Abstract: Soil phosphorus (P) fractions and transformation during the non-growing season directly affect soil P supply in the following spring in forest plantations. The species and diversity of understory vegetation can affect soil P fraction and transformation in non-growing season by altering the microenvironment and the composition and quality of litter. In order to clarify the effects of the species identity and diversity of understory vegetation on soil P fraction and availability, four understory vegetation treatments, including understory vegetation removal (UR), retention of natural diverse understory vegetation (RD), retention of single main understory vegetation species (RS), and planting single N-fixing plant species (PN), were employed in polar (Populus deltoides ‘Nanlin3804’) plantation in August 2017. Soil samples from surface layer (0-5 cm) were collected in January 2019 for P fractionation analysis. In addition, soil P transformation process was studied using in situ buried bag culture method during the nongrowing season. The results showed that soil total labile P content was significantly higher under RD treatment than under UR and RS treatment, indicating obvious improvement of soil P availability in non-growing season with the retention of understory vegetation. Soil intermediate labile P content was significantly lower, while the net production rates of soil labile P and labile organic P were significantly higher under PN treatment than under RS treatment. Soil stable P content was the highest under UN treatment, indicating a decline in P availability when understory vegetation was removed. For all the treatments, soil inorganic P occupied relatively higher proportion (43.4%-48.1%) of soil total P compared with soil organic P, which contributed to 26.9%-34.5% of soil total P. The total inorganic P content was significantly higher under PN treatment than under RS treatment. In conclusion, understory vegetation in poplar plantations significantly reduced soil stable P fractions and improved P availability in nongrowing season. Furthermore, soil labile P significantly increased with increasing species diversity of understory vegetation. In addition, planting N-fixing plants could effectively enhance soil P availability by promoting P transformation and net production of labile P.

Key words: poplar plantation, understory vegetation diversity, soil phosphorus fractionation, phosphorus availability, labile phosphorus.