Decoupled recovery of soil physicochemistry, microbiome and methane sink function during secondary forest succession in the Eastern Himalayas

Abstract

Secondary forests are increasingly important for biodiversity recovery and ecosystem functioning after disturbance. As vegetation regenerates, accompanying shifts in soil conditions and microbial communities regulate climate-relevant processes such as atmospheric methane (CH4) uptake. Whether these belowground dimensions recover synchronously, however, remains unclear. Here we show that belowground recovery followed asynchronous trajectories across a forest chronosequence in Metok, Eastern Himalayas. Soil inorganic properties and microbial community composition converged progressively on the primary-forest state as soils acidified. By contrast, soil organic carbon, microbial diversity and abundance appeared to recover by 25 years but diverged again at 40 years. Co-occurrence network topology resembled the primary forest by 40 years, yet the keystone taxa underlying it were entirely stage specific. Atmospheric CH4 uptake fluctuated: uptake recovered to primary-forest levels in 25-year stands but weakened in 40-year stands to rates comparable with the recently disturbed 5-year forest. Rather than canonical methane-cycling taxa, four amplicon sequence variants—three assigned to Mycobacterium—tracked CH4 flux closely while acting as hubs or connectors in the co-occurrence networks, warranting genomic and experimental work to resolve their function. Apparent recovery within the first 25 years therefore did not persist, cautioning against inferring restoration success from early succession alone and highlighting the need for longer chronosequences and repeated observation to resolve the dynamics of secondary-forest recovery.