochar and insect frass-derived biochar could improve plant growth in control conditions and when grown on metal-polluted soil. Additionally, this study showed the potential of local alternatives for peat replacement in soilless strawberry cultivation. Moreover, part of peat replacement can be done with biochar. Despite these positive results, it is essential to note that matching the right biochar type and concentration with the proper application for plant growth promotion is important, as this study also showed that not all biochar types and all biochar concentrations do improve plant growth. To further confirm these results, long-term field experiments should be performed.