A Combined Strategy Using Funneliformis mosseae and Phosphorus Addition for Enhancing Oat Drought Tolerance

Arbuscular mycorrhizal fungi (AMF) play a crucial role in the soil–plant interface, yet the combined effects of AMF inoculation and phosphorus (P) addition on soil–plant nitrogen (N) and P, as well as oat grain yield, under drought stress remain unclear. Experiments were conducted during the 2021 and 2022 oat-growing seasons, applying AMF (40 g inoculum per pot; sterilized inoculum as the NAMF control) and P (0, 20, and 40 mg kg−1 soil, designated P0, P1, and P2) under 75% and 55% relative water content. This study found that AMF inoculation at the P1 level significantly improved the AMF colonization rate, grain yield, and partial factor productivity of P (PFPP) of oat. The grain yield increased by 6.2% (2021) and 9.8% (2022) under drought stress compared to the AMF-free treatment. AMF inoculation and P addition showed interactive effects on soil–plant N and P dynamics, which significantly increased microbial biomass phosphorus (MBP), nitrate N, and the available P content in oat soil. P1AMF significantly increased the total N and P contents under drought stress compared to P1NAMF, with maximum increments of 40.7% (N) and 11.1% (P) in 2021 and 15.4% (N) and 32.3% (P) in 2022. Moreover, the P1AMF treatment significantly improved P recovery efficiency (PRE), achieving a maximum increase of 48.4% across the two-year study. The analysis revealed that soil MBP was the key factor influencing oat grain yield, as well as the total N and P content in oat plants. It was concluded that AMF inoculation with a moderate amount of P addition could effectively regulate soil N and P availability and enhance plant N and P contents, as well as P productivity and use efficiency, thereby improving oat drought tolerance. Soil MBP acted as a vital bridge in the oat soil–plant continuum.