Planting more than just trees may hold the key to restoring degraded tropical lands. A new study has uncovered how mixed-species afforestation works: it enhances soil multifunctionality by reshaping microbial communities and increasing the molecular diversity of soil organic carbon (SOC).

Led by the restoration ecology team at Heshan Station of the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS), the study was published in Journal of Applied Ecology on July 19.

Afforestation is a key strategy for restoring degraded ecosystems and mitigating climate change. Previous assessments have focused on vegetation recovery or individual soil functions, leaving a gap in understanding how different afforestation models jointly influence nutrient supply, enzyme activity, and microbial metabolism—collectively termed soil multifunctionality.

However, the molecular composition and diversity of SOC, which may shape microbial processes and nutrient cycling, have been underexplored.

To address this question, the team conducted the research at the Xiaoliang Tropical Coastal Ecosystem Research Station of SCBG, using a restoration sequence comprising bare land, a Eucalyptus monoculture, a secondary mixed-species forest, and an old-growth natural forest.

They measured 13 soil functional indicators associated with soil nutrient content, microbial biomass, carbon use efficiency, and hydrolytic enzyme activities, and analyzed the relationships among plant communities, soil microbes, SOC molecular composition, and soil multifunctionality.

The researchers found that afforestation significantly improved soil nutrient supply and enzyme activity, even though microbial carbon use efficiency declined.

Overall, afforestation markedly enhanced soil multifunctionality, with the secondary mixed-species forest outperforming the Eucalyptus monoculture. Mixed planting promoted microbial community reassembly and functional potential: microbial biomass and diversity increased, and functional genes related to recalcitrant carbon degradation, nitrogen fixation, ammonification, denitrification, and organic phosphorus hydrolysis were notably enriched. The microbial functional gene network also expanded in scale and connectivity, reflecting improved functional coordination.

A key finding of the study was that SOC molecular diversity serves as a critical "biochemical bridge" linking microbial attributes to soil multifunctionality.

Following afforestation, nitrogen-containing compounds, lignin-derived molecules, phenolics, and carbohydrates increased, with SOC molecular diversity significantly higher than in the bare land. Path analysis confirmed that microbial community and functional attributes influence soil multifunctionality through SOC molecular diversification, supporting the synergistic enhancement of multiple soil processes.

The study suggests that ecological restoration of tropical degraded lands should go beyond counting trees planted or carbon stocks gained. It should prioritize mixed-species configurations, belowground microbial functional organization, and the optimization of SOC molecular diversity.

For future plantation and restoration practices, the researchers suggest prioritizing multi-species mixed planting and incorporating SOC molecular diversity into restoration evaluation systems to maximize long-term ecological benefits.

Mixed-species afforestation consistently enhances soil multifunctionality through microbial reassembly and SOC molecular diversification. (Image by LI Yue)