Nitrogen and phosphorus availability affect wheat carbon allocation pathways: rhizodeposition and mycorrhizal symbiosis

Plants allocate their photosynthetic carbon (C) belowground through rhizodeposition, which can be incorporated into microbial biomass and organic matter, but can also be directly shared with arbuscular mycorrhizal fungi (AMF). In this study, we investigated how both rhizodeposition and AMF colonisation are affected by nitrogen (N) and phosphorus (P) availability in soil systems, and in turn, how these C allocation pathways influenced plant P uptake in four different wheat genotypes with variable root traits. Wheat genotypes (249, Suntop, Scout and IAW2013) were grown in pots and labelled continuously during their growth period with (CO2)-C-13 to determine rhizodeposition. We applied two levels of N (25 and 100 kg ha(-1)) and P (10 and 40 kg ha(-1)) fertiliser. Plant root traits, plant P content, soil available P and N, microbial biomass C and P, and AMF colonisation were examined. We constructed a structural equation model to show how C allocation to rhizodeposition and AMF colonisation depended on P and N availability, and how these pathways affected plant P uptake and grain yield. Wheat genotypes with fine roots (Suntop, Scout and IAW2013) were associated with AMF colonisation for plant P uptake, and the genotype with the largest root biomass (249) provided more C to rhizodeposition. Both rhizodeposition and AMF colonisation increased plant P and grain yield under low P and high N availability respectively, while root biomass and root traits, such as specific root length and proportion of fine roots, determined which C allocation pathway was employed by the plant.