Mangrove afforestation is considered an important measure in the “natural-based solution” for mitigating climate warming through sequestering massive carbon (C) into vegetation biomass, yet how the planted mangrove species facilitate soil C sequestration remains unclear. Here, we investigated the stock, source, and fraction of soil organic carbon (SOC) over 1 m depth after 20-year afforestation of five mangrove pure plantations (Acrostichum aureum, Acanthus ilicifolius, Aegiceras corniculatum, Kandelia obovate, and Excoecaria agallocha) on Qi’ao Island, South China. The results showed that SOC stocks did not significantly differ among the five plantations, with an average value of 16.7 kg C m⁻². Based on the two-end-member mixing model with plant–soil C stable isotope signatures, the autochthonous (mangrove-derived) C source accounted for 20.2–34.1% of SOC but varied significantly among the plantations. The SOC stock in particulate fraction (1.2–2.0 g C kg⁻¹) and mineral-associated fraction (14.3–16.0 g C kg⁻¹) also significantly differed among the plantations. The similar SOC stock but different source contributions and C fractions among the plantations observed here may have important implications for mangrove afforestation to optimize stand structure and maximize C sequestration.

Due to increasing anthropogenic pressure, over half of the world’s tropical forests are reforested or afforested secondary forests or plantations. The recovery pace and potential of these forests depend largely on soil microbially-mediated biogeochemical cycling. Here we measured soil extracellular enzyme activities and quantified microbial metabolic limitations using a vector analysis in a bare land (BL, representing the original state before restoration), two afforested sites [i.e. a restored secondary forest (MF) and a managed Eucalyptus exserta plantation (EP)] and a nearby undisturbed forest (UF) in south China. Results showed that soil microbial metabolisms were co-limited by carbon (C) and phosphorus (P) across the four forests. Both microbial C and P limitations were higher in BL than UF. Microbial C limitation significantly reduced after restoration only in MF when compared to BL, but it was still higher than that in UF. Interestingly, microbial P limitation significantly enhanced after restoration in both EP and MF when compared to BL, and it did not differ between the two restored forests. Structural equation modeling (SEM) showed that microbial C limitation was primarily attributed to microbial C use efficiency, while microbial P limitation was co-driven by plant biomass, microbial C use efficiency and soil P availability. These findings suggest microbial C limitation could be gradually recovered after forest restoration in southern China, which would facilitate soil organic carbon accumulation. However, the enhanced microbial P limitation after forest restoration underlines the necessity to develop optimal P management in these restored forests.

Increasing nitrogen (N) deposition has been suggested to favour arbuscular mycorrhizal (AM) plants over ectomycorrhizal (ECM) plants, but how mycorrhizal association mediates N impacts on soil organic carbon (SOC) remains unclear. By synthesizing 315 N addition studies globally, we investigated the effects of N addition on SOC content in ecosystems dominated by ECM and AM plants (ECM and AM ecosystems). Our results showed that SOC increased by 4.1% in ECM ecosystems, likely due to reduced ligninase activity. In AM ecosystems, SOC did not change during the first 5 years of N addition, but increased by 7.3% over long‐term N addition (≥5 years), driven by increased litter inputs and progressive soil acidification. These findings highlight that N addition enhances SOC in both ECM and AM ecosystems via distinct mechanisms and temporal dynamics, implying that the potential of soil C sequestration may differ between the two ecosystems from short‐ to long‐term N addition. Read the free Plain Language Summary for this article on the Journal blog.