Organic matter in density fractions responds to 10 years of experimental field warming in a temperate forest

Global temperatures could increase by approximately 4°C until 2100, according to IPCC climate scenarios, causing surface and subsoil will warm in synchrony with the atmosphere. This warming is predicted to accelerate soil carbon loss and greenhouse gas release, but also change the composition of soil organic matter in ways that affect its cycling and future vulnerability. This is important because despite low carbon concentrations, subsoils store more than half of the total global soil organic carbon. However, it remains largely unknown how this deep soil carbon will respond to warming. In this study we explore how 10 years of experimental field warming affects soil carbon quantity and quality in bulk soil and in functional (density) fractions in whole soil profiles. 

After 10 years of experimental field warming of a temperate forest (Blodgett, Sierra Nevada, CA, USA), we analyzed carbon composition of bulk soil and density fractions using Diffuse Reflectance Infrared Fourier Transform spectroscopy. Soil carbon functional pools included free and occluded particulate organic matter (fPOM, oPOM) as well as mineral associated organic matter (MAOM), at 3 different depths (10-20, 40-50, and 80-90 cm). The results showed that the relative proportion of carbon in fPOM and oPOM decreased with depth and was lower in warmed plots. Soil carbon (C) quality in fPOM and oPOM did not change with warming or depth. However, C quality in MAOM was different, with 11% more aliphatic C in the topsoil (10-20 cm), and 17% more aromatic C in the deep soil (80-90 cm). This indicated an increasing level of SOC decomposition in subsoil >50 cm. With warming, most of the remaining organic matter in the deep soil was protected by mineral association, with relatively more aromatic C present. This raises the possibility that SOC that is mineral-associated in subsoil, especially in the form of aromatic C, might resist future warming more than SOC in other functional fractions.