Soil warming influences organic carbon composition at the microscale in Blodgett Forest

The impact of warming on the soil organic carbon (SOC) cycle and its potential positive feedback with increasing atmospheric CO₂ concentrations is of global concern. Earth System Models currently predict that warming will increase soil CO2 efflux faster than net primary productivity (Crowther et al., 2016); yet there are still large uncertainties associated with these modelled estimates, which can be reduced by process-based observations from whole-soil warming experiments. To identify the influence of 7.5 years of +4°C whole-soil warming on SOC and its elemental associations at the microscale, we used scanning transmission X-ray spectromicroscopy at the carbon K-edge (STXM C NEXAFS). We focused our analyses on soils collected from three depth intervals (10-20, 40-50, 60-70 cm) at the control and warmed plots from the whole-soil warming project at Blodgett Experimental Forest (granitic Alfisols). Relative to control plots, samples from the warmed plots had elevated aromatic and phenolic C content, and this observation was most pronounced in the 40-50 cm depth samples. This result differed from previous observations at the bulk-soil level (Ofiti et al., 2021), which demonstrated a decrease in the relative abundance of these compound classes with warming, particularly at depth. These contrasting results may be explained by a difference in SOC dynamics at the bulk scale relative to the microscale, with STXM investigating SOC bound in organo-mineral assemblages at the microscale, while bulk soil measurements include larger partulate organic matter. It could also be indicative of the changes in root dynamics with warming that were also recorded in Ofiti et al. (2021). The STXM data also showed that organic carbon was strongly associated with calcium in these acidic soils, which had a more plant-like nature than C associated with iron. This supports similar observations, which were recently made in soils from an acidic grassland soil series at Point Reyes, California that had developed in a different parent material (sandstone; Rowley et al., 2023). This study highlights the importance of investigating how organo-mineral or -metal associations will respond to changing environmental conditions at various analytical scales. 

References

Crowther et al., 2016. Quantifying global soil carbon losses in response to warming. Nature 540(7631), 104-108.
Ofiti, et al., 2021. Warming promotes loss of subsoil carbon through accelerated degradation of plant-derived organic matter. Soil Biology and Biochemistry 156, 108185.
Rowley et al., 2023. Association between soil organic carbon and calcium in acidic grassland soils from Point Reyes National Seashore, CA. Biogeochemistry 165, 91-111.