| 22 Apr 2026
Quantifying temperature dependence of Fe reduction in humid tropical soils: a Bayesian model-data integration
Abstract. Humid tropical forests are critical regulators of the global carbon (C) cycle, yet their soil C stocks are increasingly vulnerable to warming. Predicting potential losses requires a mechanistic understanding of the processes that govern soil C stabilization and mineralization, particularly in Fe-rich soils, where iron (Fe) redox cycling plays a dual role in both protecting and decomposing organic matter. However, the temperature dependency of these Fe-mediated processes remains poorly understood. In this study, we quantified the temperature dependence of FeIII reduction by conducting anoxic incubations at 23, 27, and 33 °C and calibrating four kinetic models of increasing complexity to estimate the Q10 and Arrhenius (Ea) using a Markov Chain Monte Carlo (MCMC) framework. Model performance was evaluated using Bayesian information criteria (WAIC, LOO, and LPML) to assess fit, complexity and uncertainty. Short-term warming significantly accelerated Fe-reduction rates, potentially destabilizing mineral-associated organic carbon and enhancing microbial respiration. Estimated Q10 and Ea values ranged from 1.5 to 2.1 and 30.8 to 56.5 kJ mol-1 respectively, comparable to the temperature sensitivity values measured in temperate and tropical biomes. With the available data, Bayesian information criteria preferred the simplest one pool FeII model due to its parsimony. In contrast, the most complex (three pool) model, which includes dissolved organic carbon (DOC) dynamics alongside Fe reduction and oxidation, was generally the least preferred by Bayesian information criteria due to increased uncertainty from unconstrained additional processes. These results underscore the importance of temperature-dependent Fe redox processes in regulating soil C cycle in humid tropical soils and emphasize the need to balance model complexity with data availability when modeling coupled C-Fe interactions.
This manuscript investigates the temperature dependency of Fe reduction in a soil sample from a humid tropical system. This was assessed experimentally using batch incubation experiments and subsequently interpreted using Bayesian modelling approaches to identify the model with the best performance. To this end, four different models with increasing levels of complexity were evaluated. Overall, I consider the manuscript to be well structured and clearly written, and it presents interesting results that contribute to understanding how increasing temperature may influence Fe reduction processes in this type of system. The application of a Bayesian framework to evaluate model performance in terms of parsimony and uncertainty is also a relevant and valuable contribution.
However, there are several points of concern that, in my opinion, should be addressed before the manuscript can be considered for publication.
A major concern relates to the framing and interpretation of the study in relation to soil C dynamics. The authors state that one of the aims of the manuscript is to enhance the mechanistic understanding of how temperature influences C cycling through its coupling with Fe redox processes. Furthermore, the manuscript claims that the results underscore the importance of temperature-dependent Fe redox processes in regulating soil C cycling. While the coupling between Fe and C cycles is undoubtedly important in these soils, the results and subsequent discussion do not provide direct evidence or substantial new insights into C cycling dynamics. The study is primarily focused on the temperature dependency of Fe redox processes, and no direct measurements related to C destabilization, decomposition, or other C dynamics were performed. Therefore, I recommend that the authors moderate the claims regarding soil C cycling and instead place greater emphasis on discussing Fe redox processes and the modelling framework, which constitute the strongest aspects of the study.
A second major concern relates to methodological aspects. Throughout the manuscript, the authors state that the experiments were conducted under “constant anoxic conditions.” However, no direct evidence or measurements are provided to verify the establishment and maintenance of anoxic conditions (eg., pE, dissolved O2, or O2 saturation). In addition, important methodological details are missing, such as the gas-to-liquid ratio, procedures used to remove O2 (e.g., gas purging), the use of airtight vessels, and how anoxic conditions were maintained throughout repeated sampling events. Without these details, the assumption of constant anoxic conditions is insufficiently supported, which is particularly important given that the interpretation of Fe reduction dynamics strongly depends on this assumption.
In the attached file, I also include several specific comments aimed at improving clarity and interpretation of the results.