Response of soil bacterial communities to nitrogen and phosphorus additions in an age-sequence of subtropical forests

With global nitrogen (N) deposition continuously increasing, few reports exist describing how soil bacteria respond at the molecular level to long-term addition of excess N in variously aged forests. To reveal the responses of soil bacteria to the elevated N, an experiment was conducted with a chronic nutrient addition within differently aged stands (46-, 78-, and 200-years-old) in the northern subtropical China since 2011, including three treatments, CK (no N nor phosphorus (P) additions), N treatment (N, 100 kg N ha-1), and N with P (N+P, 100 kg N ha-1 + 50 kg P ha-1) to examine potential P limitation under N deposition. Metagenomic sequencing was used to examine the snapshot responses of soil bacterial communities. Soil moisture and texture, ammonium, nitrate, SOC (soil organic carbon), TN (soil total nitrogen), TP (total phosphorus), DOC (dissolved organic carbon), DON (dissolved organic nitrogen) were measured to explain the influence mechanism of forest age and fertilization on changes of microbial community. Following N addition, soil bacterial community diversity and most dominant phyla increased, but they showed a decrease with increasing stand age. The effects of fertilization on the same taxa were variable across forest ages. Soil bacterial community responded differently in 7-year fertilization, with distinct shift in 46-year-old forest and adaptability to long-term N addition in the 200-year-old forest. Soil texture and moisture, DOC, DON, pH, SOC/TN and TP were significantly correlated with bacterial community across stand ages, while N fertilization affected the bacterial community mostly via inducing soil moisture, NO3--N, DOC and pH in the 46-year-old forest, whose effects decreased with increasing stand age. Our results suggest that due to the variation of soil physicochemical properties among forest ages, soil bacterial communities are more stable and resilient to N deposition with increasing the age of stands. Soil bacterial communities might not encounter P limitation following the long-term addition of N in the subtropics.