Tree Planting Boosts Soil Life and Carbon Storage, Global Study Finds

Planting trees has become one of the most widely promoted tools in the fight against climate change, but a persistent question has shadowed the world’s multibillion-dollar restoration ambitions: what actually happens beneath the surface of a new forest? A major global analysis published in Nature Ecology & Evolution now offers the most comprehensive answer yet, showing that both afforestation, the planting of trees on land that has never been forested, and reforestation, the re-establishment of forest on previously cleared land, can deliver substantial gains in two of the planet’s most important hidden assets: soil carbon and the living microbial biomass that underpins it. Drawing on 1,158 paired sites spanning six continents and stand ages of up to a century, the study reveals that the fate of carbon in newly planted forests is written largely underground, in the bodies and activities of soil microorganisms.

The research team, led by Zexin Meng and Yiping Wu of Xi’an Jiaotong University and Central South University of Forestry and Technology, together with Manuel Delgado-Baquerizo of the Spanish National Research Council and an international consortium of co-authors, compiled paired comparisons in which each planted or regenerating site was matched against a nearby reference ecosystem with comparable soil, climate and land-use history. This paired design is critical because soil carbon is notoriously variable across landscapes, and differences between adjacent plots can easily be mistaken for the effects of tree planting itself. By comparing each planted site with its own reference, the authors could isolate the signal of forest establishment from the background noise of geography, geology and land management.

The headline finding is that the two dominant restoration pathways behave very differently over time. Afforestation, the conversion of non-forest land such as grassland, cropland or degraded terrain into new forest, showed the potential to support greater soil microbial biomass and larger topsoil carbon storage across the full span of stand development, with benefits that persisted or continued to build for as long as a century. Reforestation, by contrast, told a more complicated story. On land where forest had been lost and was being re-established, both soil carbon storage and microbial biomass recovered substantially, approaching the levels of the original forest within roughly the first three decades. But in older reforested stands, those gains were not sustained, suggesting that the early carbon rebound of second-growth forests can stall or reverse as stands mature.

That divergence matters because soil is not merely a passive vault for carbon. The organic carbon held in topsoil is intimately bound to the soil microbiome, the vast community of bacteria, fungi and other microorganisms that decompose plant litter, transform nutrients and, when they die, contribute their own carbon-rich remains to the soil matrix. Microbial biomass is simultaneously a living reservoir of carbon and an engine of carbon processing, and decades of research have shown that the two quantities tend to rise and fall together. The new synthesis confirms this coupling on a planetary scale: across prior land uses, climatic zones and planted tree species, increases in soil carbon storage generally went hand in hand with increases in microbial biomass, whether the mechanism at work was afforestation or reforestation.

The authors found that the strength of these coupled gains was not uniform across the globe. The most pronounced benefits emerged on degraded lands, where starting conditions were poor and the arrival of trees represented a dramatic improvement in the quantity and quality of organic inputs entering the soil. Fallen leaves, fine roots and root exudates from growing trees feed microbial communities, and as those communities expand and turn over, they stabilise carbon in the soil in forms that can persist for years to decades. On already fertile or carbon-rich soils, by contrast, the marginal gains from planting trees were smaller, a pattern with direct implications for where restoration dollars are best spent.

To move beyond the individual field sites and ask where these effects might play out worldwide, the team built predictive models of long-term soil carbon and microbial biomass dynamics, incorporating environmental layers such as temperature and precipitation from the WorldClim database, aridity indices, elevation data, soil texture, pH and initial carbon content from SoilGrids, and land-use information from satellite-derived global forest maps. The resulting global mapping of coupled soil carbon and microbial responses provides a spatial blueprint that could help governments and restoration practitioners identify the landscapes where tree planting is most likely to lock carbon into the ground while simultaneously rebuilding the biological engine of soil fertility.

The study arrives at a moment of intense scrutiny for nature-based climate solutions. Global pledges such as the Bonn Challenge and the United Nations Decade on Ecosystem Restoration have committed vast areas to forest restoration, and earlier work has estimated enormous theoretical potential for tree planting to sequester carbon. Yet a series of recent studies has warned that the reality is messier: tree planting in the wrong places can harm biodiversity, deplete water resources, or even reduce rather than increase carbon storage, particularly at northern high latitudes where darkening the land surface can offset the carbon absorbed by trees. By focusing specifically on the soil compartment and its microbial inhabitants, the new analysis adds a dimension that above-ground carbon accounting has largely ignored, and it does so with an unusually broad empirical foundation.

The technical rigour of the synthesis is notable. The authors applied meta-analytic frameworks to quantify effect sizes across the paired sites, used piecewise regression to detect ecological thresholds in the trajectories of carbon and microbial biomass over stand age, and employed random forest machine-learning models to identify the environmental variables that best explain variation in the observed responses. Spatial prediction was handled with explicit attention to model validity, including nearest-neighbour distance-matching cross-validation, a technique designed to guard against the over-optimistic map accuracy that has plagued many large-scale ecological models. The underlying dataset has been deposited in a public figshare repository, allowing other researchers to interrogate, extend or challenge the findings.

For policymakers, the practical message is twofold. First, afforestation on degraded, non-forest land appears to offer the most durable coupled benefits for soil carbon and soil life, with gains that can accumulate over a century of stand development. Second, reforestation delivers a rapid early recovery of below-ground carbon and microbial biomass, but that recovery may not persist in older stands, meaning that second-growth forests should not be assumed to be carbon-equivalent to the primary forests they replace. The authors argue that their results provide critical evidence for refining nature-based climate solutions: if tree-planting programmes are designed with soil processes in mind, prioritising degraded landscapes and managing reforested stands for long-term soil carbon retention, they can simultaneously sequester carbon and restore the biological foundation of soil health. In an era when every tonne of carbon counts, the study suggests that the most reliable climate allies of the world’s new forests may be the invisible organisms working in the dark beneath them.

Subject of Research: Long-term global responses of soil carbon storage and soil microbial biomass to afforestation and reforestation

Article Title: Afforestation and reforestation support coupled gains in soil life and carbon storage worldwide

Article References: Meng, Z., Wu, Y., Eisenhauer, N., Cui, Y., Abalos, D., Li, H., Zhen, H., Wang, P., Zhou, G., Zhang, F., Liu, S., Chen, J., Zhou, G., Wang, Y.-P., Xu, J., Qiu, L., Zhao, F., Sáez-Sandino, T., & Delgado-Baquerizo, M. (2026). Afforestation and reforestation support coupled gains in soil life and carbon storage worldwide. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03183-2

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03183-2

Keywords: afforestation, reforestation, soil carbon, soil microbial biomass, nature-based climate solutions, carbon sequestration, soil microbiome, forest restoration, global synthesis, degraded lands, climate change mitigation, ecosystem restoration