Fungal Dead Matter Responds First as Young Forests Rewire Their Soils

When scientists plant a forest, they usually watch the trees. Height, trunk diameter, canopy cover — these are the visible currencies of restoration success. But a decade-long experiment in Germany suggests that the most sensitive early warning signals of a young forest’s development lie not in the trees above ground, nor even in the bulk carbon and nitrogen stored in the soil, but in something far less glamorous: the dead bodies of fungi. A new study published in the journal Plant and Soil reports that tree diversity and the type of mycorrhizal partnership a tree forms produce measurable shifts in fungal necromass — the residual cell-wall material left behind when fungi die — long before any comparable change appears in total soil carbon or nitrogen pools.

The research, led by Tengteng Li of the South China Botanical Garden and the German Centre for Integrative Biodiversity Research (iDiv), together with Olga Ferlian, Panpan Wu, Zhanfeng Liu, and senior author Nico Eisenhauer, took advantage of a unique experimental site known as MyDiv. There, trees were planted in plots containing either one, two, or four species, and the researchers deliberately manipulated the mycorrhizal composition of the plots: some contained only arbuscular mycorrhizal (AM) trees, some only ectomycorrhizal (ECM) trees, and some a mixture of both types. Mycorrhizal fungi are symbionts that colonize tree roots, trading soil nutrients such as nitrogen and phosphorus for plant-derived carbon. The two major types differ profoundly in their biology — AM fungi penetrate root cells and are relatively fast-cycling, while ECM fungi form sheaths around roots, build persistent hyphal networks, and are often associated with slower, more fungal-dominated decomposition pathways.

What makes the new study unusual is its vertical ambition. Rather than sampling only the top few centimeters of soil, the team quantified total carbon, total nitrogen, and microbial necromass along a full one-meter soil profile, tracking how these pools changed over the first ten years of stand development. Microbial necromass was estimated using amino sugar biomarkers — molecular fingerprints that allow researchers to distinguish fungal residues from bacterial residues in soil. This matters because a growing body of research indicates that microbial residues, not just dead plant litter, constitute a major and often dominant fraction of stable soil organic matter. If biodiversity influences how much microbial necromass accumulates, it may ultimately shape how much carbon soils can lock away for decades or centuries.

The headline finding is one of decoupling. Over the decade, total soil carbon declined by roughly 3 percent across the profile, total nitrogen by about 17 percent, and microbial necromass by around 14 percent. Soil depth was the single strongest driver of overall variation, which is perhaps unsurprising given that organic matter concentrates near the surface. But the temporal trajectories of the three pools diverged in revealing ways. Nitrogen declined first in the upper 40 centimeters during the initial five years, then its losses extended deeper even as the topsoil began to recover. Total carbon declined later, mainly at intermediate depths between 5 and 40 centimeters. Microbial necromass, meanwhile, decreased later still, but across the entire one-meter profile. These staggered patterns suggest that the early years of afforestation are not a uniform period of soil building; instead, different pools respond on different clocks and at different depths.

The most striking result, however, concerns biodiversity. Effects of tree species richness and mycorrhizal type were detected only for microbial necromass — not for bulk carbon or nitrogen — and those effects varied with mycorrhizal identity, soil depth, and time. Fungal necromass dominated the dynamics of total microbial necromass and showed the strongest biodiversity responses of any measured pool. In mixed AM-plus-ECM communities, fungal necromass in the topsoil initially increased by 21 to 27 percent. In pure ECM communities, a similar magnitude of increase — around 24 percent — emerged only after a full decade. In other words, the composition of the below-ground symbiotic community determined both the timing and the magnitude of the fungal necromass response.

Why should mixed mycorrhizal communities act early while ECM-dominated communities act late? The authors’ findings are consistent with a mechanistic picture in which complementary mycorrhizal strategies accelerate the early build-up of fungal biomass and its subsequent turnover. Mixing AM and ECM trees may broaden the range of nutrient acquisition strategies operating in a plot, stimulating overall productivity and, with it, the flux of plant carbon into fungal tissue. When that tissue dies, its chitin-rich cell walls become necromass, a chemically stabilized precursor to soil organic matter. ECM-dominated systems, by contrast, may require longer for their slower-cycling hyphal networks to accumulate enough standing biomass and turnover to register as a detectable necromass signal. The decade-long delay in the ECM response fits this slower tempo.

The study’s implications reach well beyond soil ecology’s internal debates. Global assessments consistently identify forest restoration as one of the most promising nature-based strategies for carbon removal, and recent work has shown that tree diversity can enhance decadal soil carbon and nitrogen accrual in forests. But if biodiversity effects on the stable, microbial-derived fraction of soil organic matter appear in necromass years before they appear in bulk carbon pools, then standard soil carbon monitoring may systematically undercount the early benefits of diverse plantings. A restoration project could be quietly building long-term carbon storage capacity through fungal pathways while its measured soil carbon stock remains essentially flat, potentially leading managers and policymakers to undervalue mixed-species, mixed-mycorrhizal designs.

The depth-resolved design also carries a caution for how soil carbon is accounted for. Because the temporal trajectories of carbon, nitrogen, and necromass diverge across the one-meter profile — with nitrogen losses penetrating deeper even as topsoil recovers, and carbon losses concentrated at intermediate depths — shallow sampling alone could paint a misleading picture of whether a young forest is gaining or losing soil fertility. The finding echoes a broader concern in the literature that deep soil organic carbon responds to global change in ways that topsoil-focused studies miss, and that afforestation effects on soil carbon and nitrogen depend strongly on initial soil conditions and profile position.

For the growing community of researchers working on microbial necromass, the study adds an important temporal dimension. Much of the existing work has established that microbial residues contribute substantially to stabilized soil organic carbon and that necromass formation depends on microbial death pathways and clay-organic matter interactions. What this experiment demonstrates is that the necromass pool is not merely a passive integrator of past microbial activity; it is an early, biodiversity-sensitive responder that can move independently of the bulk pools it will eventually feed. That decoupling — fungal necromass shifting by 20 percent or more while total carbon barely budges — is precisely the kind of signal that could serve as a leading indicator of future soil carbon change.

The authors conclude that biodiversity effects during afforestation emerge through fungal-mediated pathways before becoming detectable in bulk soil carbon pools. If that sequence holds in other systems and over longer timescales, it suggests a reframing of how restoration success should be judged. The trees are the visible half of the story; the fungi that partner with their roots, live, die, and leave their walls in the soil are the other half — and on the evidence of this decade-long experiment, the fungal half speaks first. For anyone betting on forests to draw down carbon, listening to that early signal, at depth and over time, may prove essential.

Subject of Research: Effects of tree diversity and mycorrhizal type on fungal necromass, soil carbon and nitrogen across the soil profile during a decade of afforestation

Article Title: Tree diversity and mycorrhizal type induce stronger shifts in fungal necromass than in soil carbon and nitrogen across the soil profile over a decade of afforestation

Article References: Li, T., Ferlian, O., Wu, P., Liu, Z., & Eisenhauer, N. (2026). Tree diversity and mycorrhizal type induce stronger shifts in fungal necromass than in soil carbon and nitrogen across the soil profile over a decade of afforestation. Plant and Soil. https://doi.org/10.1007/s11104-026-09082-6

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09082-6

Keywords: tree diversity, mycorrhiza, fungal necromass, microbial necromass, soil carbon, soil nitrogen, afforestation, soil profile, amino sugar biomarkers, ectomycorrhiza, arbuscular mycorrhiza, soil organic matter