Maturing forests redistribute soil carbon rather than simply storing more

Forests are widely valued for capturing atmospheric carbon, but their climate benefit depends not only on how much carbon enters the soil. It also depends on where that carbon is stored and how long it remains there. These features can change in different ways as forests develop.

A new study published in Forest Ecosystems shows that forest maturation does not simply increase soil carbon. Instead, it redistributes carbon between soil pools, with contrasting patterns near the surface and deeper underground. The findings identify microorganisms as an important link between forest succession and persistent soil carbon formation.

The researchers examined temperate forests in Northeast China representing four stages of a 300-year succession, from young birch stands to mature mixed broad-leaved and Korean pine forest. They analyzed soils at depths of up to 30 centimeters, focusing on two major carbon pools.

1. Particulate organic carbon (POC) comes mainly from incompletely decomposed plant and microbial material and remains relatively vulnerable to further decomposition.
2. Mineral-associated organic carbon (MAOC) forms when processed organic compounds bind to mineral surfaces, where they can remain protected for much longer periods.

Across the succession, POC declined at every sampled depth. In the upper 10 centimeters, however, MAOC increased by about 55% and accounted for a growing share of total soil carbon, shifting surface soil toward a more persistent form of carbon. Below this surface layer, both POC and MAOC declined. Mature forests therefore contained a more stable form of carbon near the surface, but did not accumulate carbon uniformly throughout the soil profile.

To understand why, the researchers applied microbial carbon pump theory, which describes how microbes transform plant-derived carbon into soil organic matter. This occurs through two routes. During ex vivo modification, extracellular enzymes transform organic matter outside microbial cells. During in vivo turnover, microbes incorporate carbon into biomass that can later enter the soil as dead microbial material, or necromass.

Microbial life-history strategies influence these routes. The resource-acquisition (A) strategy invests in extracellular enzymes and is linked to ex vivo modification. As forests matured, reduced investment in this strategy may have slowed decomposition and favored the binding of organic compounds to minerals, supporting surface MAOC accumulation. The high growth yield (Y) strategy channels carbon into microbial biomass and is linked to in vivo turnover. Declining necromass was closely associated with POC loss. In deeper, resource-poor soils, limited carbon restricted biomass and necromass formation, so stronger Y-strategy traits did not produce greater carbon storage.

These findings extend microbial carbon pump theory by showing that both ex vivo modification and in vivo turnover play important roles, but their contributions vary with forest succession and soil depth. Incorporating these depth-specific mechanisms into carbon models could improve predictions of forest carbon storage and support restoration aimed at building more persistent soil carbon reserves.

DOI Link:

https://doi.org/10.1016/j.fecs.2026.100498