Planting Trees Builds up Soil Carbon and Soil Life at the Same Time

Soil carbon and the mass of microbes living in soil generally rise together where forests are planted, but how long the gains keep building depends on what the land was before, Zexin Meng of Xi'an Jiaotong University and colleagues report, based on a compilation of 1,158 paired planted and reference sites on six continents.

Planting forests is widely promoted as a nature-based climate solution, the authors write. Whether different planting strategies follow similar paths in soil carbon, and what that does to soil life, has been poorly understood. Their study, published Sept. 18 in Nature Ecology & Evolution, sets the two main strategies side by side.

The two are not the same thing. Afforestation puts trees on land that was not forest. Across those sites, the paper reports, afforestation has the potential to support more soil microbial mass and more topsoil carbon as stands age over spans of up to a century.

Reforestation replants forest that has been lost. There, soil carbon and microbial mass recovered substantially, approaching the levels of the original forest during the first three decades. Beyond that, the paper reports, the gains were not sustained in older stands.

Carbon and microbial mass moved together in both cases, across previous land uses, climate zones and the tree species planted, with greater gains on degraded land.

The authors conclude that afforestation and reforestation could each add to soil carbon storage and support soil microbial growth under global change, and they offer the comparison as a way to refine nature-based planting programs. The figures come from measurements assembled at sites other researchers had already studied, not from new fieldwork; the compiled dataset is deposited in the public repository figshare.