Abstract. Global average temperatures are forecast to increase by 4 °C by 2100 under the Intergovernmental Panel on Climate Change SSP5-8.5 scenario. This warming could accelerate soil organic carbon (SOC) mineralization, net loss of soil carbon to the atmosphere, and consequently exacerbate global warming through a positive feedback loop. It is generally assumed that mineral-associated organic matter (MAOM) stocks are less sensitive to warming compared to particulate organic matter (POM), especially in the subsoil; yet more empirical data investigating the whole-soil responses to warming is still required to rigorously test this assumption. Our study was conducted in a whole-soil field warming experiment in a temperate mixed-conifer forest at Blodgett Forest Research Station, University of California, Berkeley, which had been subjected to 9.5 years of warming. Soils from three depths (10–20, 40–50, and 80–90 cm) were separated into three density fractions, free POM (fPOM), occluded POM (oPOM), and MAOM. We then investigated the SOC concentration and composition of bulk soil and fractions with elemental analysis and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. In subsoils below 50 cm, a decade of experimental warming caused bulk carbon loss and shifted its composition towards lignin and C–H aromatic bonds. Warmed plots had significantly lower mass of the POM fractions relative to control plots in the deep soil (80–90 cm) with no significant difference at or above 50 cm. fPOM composition showed a significant shift towards enrichment in lignin at 40–50 cm but not in topsoils. Meanwhile, MAOM mass and composition shifted along the depth gradient but were not significantly affected by warming. This study at Blodgett Forest thus supports the assumption that POM will be more responsive to warming in the subsoil than MAOM.
Sun et al. (Mon,) studied this question.