Contrasting responses of soil phosphorus pool and bioavailability to alder expansion in a boreal peatland, Northeast China

Dinitrogen (N2)-fixing woody plants, mainly alder (Alnus) species, have widely expanded to boreal peatlands. However, little is known about the effect of N2-fixing plant expansion on soil phosphorus (P) dynamics and its potential mechanisms in these ecosystems. To clarify the response of soil P status to N2-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 (CaCl2, 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 CaCl2 and enzyme extractable P fractions, despite unchanged P sorption index and citrate extractable P fraction. Both CaCl2 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 N2-fixing tree expansion, and suggest that N2-fixing tree expansion would reduce soil P pool via enhanced P bioavailability and subsequently increased plant P uptake in boreal peatlands.