Alder encroachment alters subsoil organic carbon pool and chemical structure in a boreal peatland of Northeast China

Boreal peatlands have been experiencing increased abundances of symbiotic dinitrogen-fixing woody plants (mainly alder species). However, how alder encroachment alters soil organic carbon (C) pool and stability is unclear. To examine the effects of alder encroachment on soil organic C, we measured soil organic C pool, phenol oxidase (POX) activity, organic C mineralization rate, and organic C chemical structure (alkyl C, O-alkyl C, aromatic C, and carbonyl C) using solid-state 13C nuclear magnetic resonance spectroscopy in the 0-10 cm, 10-20 cm, and 20-40 cm depths in the Alnus sibirica islands and adjacent open peatlands in the north of Da'xingan Mountain, Northeast China. A. sibirica islands had 28 %, 25 %, and 30 % greater POX activity and 36 %, 31 %, and 100 % higher organic C mineralization than open peatlands in the 0-10 cm, 10-20 cm, and 20-40 cm soil depths, respectively. Despite no significant changes in the 0-10 cm and 10-20 cm depths, alder encroachment reduced soil organic C pool in the 20-40 cm depth. Soil organic C pool in the 0-40 cm depth was lower in A. sibirica islands (298 Mg ha-1) than in the open peatlands (315 Mg ha-1). Moreover, alder encroachment increased alkyl (7 %) and carbonyl (57 %) C fractions but reduced O-alkyl C fraction (16 %) in the 20-40 cm depth, resulting in increased aliphaticity and recalcitrance indices. These findings suggest that alder encroachment will reduce soil organic C accumulation by accelerating microbial decomposition, and highlight that increased biochemical stabilization would attenuate soil organic C loss after alder expansion in boreal peatlands. Our results will help assess and project future C budgets in boreal peatlands.