BACKGROUND Since the 1990s, drylands have been extensively converted to rice paddy fields on the former wetlands in the Sanjiang Plain of northeast China. However, the influence of this successiveland‐use change from native wetlands to drylands to rice paddy fields on soil organic carbon (C) dynamics remains unexplored. Here, we compared the difference in soil organic C stock among native wetlands, drylands, and paddy fields, and then used a two‐step acid hydrolysis approach to examine the effect of this land‐use change on labile C I (LPI‐C), labile C II (LPII‐C), and recalcitrant C (RP‐C) fractions at depths of 0–15 cm, 15–30 cm, and 30–50 cm. RESULTS Soil organic C stock at a depth of 0–50 cm was reduced by 79% after the conversion of wetlands to drylands but increased by 24% when drylands were converted to paddy fields. Compared with wetlands, paddy fields had 74% lower soil organic C stock at a depth of 0–50 cm. The conversion of wetlands to drylands reduced the concentrations of LPI‐C, LPII‐C, and RP‐C fractions at each soil depth. However, land‐use change from drylands to paddy fields only increased the concentrations of LPI‐C and LPII‐C fractions at the 0–15 cm and 30–50 cm depths. CONCLUSION The conversion of drylands to paddy lands on former wetlands enhances the soil organic C stock by promoting labile C fraction accumulation, and labile C fractions are more sensitive to this successive land‐use change than recalcitrant C fractions in the Sanjiang Plain of northeast China. © 2022 Society of Chemical Industry.

Aim: Boreal peatlands are very sensitive to nitrogen (N) enrichment due to the low soil N availability. Our aim was to reveal how increased N availability altered plant composition in boreal permafrost peatlands with dense vascular plants (>50%).Location: Greater Khingan Mountains, Northeast China.Methods: A nitrogen addition experiment with four N addition levels (0, 3, 6, and 12 g N m −2 year −1 ) was conducted in a poor fen in the continuous permafrost zone with vascular plant coverage above 60%. Plant diversity, community composition, and above-ground biomass were investigated after four, six, and eight years of N addition.Results: Nitrogen addition decreased species richness and diversity but enhanced total above-ground biomass. Plant height determined the plants' responses to N addition, and the magnitudes increased with N addition level and experimental duration. The relative coverage of high-stature (>50 cm) species increased, but the relative coverage of low-stature (<50 cm) species decreased with elevating N addition levels.When N addition proceeded, the relative coverage and above-ground biomass of tall deciduous shrubs and nitrophilous grasses increased, whereas the relative coverage and above-ground biomass of dwarf evergreen shrubs, mosses, and lichens declined.Regardless of N addition levels, cryptogams (i.e., mosses and lichens) and a dwarf evergreen shrub (Chamaedaphne calyculata) disappeared after eight-year N addition.Conclusions: Cryptogams and dwarf evergreen shrubs are highly vulnerable to increased N availability, and N enrichment-induced biodiversity loss, especially the disappearance of cryptogams, will deteriorate ecosystem structure and function in boreal permafrost peatlands.