Process‐based modeling of silicate mineral weathering responses to increasing atmospheric CO₂ and climate change

Abstract

[1] A mathematical model describes silicate mineral weathering processes in modern soils located in the boreal coniferous region of northern Europe. The process model results demonstrate a stabilizing biological feedback mechanism between atmospheric CO 2 levels and silicate weathering rates as is generally postulated for atmospheric evolution. The process model feedback response agrees within a factor of 2 of that calculated by a weathering feedback function of the type generally employed in global geochemical carbon cycle models of the Earth's Phanerozoic CO 2 history. Sensitivity analysis of parameter values in the process model provides insight into the key mechanisms that influence the strength of the biological feedback to weathering. First, the process model accounts for the alkalinity released by weathering, whereby its acceleration stabilizes pH at values that are higher than expected. Although the process model yields faster weathering with increasing temperature, because of activation energy effects on mineral dissolution kinetics at warmer temperature, the mineral dissolution rate laws utilized in the process model also result in lower dissolution rates at higher pH values. Hence, as dissolution rates increase under warmer conditions, more alkalinity is released by the weathering reaction, helping maintain higher pH values thus stabilizing the weathering rate. Second, the process model yields a relatively low sensitivity of soil pH to increasing plant productivity. This is due to more rapid decomposition of dissolved organic carbon (DOC) under warmer conditions. Because DOC fluxes strongly influence the soil water proton balance and pH, this increased decomposition rate dampens the feedback between productivity and weathering. The process model is most sensitive to parameters reflecting soil structure; depth, porosity, and water content. This suggests that the role of biota to influence these characteristics of the weathering profile is as important, if not more important, than the role of biota to influence mineral dissolution rates through changes in soil water chemistry. This process-modeling approach to quantify the biological weathering feedback to atmospheric CO 2 demonstrates the potential for a far more mechanistic description of weathering feedback in simulations of the global geochemical carbon cycle.

Evaluating the effects of terrestrial ecosystems, climate and carbon dioxide on weathering over geological time: a global-scale process-based approach

“…the volume immediately around the root and mycorrhizal fungal hypha with an effective radius of several millimetres) is largely controlled by the 'biological proton cycle' describing the stoichiometry of proton and base cation cycling between soil, organisms and organic matter [16]. Overall, these reactions determine the net flux of protons into and out of the sub-surface environment and therefore the acid-base balance and its influence on the pH of the mineral-weathering environment.…”

“…Fine roots and EM hyphae also exude low molecular weight organic acids at rates of 10 mol m 21 fine roots day 21 [38] and 10 mol m 21 hyphae day 21 [39], respectively. A further contribution of organic acids is linked to leached carbon provided by SDGVM, because removal of organic matter is associated with permanent loss of alkalinity from the soil [10,16]. Because SDGVM produces steady-state vegetation, we do not model the effects of aggrading ecosystems in this paper.…”

“…Aqueous speciation reactions such as inorganic carbon speciation and pH buffering are treated as thermodynamic equilibria. The model includes representation of the biological cycling of protons and alkalinity [16] occurring in discrete regolith zones: (i) the soil that is in contact with or under the influence of living roots and mycorrhizal hyphae (the mycorrhizosphere) and (ii) the remaining bulk soil. Weathering in both zones is influenced by abiotic processes and decomposition [10].…”

“…Roelandt et al [25] coupled a DGVM into a sophisticated n-layer mechanistic weathering model [26] requiring detailed information about soil characteristics for each grid point, that accounts for both evapotranspiration and root respiration effects on weathering via hydrology and soil pCO 2 . Unlike the simpler one-layer box model of Taylor et al [10], it takes no account of the actions of mycorrhizal fungi on the biological proton cycle that control mineral weathering [16]. It is important to note that this study focused on the catchment scale.…”

“…The model describes how the primary productivity of terrestrial vegetation and associated nutrient uptake by fine roots and mycorrhizal fungi influences soil chemistry and mineral weathering via its effects on the biological proton cycle, and integrates these processes into a leading global geochemical carbon cycle model (GEOCARBSULF). The biological proton cycle is controlled by the stoichiometry of reactions during the uptake of mineral nutrients by plants and symbiotic root fungi during growth, and the subsequent return of these nutrients during decomposition and leaching of organic matter [16]. Overall, these reactions determine the net flux of protons into and out of the sub-surface environment and, therefore, the acid-base balance and its influence on the pH of the mineral weathering environment.…”