<scp>BLOSOM</scp> : A Plant Growth Facility Optimised for Continuous <scp> ¹³ C </scp> Labelling and Measurement of Soil Organic Matter Dynamics

ABSTRACT Changes in soil carbon (C) stocks are largely driven by rhizosphere processes forming new soil organic matter (SOM) or stimulating SOM decomposition by rhizosphere priming effects (RPEs). Quantifying these changes is challenging and requires high spatial sampling densities or plant–soil experiments with highly distinct C isotopic signatures for plants and soils. Current methods for quantifying new SOM formation and RPEs rely on low labelling intensities, which introduces high levels of uncertainty. Here, we describe the design and operation of an experimental laboratory facility—BLOSOM (Botanical Labelling Observatory for Soil Organic Matter)—optimised for continuous 13 C labelling of plants at high labelling intensities (&gt; 500‰) to quantify new SOM formation and RPEs in temperature‐controlled soils from 216 experimental units. Throughout a &gt; 6‐month experimental period, independent control of soil and air temperature was achieved across diurnal cycles averaging at 5.24°C ± 0.05°C and 21.4°C ± 1.2°C, respectively. BLOSOM can maintain stable CO 2 concentrations and δ 13 C isotopic composition within 5% of setpoints (CO 2 : 440 ppm, δ 13 C: 515‰) across a &gt; 6‐month period. This high‐precision control on atmospheric enrichment enables the detection of new SOM formation with a total uncertainty of ±39% to ±3% for a theoretical range of 0.5%–10% new SOM formation, respectively. BLOSOM has the potential improve quantification and mechanistic understanding of new SOM formation and RPEs across many different combinations of plants, soils and simulated climatic conditions to mimic a wide range of ecosystems and climate scenarios.