Climate regulates ecosystem carbon transfer efficiency from atmosphere to soil via soil nitrogen

Terrestrial soil carbon (C) accumulation is governed by the balance between plant C uptake through gross primary production (GPP) and ecosystem respiration (expressed as net ecosystem production, NEP), and by the proportion of this assimilated C that is transferred into soil organic carbon (SOC) pools. However, the efficiency with which assimilated C becomes stabilised in soils remains poorly quantified, and its drivers unknown. This knowledge gap constrains our ability to properly evaluate the long-term C sink potential of forests. Here, we integrated 1990–2020 soil C stock data from forest monitoring networks and ecosystem C flux data from upscaled and remotely-sensed-based C flux products across Europe to quantify and investigate the controls of SOC stocks, accumulation rates, and C transfer efficiencies from GPP and NEP (GSE and NSE). On average and across all studied forests, annual SOC accumulation rates, in the 0–80 cm soil profile, represented 11.9% and 31.2% of mean annual GPP (GSE) and NEP (NSE), respectively. SOC stocks and accumulation rates increased with higher NEP, whereas SOC accumulation rates declined with higher ecosystem respiration. Sites with cool and wet climates and fine-textured soils exhibited higher GSE and NSE, while high vapour pressure deficit, precipitation, and coarse soils were consistently related to reduced C stabilisation. Structural equation models showed that soil nitrogen exerted the strongest total positive effects on SOC stock (0.79, 95% CI: 0.69–0.85), SOC accumulation rate (0.87, 95% CI: 0.79–0.93), GSE (0.86, 95% CI: 0.79–0.92), and NSE (0.62, 95% CI: 0.56–0.65). Overall, these results provide a large-scale empirical benchmark of ecosystem C transfer efficiency, highlighting soil nitrogen as the dominant regulator of long-term soil C sequestration. Incorporating the observed variability in C transfer efficiency driven by soil nitrogen and climate into Earth system models will enhance predictions of forest C sequestration potential.