Scientists have revealed that tree species diversity affects microbial necromass before changes become detectable in total soil carbon and nitrogen during temperate afforestation. The findings suggest that fungal necromass may serve as a sensitive indicator of early belowground carbon changes.

Afforestation is an important strategy for restoring degraded ecosystems and strengthening terrestrial carbon sinks. However, in temperate regions, it remains unclear how soil carbon and nitrogen pools change during the early stages of afforestation and when tree species diversity and mycorrhizal types begin to influence belowground carbon processes, partly because long-term observations covering the entire soil profile are scarce. Microbial necromass is a major component of soil organic matter and may respond to vegetation restoration earlier than total soil carbon and nitrogen.

A research team from the Restoration Ecology Research Group at the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences, in collaboration with the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig and Leipzig University, conducted a nearly 10-year study across the 0–100 cm soil profile at the MyDiv tree diversity experiment in Germany.

The researchers examined temporal and vertical changes in soil carbon, nitrogen, and microbial necromass across tree communities that differed in species richness and mycorrhizal composition. The findings were recently published in the international journal Plant and Soil.

The results revealed asynchronous and vertically stratified changes in soil carbon, nitrogen, and microbial necromass during early temperate afforestation.

Over the first decade, concentrations of total carbon, total nitrogen, and microbial necromass across the 1 m soil profile decreased by approximately 3%, 17%, and 14%, respectively. During the first five years, total nitrogen declined primarily in the 0–40 cm soil layer, whereas total carbon and microbial necromass remained relatively stable. Thereafter, total carbon decreased mainly at 5–40 cm depth, while microbial necromass declined throughout the soil profile.

According to the researchers, these findings show that soil carbon, nitrogen, and microbial necromass do not change synchronously during early afforestation, but instead follow distinct temporal trajectories and undergo pronounced vertical reorganisation.

Importantly, effects of tree species diversity emerged in microbial necromass before they became detectable in total soil carbon and nitrogen. Over the 10-year period, tree species richness and mycorrhizal composition had not yet significantly altered total soil carbon or nitrogen concentrations, but they had already affected microbial necromass, particularly fungal necromass.

In mixed communities containing both arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species, microbial and fungal necromass in the topsoil were 21–27% higher in four-species plots than in two-species plots. Approximately 10 years after planting, fungal necromass was also about 24% higher in four-species than in single-species ECM communities.

These findings suggest that biodiversity effects on soil organic matter formation may develop gradually, with microbial processes responding before measurable changes become evident in total soil carbon pools. Fungal necromass may therefore provide a sensitive early indicator of belowground responses to tree species diversity.

Implications for Soil Carbon Monitoring and Forest Restoration

According to the researchers, the study enhances understanding of how biodiversity affects belowground carbon dynamics during early temperate afforestation and provides new insights into plantation restoration and soil carbon monitoring.

The findings also suggest that short-term observations focused only on topsoil, or assessments based solely on total soil carbon, may overlook early effects of tree species diversity belowground. The researchers noted that incorporating deep-soil sampling, mycorrhizal functional types, and microbial necromass into long-term monitoring programmes could provide a more comprehensive assessment of soil carbon dynamics during forest restoration.

The results further indicate that increasing tree species diversity and combining tree species with contrasting mycorrhizal associations can influence microbial pathways involved in soil organic matter formation, potentially contributing to long-term soil carbon sequestration.

However, the researchers added that because the study was conducted at a single temperate experimental site, further long-term and cross-site studies are needed to determine whether these patterns apply more broadly across temperate, subtropical, and tropical forests.

LI Tengteng, a postdoctoral researcher at the SCBG, is the first author of the study. Prof LIU Zhanfeng and Prof Nico Eisenhauer are co-corresponding authors. The research was supported by the National Natural Science Foundation of China and the Guangdong Provincial Basic and Applied Basic Research Programme.