Recycling of phosphorus (P) from waste streams in agriculture is essential to reduce the negative environmental
effects of surplus P and the unsustainable mining of geological P resources. Sewage sludge (SS) is an important P
source; however, several issues are associated with the handling and application of SS in agriculture. Thus, post-
treatments such as pyrolysis of SS into biochar (BC) could address some of these issues. Here we elucidate how
patches of SS in soil interact with the living roots of wheat and affect important P-related rhizosphere processes
compared to their BC counterparts. Wheat plants were grown in rhizoboxes with sandy loam soil, and 1 cm Ø
patches with either SS or BC placed 10 cm below the seed. A negative control (CK) was included. Planar optode
pH sensors were used to visualize spatiotemporal pH changes during 40 days of plant growth, diffusive gradients
in thin films (DGT) were applied to map labile P, and zymography was used to visualize the spatial distribution of
acid (ACP) and alkaline (ALP) phosphatase activity. In addition, bulk soil measurements of available P, pH, and
ACP activity were conducted. Finally, the relative abundance of bacterial P-cycling genes (phoD, phoX, phnK)
was determined in the patch area rhizosphere. Labile P was only observed in the area of the SS patches, and SS
further triggered root proliferation and increased the activity of ACP and ALP in interaction with the roots. In
contrast, BC seemed to be inert, had no visible effect on root growth, and even reduced ACP and ALP activity in
the patch area. Furthermore, there was a lower relative abundance of phoD and phnK genes in the BC rhizosphere compared to the CK. Hence, optimization of BC properties is needed to increase the short-term ef-
ficiency of BC from SS as a P fertilizer.