Biochar-root interactions are mediated by biochar nutrient content and impacts on soil nutrient availability

Roots are the first point of contact between biochar particles and growing plants, yet detailed studies of biochar-root interactions are few. Biochar may affect root growth, and therefore plant performance, through two mechanisms: (i) as a direct nutrient source and (ii) through impacts on nutrient availability. To test the hypothesis that biochar-root interactions occur and are determined by biochar nutrient supply and impacts on soil nutrients, spring barley (Hordeum vulgareL.) was grown with and without biochar addition in rhizobox mesocosms. Biochar from unaltered and artificially weathered Miscanthus or willow (Salix sp) biochar types was used and was manipulated to alter its structure and nutrient content. After 28days of plant growth, biochar nutrient content, soil nutrient content and the amount of biochar were measured in the bulk soil, the rhizosphere and the rhizosheath. Plants in biochar-amended soils had larger rhizosphere zones than the control treatment. The rhizosphere contained more biochar particles than the bulk soil, an indication that roots preferred soil containing biochar particles. Biochar particles retained soil nitrogen (N) in the form of nitrate, and also supplied phosphorus (P) to the soil and plant. Miscanthus biochar had a larger extractable P content than the Salix biochar, with different effects on plant growth and root responses. Although artificial physical weathering had no effect on overall plant growth, weathering effects on N retention and P content were dependent on biochar type. Our results indicate that roots are attracted towards biochar, resulting in its partitioning between bulk and rhizosphere soil. Biochar thus controls plant root nutrient acquisition directly as a nutrient source and indirectly by altering soil nutrient content.