Background and aims <p>Arbuscular mycorrhizal fungi (AMF) are integral to the global carbon and nutrient cycles, primarily through the production of glomalin-related soil protein (GRSP), which contributes significantly to soil organic carbon (SOC) accumulation and ecosystem stability. However, the distribution pattern and environmental controls of GRSP during long-term ecosystem development are poorly understood.</p> Methods <p>Here, we investigated the dynamics of GRSP and its contribution to SOC accumulation along a 2-million-year chronosequence at Jurien Bay, south-western Australia, a biodiversity hotspot with severe phosphorus (P) deficiency.</p> Results <p>Our results revealed a progressive decline in AMF biomass with increasing soil age along the chronosequence, driven by P depletion and a reduction in the relative dominance of mycorrhizal plants (indicated by their relative canopy cover). Paradoxically, GRSP concentrations, especially easily-extractable GRSP (EE-GRSP), increased significantly along the chronosequence and peaked in the most weathered and severely P-impoverished soils. In addition, GRSP contributed up to 142 ± 15 mg SOC g⁻¹, with increased production and stability facilitated by interactions with soil acidity, fine texture, nutrient stoichiometry, and mycorrhizal plant richness (the number of plant species that can form a symbiosis with AMF).</p> Conclusions <p>These results demonstrate that GRSP dynamics is primarily determined by AMF turnover, mycorrhizal plant species richness, and nutrient limitation, underscoring its critical role in SOC accumulation under nutrient-depleted conditions. This study advances our mechanistic understanding of AMF-mediated soil processes, with implications for sustainable land management and climate change mitigation in nutrient-limited yet biodiverse ecosystems.</p>