Enhanced weathering (EW) is a promising strategy to remove atmospheric CO2 by amending agricultural and forestry soils with ground silicate materials. However, the current model-based assessments of EW potential face uncertainties stemming from the intricate interplay among soil physical, chemical, and biotic processes, compounded by the absence of a detailed model-data comparison, mostly due to the limited availability of comprehensive data. Here, we address this critical gap by advancing and validating an ecohydrological and biogeochemical model for EW dynamics in soils. We conduct a hierarchical validation in which model results are critically compared to four experimental datasets of increasing complexity, from simple closed incubation systems to open mesocosm experiments. The comparison demonstrates the model ability to capture the dynamics of primary variables, including rock alkalinity release and CO2 sequestration. The comparison also reveals that weathering rates are consistently lower than traditionally assumed by up to two orders of magnitude. We finally discuss avenues for further theoretical and experimental explorations.

Ecohydrology engineering provides a valuable framework for addressing emerging environmental challenges by integrating ecological and environmental engineering principles. In this study, we discuss the potential of parsimonious, physically-based ecohydrological models through the lens of three case studies: sustainable irrigation, urban heat island mitigation via green roofs, and mangrove restoration for climate change mitigation. First, we investigate sustainable irrigation strategies, illustrating the trade-offs between water conservation and soil salinization. This highlights the delicate balance required to optimize crop yield while mitigating soil degradation. Second, we explore the role of green roofs in urban heat island mitigation, showing how vegetation and water dynamics on rooftops can enhance latent heat flux, thereby potentially reducing urban temperatures and improving building energy efficiency. Lastly, we assess the climate mitigation potential of mangrove restoration, accounting for the impacts of salinization and sea-level rise. We demonstrate that carbon sequestration in mangrove ecosystems may be strongly limited by productivity reduction due to salinity and reduced area availability under sea-level rise. These examples highlight the value of simple ecohydrological models in providing critical insights into sustainable environmental management. Ecohydrological engineering, therefore, offers promising avenues for developing innovative solutions that leverage the intricate connections between water and biota to address emerging challenges.

Alkalinization of natural waters by the dissolution of natural or artificial minerals is a promising solution to sequester atmospheric CO$_2$ and counteract acidification. Here we address the alkalinization carbon capture efficiency (ACCE) by deriving an analytical factor that quantifies the increase in dissolved inorganic carbon in the water due to variations in alkalinity. We show that ACCE strongly depends on the water pH, with a sharp transition from minimum to maximum in a narrow interval of pH values. We also compare ACCE in surface freshwater and seawater and discuss potential bounds for ACCE in the soil water. Finally, we present two applications of ACCE. The first is a local application to 156 lakes in an acid-sensitive region, highlighting the great sensitivity of ACCE to the lake pH. The second is a global application to the surface ocean, revealing a latitudinal pattern of ACCE driven by differences in temperature and salinity.

Incipient channelization in mountainous landscapes is often associated with the presence of first-order valleys at a regular wavelength under diverse hydroclimatic forcings. Here we provide a formal linear stability analysis of a landscape evolution model in detachment-limited erosion conditions to quantify the impact of the erosion law on the regular valley formation. The linear stability analysis is conducted for the unchannelized hillslope solutions along a long mountain ridge, where the perturbed equations constitute a third-order differential eigenvalue problem. The solutions to the posed eigenvalue problem are obtained by a spectral Galerkin technique with numerical quadrature. Results reveal the dependence of the erosion threshold and the emergent ridge/valley wavelength on the exponents in the power-law scaling coupling fluvial erosion with specific drainage area (m) and local slope (n). As the exponent m increases for a fixed n, the emergent valley spacing expands and the erosion limit for the first channel instability declines. Conversely, the erosion threshold for the first channelization rises with an increase in n at a particular value of m. We also show that predictions of the stability analysis conform with numerical simulations for different degrees of nonlinearity in the erosion mechanism and agree well with topographic data of a natural landscape.

Soil salinization represents one of the most widespread forms of land degradation worldwide, posing an enormous threat to food security, sustainable development, and ecosystem resilience across a wide range of climatic, hydrological, and socio-economic conditions. Globally, it already affects an area about the size of Continental United States (circa 1.1 Gha) and is expected to further intensify in response to climate change, sea-level rise (SLR), and increasing demand for crop production. Salt-affected soils are prevalent in arid regions, which are naturally prone to salt accumulation due to elevated evaporative demand and low precipitation input. However, salinity also plays a crucial role in regulating vegetation-climate interactions in highly carbon-intense tidal ecosystems - now threatened by SLR and coastal salinization. This contribution explores the hydrological controls of soil salinization across a wide range of temporal and spatial scales - ranging from the plant to the catchment scale. We focus, in particular, on how salinity affects the bi-directional interaction between vegetation and climate in both the dry land and tidal critical zone. Using both process-based models and observations, we unveil the central role of plant-salt tolerance in regulating soil-plant-atmosphere interactions and show how salinity acts as an aridity enhancer, able to exert major controls on vegetation-climate interactions in the critical zone.

In the predominantly oxic, upland soils, periods of high wetness trigger anaerobic processes such as iron (Fe) reduction within the soil microsites, with implications for organic matter decomposition, the fate of pollutants, and nutrient cycling. In fluctuating O conditions, Fe reduction is maintained by the re-oxidation of ferrous iron, which renews the electron acceptor, Fe, for microbial Fe reduction. To characterize such processes, it is fundamental to relate the redox cycling of iron between the two redox states to the hydro-climatic conditions. Here, we link iron cycling to soil moisture variability through a model of iron-redox dynamics and find the hydrologic regime that maximizes Fe reduction, under non-limiting organic carbon availability. Away from the optimal cycle, the duration of the oxic or the anoxic phase limits the regeneration of Fe or its reduction rate, respectively. We relate the average duration of the oxic and anoxic intervals to the frequency and mean depth of precipitation events that drive the dynamics of soil moisture, effectively linking iron cycling to the hydrologic regime. We then compare a tropical (Luquillo CZO) and a subtropical (Calhoun CZO) forest to provide insights into the soil moisture control on iron-redox dynamics in these ecosystems. The tropical site maintains a high potential for iron reduction throughout the year, due to quick and frequent transitions between oxic and anoxic conditions, whereas the subtropical site is strongly affected by seasonality, which limits iron reduction to winter and early-spring months with higher precipitation and lower evaporative demand.

In this comment, we clarify that the specific contributing area is defined in the limit of an infinitesimal contour length. We also confirm that not all solutions obtained from the minimalist landscape evolution model of Bonetti et al. (2020) are rescaled copies of each other because of the crucial role of the boundary conditions. We use dimensional analysis and numerical simulations for a case where only one horizontal length scale enters the physical law to establish this point.

Channel networks increase in complexity as the importance of erosion grows compared to diffusion by soil creep, giving rise to a channelization cascade. In this cascade, smaller channels join to form progressively larger ones with an alternation of ridges and valleys involving a multitude of wavelengths. Simulations of landscape evolution models and laboratory experiments are used to uncover the signature of such a cascade in the wavenumber spectrum of elevation fluctuations. Power spectra at intermediate distances from the boundaries are characterized by a peak wavenumber (the most energetic mode) that is related to the quasi-cyclic valleys superimposed on power-law scaling with exponent ($\alpha$) across a wide range of smaller scales. Dimensional analysis and self-similarity arguments are used to reveal the controlling factors on $\alpha$, showing that $\alpha$ is uniquely linked to the power-law relation (with exponent $m$) between erosion potential and the specific drainage area via $\alpha = 2m -3$.