The molecular composition of humus carbon: Recalcitrance and reactivity in soils

Soil organic C (SOC) is the largest terrestrial reservoir in the biosphere, accounting for 1500–1770 Pg, as compared to C stocks of vegetation (450– 650 Pg) (IPCC, 2013). Although humus C represents from 60% to 80% of SOC, its dynamics still remain poorly understood after nearly a century of study, due to the multiplicity of factors that affect stabilization of humic matter. Industrial agricultural practices accelerate the decline of humus content in soil, and, consequently, the reduction of soil fertility, biodiversity, and soil structural stability (Fontaine et al., 2007; Reeves, 1997), while enhancing greenhouse gases (GHG) emissions from soil (Smith et al., 2014). Because it is the specific molecular composition of the soil Humeome that significantly affects SOC storage dynamics (Woo et al., 2014), soil basal respiration (Fang et al., 2005), and humus-plant relationships (Canellas and Olivares, 2014), a rigorous identification of the molecular structure of the components of soil humus C is necessary, if any technological control of its content and dynamics can ever be introduced.