Field reality: short-scale soil heterogeneity impacts mycorrhizal contribution to maize P nutrition under contrasted fertilization

The relationship between root colonization by arbuscular mycorrhiza (AM) fungi and phosphorus (P) fertilization remains controversial and soil characteristics are still rarely considered. To assess the contribution of AM to maize P nutrition in contrasted field con-ditions, a range of hybrids were grown at two contrasting P levels of a long-term P-ferti-lizer trial in two adjacent soil types: alkaline and neutral. Root colonization was assessed by microscopic observation and qPCR targeting two common AM species, Funneliformis mosseae and Rhizophagus irregularis. Functionality of AM colonization was assessed by measuring the expression of two P-transporters by qPCR. Regardless of the soil type, AM root colonization was highest in the non-fertilized treat-ment compared to high-P fertilization. Looking closer, this drop was driven by the neutral soil and no variation was observed in the alkaline soil with P fertilization. The non-ferti-lized treatment was particularly harsh for maize development in the neutral soil, trigger-ing the highest expression of the plant P starvation inducible gene (ZmPT1:3), coupled to the lowest plant growth and P content. Under these conditions, the mycorrhizal P uptake pathway was particularly activated with the highest expression of the AM inducible P transporter (ZmPT1;6). On contrary, its expression remained constant among fertilization levels in the alkaline soil. The observed resilience to P fertilization in the alkaline soil could partially be due to high abundance of F. mosseae, which increased under P fertilization. Our results emphasize the importance of considering soil characteristics which differentially impacted AM sym-bioses and their implication in plant nutrition.