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

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 (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 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.