Arbuscular mycorrhizal mycelia ameliorate aggravated phosphorus limitation caused by nitrogen deposition in a subtropical karst forest

Increasing nitrogen (N) deposition tends to aggravate phosphorus (P) limitation in subtropical forest ecosystems. Arbuscular mycorrhizal (AM) fungi are believed to improve the plant P supply under P-depleted soil conditions. However, how the AM fungi and their extraradical mycelia impact soil P transformation and subsequent P availability under N-induced P limitation is not fully understood. Using an ingrowth core design, we quantified the effects of AM mycelia on different soil P pools and potential drivers controlling the transformation and availability of soil P in a subtropical forest receiving N fertilization. N addition had greater positive and negative mycelial effects on the soil labile P pools and moderately labile P pools, respectively. This finding indicated that AM mycelia increased the availability of soil P under N deposition by promoting transformation from moderately labile P to labile P. Additionally, we observed diverse mycelial effects under N addition on multiple microbial (P transformation genes and phosphatase activities) and physiochemical drivers (Al/Fe oxyhydroxides and soil pH) involved in driving soil P transformation. These results suggest that AM mycelia can improve soil P availability to counteract increased P limitation due to N deposition by controlling microbial and physiochemical processes that co-regulate soil P transformation. The positive feedback effects of mycorrhizal fungi on soil P transformation and availability as well as the drivers controlling these effects should be incorporated into ecosystem biogeochemical models. This is crucial for accurately predicting forest productivity and function under future N deposition scenarios.