Biochar Effects On Plant-Microbe Interactions: Mycorrhizae and Rhizobia.

Biochar has in several experiments shown to change the abundance and composition of the microbial community in soils. However, the functional relationship between the diverse properties of biochars and any observed changes are still obscure. Enhancing symbiotic microorganisms such as mycorrhizae and rhizobia would aid in resource use and potentially greater crop yields or lower fertilizer needs. In a series of experiments, we investigated the effect of biochars with different nutrient and surface properties on the association of mycorrhizae and rhizobia with a bean crop. Mycorrhizal colonization of roots was greater in phosphorus-constrained iron-oxide rich soil when phosphorus-depleted biochar from woody material was added. If phosphorus was added uniformly to soil, mycorrhizal colonization did not significantly increase, while colonization decreased when soluble P was coated onto the biochar before soil addition. Rhizobia infection increased several fold with additions of biochars that were either depleted of nutrients, oxidized or from which labile organic materials were extracted before they were added to an oxisol. The greatest increases were observed after leaching biochars with acetone which may either suggest a negative effect of labile organics (often called volatiles) or a positive effect of adsorption of soil-borne organic matter. The results point out challenges in distinguishing effects of nutrient additions contained in the biochar and pH changes from other and more lasting effects of biochar on plant-microbe interactions.