Alder expansion increases soil microbial necromass carbon in a permafrost peatland of Northeast China

Northern peatlands have undergone widespread expansion of nitrogen (N)-fixing woody plants such as alder species. However, how alder expansion alters soil microbial necromass carbon (C) is unclear in these ecosystems. We compared the differences in soil stoichiometric ratios among C, nitrogen (N), and phosphorus (P), microbial biomass C (MBC) and N (MBN), basal respiration rate, net N mineralization rate, glucosamine, muramic acid, and galactosamine at 0–10 cm, 10–20 cm, and 20–40 cm layers between Alnus sibirica patches (alder patches) and open peatlands in a permafrost peatland in the Heilongjiang Province of China, and assessed the impact of alder expansion on soil bacterial, fungal, and total necromass C in this peatland. Compared with open peatlands, alder patches have lower C:N ratio at each soil layer but have higher C:P ratio at 10–20 cm layer. Across soil layers, alder patches had higher soil MBC, MBN, basal respiration rate, and net N mineralization rate, but lower soil MBC:MBN ratio than open peatlands. Moreover, alder expansion enhanced the concentrations of glucosamine and muramic acid at 0–10 cm layer, but only increased muramic acid concentration at 20–40 cm layer. Accordingly, soil bacterial, fungal, and total necromass C were increased by alder expansion at 0–10 cm layer, whereas only soil bacterial necromass C was enhanced at 20–40 cm layer. In addition, bacterial necromass C correlated negatively with both soil C:N and MBC:MBN ratios. These results suggest that alder expansion promotes microbial necromass C accumulation in surface soils, and highlight that altered C:N stoichiometry drives alder expansion-induced shifts in soil bacterial necromass C in northern peatlands.