Beyond clay: towards an improved set of variables for predicting soil organic matter content

Abstract

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO 2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron-and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to

Arctic Soil Governs Whether Climate Change Drives Global Losses or Gains in Soil Carbon

“…The actual amount of protected C in any location is difficult to quantify empirically, much less define globally. Recent observational work emphasizes the importance of climate thresholds determined by ecosystem water balance, which influences mineral weathering and mechanisms of organic matter stabilization (Kramer & Chadwick, ; Rasmussen et al, ). Roughly one quarter of global soil C stocks are bound to reactive minerals in subsurface soils (Kramer & Chadwick, ), with a spatial distribution that is distinct from the protected C fractions simulated by models in our testbed (Figure S7).…”

“…Despite these differences, models in our testbed all use clay content as a proxy to determine the capacity of soils to physicochemically protect soil C (Bailey et al, ). Although generalizability of this assumption is being questioned (Rasmussen et al, ; Rowley et al, ), the protected soil C simulated here broadly corresponds to mineral associated organic matter (MAOM) that could be isolated by particle size or density fractionation (Sohi et al, ). This MAOM typically has older radiocarbon ages, consistent with longer turnover times and greater persistence of mineral‐associated organic C (Trumbore, ), which corresponds to the passive, physicochemically protected, and protected soil C pools simulated by CASA, MIMICS, and CORPSE, respectively (Text S1).…”

Drivers of postfire soil organic carbon accumulation in the boreal forest

“…One explanation could lie in the low Nordic soil data availability. Indeed, most of our knowledge on the control of soil C stocks by soil texture comes from warm agricultural regions (Rasmussen et al., ). Also, it is possible that the weathering of the granitic bedrock of the Canadian Shield did not produce reactive surface silt and clay particles considering that the studied soils are young and have been developing since the end of the last glaciation (<10k years; Minasny, McBratney, and Salvador‐Blanes ()).…”

Variation in the Molecular Structure and Radiocarbon Abundance of Mineral-Associated Organic Matter across a Lithosequence of Forest Soils

“…Recent work highlighting the microbial community as drivers of SOM production and character is creating a new understanding of how C is transformed and stabilized in soils [26,47]. Understanding of the relative importance of physicochemical stabilization mechanisms across ecosystem types is also improving [48]. However, the role of mineral chemistry in C stabilization is still not well defined in natural soils, despite decades of lab work with model systems showing sorptive fractionation and selective preservation effects.…”