Climate-driven thresholds in reactive mineral retention of soil carbon at the global scale

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

“…Measurement and theory suggest that the association of organic matter with mineral surfaces confers persistence to otherwise decomposable substrates (Cotrufo et al, ; Dungait et al, ; Kleber et al, ; Mikutta et al, ). To our knowledge, results presented by Kramer and Chadwick () are the first attempt to estimate global stocks of MAOM (Figure S7) that may be comparable to protected C stocks represented in our testbed (Figure ). The differences in protected stocks among the models presented here, and the observationally derived estimates of Kramer and Chadwick (), exemplify uncertainties arising as our understanding develops about the mechanisms that confer persistence of organic matter in soils.…”

“…To our knowledge, results presented by Kramer and Chadwick () are the first attempt to estimate global stocks of MAOM (Figure S7) that may be comparable to protected C stocks represented in our testbed (Figure ). The differences in protected stocks among the models presented here, and the observationally derived estimates of Kramer and Chadwick (), exemplify uncertainties arising as our understanding develops about the mechanisms that confer persistence of organic matter in soils. They highlight the need to develop, apply, and evaluate model formulations that seek to test and advance these new theories emerging from empirical data.…”

“…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).…”

“…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). To our knowledge, these spatial distributions are not well represented in any global‐scale model, including those in our testbed, and improving these representations should be considered a high priority if we are to confidently extrapolate emerging mechanistic insights for C cycle projections.…”

“…Models in our testbed all use clay content as a proxy for mineralogical controls on potential physicochemical protection of soil C. This common assumption (Bailey et al, ; Rasmussen et al, ) produces latitudinal gradients in protected soil C in all three of the soil models evaluated here (Figure ), but recent work highlights shortcomings of this assumption, with pH and weathering thresholds that allow either calcium or iron‐ and aluminum‐oxides to stabilize organic matter in soils. (Kramer & Chadwick, ; Rasmussen et al, ; Rowley et al, ). Both the use of clay and chemical extractions simplify the diversity of chemical and physical mechanisms by which soil C may be stabilized (Chadwick & Chorover, ).…”