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• Alder expansion reduced soil P pool in the 0–40 cm depth. • Total P concentration increased in surface soils but reduced in deep soils after alder expansion. • Alder expansion increased plant-available P fractions by elevating acid phosphatase activity. • Alder expansion caused about ninefold increase in aboveground plant biomass. Dinitrogen (N 2)-fixing woody plants, mainly alder (Alnus) species, have widely expanded to boreal peatlands. However, little is known about the effect of N 2 -fixing plant expansion on soil phosphorus (P) dynamics and its potential mechanisms in these ecosystems. To clarify the response of soil P status to N 2 -fixing plant expansion, we compared the differences in aboveground plant biomass, soil total P pool, acid phosphatase activity, P sorption index, and four bioavailable P fractions (CaCl 2 , citrate, phosphatase enzyme, and HCl extractable P fractions) in the 0–10 cm, 10–20 cm, and 20–40 cm depths between A. sibirica islands and adjacent open peatlands in a boreal peatland, Northeast China. While alder expansion hardly affected bulk density and associated volume of the peats, it strongly decreased total P pool in the 0–40 cm soil depth because of approximately ninefold increases in aboveground plant biomass and associated P transfer from soils to plants. Soil total P concentration increased in the 0–10 cm depth, but declined in the 10–20 cm and 20–40 cm depths. At each depth, alder expansion increased soil acid phosphatase activity, and CaCl 2 and enzyme extractable P fractions, despite unchanged P sorption index and citrate extractable P fraction. Both CaCl 2 and enzyme extractable P fractions correlated positively with acid phosphatase activity, indicating that increased P bioavailability was primarily caused by elevated microbial mineralization of organic P. These findings highlight the contrasting responses of soil P pool and bioavailability to N 2 -fixing tree expansion, and suggest that N 2 -fixing tree expansion would reduce soil P pool via enhanced P bioavailability and subsequently increased plant P uptake in boreal peatlands. [ABSTRACT FROM AUTHOR]