The diversity of non-rhizobial endophytes and abundance of AMF in response to P fertilizers in Pisum sativum

Introduction
Non-rhizobial endophytes (NRE) enhance plant growth via functions like nutrient mobilization and pathogen suppression. NRE are strongly influenced by several abiotic and biotic factors. In legumes, for example, phosphorus (P) availability and concurrent arbuscular mycorrhizal fungi (AMF) root colonization may affect NRE diversity and nodulation. Recently, NRE have been detected in the root interior and as an integral part of root nodules.
Goals
Here, we studied how different P fertilizers and AMF colonization rates affect NRE communities in nodules. It was hypothesized that increased P availability would increase the number of nodules for stochiometric reasons while reducing the abundance of AMF and the diversity of NRE in the nodules.
Materials & Methods
In a field trial, four fertilizer treatments with increasing solubility were tested, namely no P fertilizer (P0), bone char (BC) and bone char+ (BC+) as alternative fertilizer and triplesuperphosphate (TSP) (n=3). Bulk soil, rhizosphere soil, surface sterilized nodules and roots of Pisum sativum were sampled, followed by DNA extraction, quantitative PCR (16S rRNA, AMF, nifH, gcd), and amplicon sequencing for 16S rRNA.
Results
Contrary to our expectations, TSP resulted in the highest NRE diversity, number of nodules and abundance of AMF. Further, the overlap between nodules, roots and rhizosphere soil was most pronounced in this treatment, indicating a less strict filtering effect. In contrast, the BC+ treatment had the highest number of vital nodules, abundance of Rhizobiaceae, levels of nifH and gcd. The pronounced overlap between the roots and nodules suggested a significant influence of root endophytes on NRE. The BC and P0 treatments showed the lowest diversity and minimal overlap between compartments but had increased levels of AMF.
Summary
This study provides insights into the impacts of different P availability on symbiosis and NRE. While TSP shows a tripartite symbiosis, the recruitment of NREs is less targeted. BC+ enhances nodulation with focused NRE recruitment, possibly linked to P turnover. Conversely, BC and P0 promote symbiosis with AMF and show fewer nodules to conserve energy.