The electron transfer capacities (ETCs) of soil humic substances (HSs) are linked to the type and abundance of redox-active functional moieties in their structure. Natural temperature can affect the chemical structure of natural organic matter by regulating their oxidative transformation and degradation in soil. However, it is unclear if there is a direct correlation between ETC of soil HS and mean annual temperature. In this study, we assess the response of the electron-accepting and -donating capacities (EAC and EDC) of soil HSs to temperature by analyzing HSs extracted from soil set along glacial-interglacial cycles through loess-palaeosol sequences and along natural temperature gradients through latitude and altitude transects. We show that the EAC and EDC of soil HSs increase and decrease, respectively, with increasing temperature. Increased temperature facilitates the prevalence of oxidative degradation and transformation of HS in soils, thus potentially promoting the preferentially oxidative degradation of phenol moieties of HS or the oxidative transformation of electron-donating phenol moieties to electron-accepting quinone moieties in the HS structure. Consequently, the EAC and EDC of HSs in soil increase and decrease, respectively. The results of this study could help to understand biogeochemical processes, wherein the redox functionality of soil organic matter is involved in the context of increasing temperature.

Decomposition of soil organic matter (SOM) plays an important role in the global carbon cycle because the CO2 emitted from soil respiration is an important source of atmospheric CO2. Carbon isotopic fractionation occurs during SOM decomposition, which leads to 12C to enrich in the released CO2 while 13C to enrich in the residual SOM. Understanding the isotope fractionation has been demonstrated to be helpful for studying the global carbon cycle. Soil and litter samples were collected from soil profiles at 27 different sites located along a vertical transect from 1200 to 4500 m above sea level (a.s.l.) in the south-eastern side of the Tibetan Plateau. Their carbon isotope ratios, C and N concentrations were measured. In addition, fiber and lignin in litter samples were also analyzed. Carbon isotope fractionation factor (α) during SOM decomposition was estimated indirectly as the slope of the relationship between carbon isotope ratios of SOM and soil C concentrations. This study shows that litter quality and soil water play a significant role in isotope fractionation during SOM decomposition, and the carbon isotope fractionation factor, α, increases with litter quality and soil water content. However, we found that temperature had no significant impact on the α variance.

Organic carbon (OC) associated with reactive metal oxides, especially noncrystalline ferric iron [Fe(III)] and aluminum (Al) (hydr)oxides, is commonly assessed by the citrate-bicarbonate-dithionite (CBD) method (termed "metal-bound OC" or abbreviated as "OC CBD " hereafter) and estimated to constitute 15%-38% of total OC in sediments and upland soils (Shields et al., 2016;Zhao et al., 2016). However, the relative importance of OC CBD in different land types (e.g., uplands vs. wetlands) and soil depths remains unclear (Mu et al., 2016;Rasmussen et al., 2018;Wan et al., 2019;Zhao et al., 2016), hampering our ability to predict and protect this important OC pool that is vital for the long-term preservation of soil organic carbon (SOC) (Hemingway et al., 2019;Schmidt et al., 2011).While reactive Fe is assumed to play a central role in binding OC, its content has no relationship with the SOC-normalized concentration of OC CBD at regional scales (Mu et al., 2016;Zhao, et al., 2016). Recent evidence suggests that other polyvalent cations and/or mineral phases including manganese (Mn) and silicate (Si) may also be solublized during the CBD treatment and contribute to OC binding (Jugsujinda et al., 1995;Paterson et al., 1993). In particular, calcium (Ca), one of the most abundant polyvalent cations in soils (Rowley et al., 2018), can promote Fe-OC association by forming Fe-OC-Ca ternary co-precipitates (Adhika-