A large proportion of phosphate (P) fertilizer applied to Andosols reacts with reactive aluminum (Al) and iron (Fe) to become unavailable for plant uptake. We investigated whether biochar could enhance plant growth by (i) mobilizing soil P through changing soil pH or facilitating the growth of arbuscular mycorrhizal fungi (AMF), and/or (ii) introducing additional P.

We grew Lotus pedunculatus cv. barsille in two Andosols of contrasting P status amended with three biochars (with distinct porosity, nutrient and liming properties) at a dose of 10 t ha–1 for 32 weeks. The growth medium was divided into a root and a hyphal zone through a nylon mesh and a tephra layer that allowed the P in the hyphal zone to be transferred only by AMF hyphae.

The addition of a relative nutrient–rich biochar (e.g. made from willow woodchips) with liming properties to the root zone of the P–deficient soil increased plant growth by 59% and P uptake by 73%. Pine–based biochar provided no extra nutrient acquisition and no plant–growth stimulation when added to the root zone of the P–deficient soil. However,when hyphae of those plants had access to a P–rich soil patch, the presence of pine biochar in the soil patch greatly enhanced P uptake and plant growth (e.g., by 76% and 40% when using biochar produced at 450oC compared to the absence of it). None of the tested biochars conferred advantages in the root zone of a high–P soil.

We concluded that the benefits from biochar addition to nutrient uptake and plant growth are biochar– and soil–specific. Thus, biochars need to be tailored–made for certain soils by optimizing feedstock and pyrolysis conditions before application.

Keywords: Arbuscular mycorrhizal fungi, Andosols, Biochar, Lotus, Phosphorus bioavailability, Rhizosphere