In upland soils in humid climates, mineral stabilization of organic matter (OM) on millennial scales is often driven by the abundance of poorly crystalline, metastable chemical weathering products. Studies of volcanic ash soils have demonstrated that these metastable materials transform into increasingly crystalline minerals at advanced stages of weathering, so that the overall affinity of mineral surfaces for OM declines with time. However, the abundance of clay-sized (<2 𝜇m diameter) particles tends to increase with weathering, enhancing soil specific surface area (SSA) and potentially compensating for the loss of mineral affinity for OM. As a first step towards understanding the net effects of these simultaneous transformations on OM stabilization, we compared the coverage of SSA by OM in A and B horizons of ash-derived soils sampled along an elevation gradient in Veracruz, Mexico. N2 adsorption isotherms and Brunauer–Emmett–Teller (BET) theory were used to estimate SSA of bulk soil versus samples from which OM had been removed via combustion (muffling) and chemical oxidation (bleaching). In addition to comparing the effectiveness of the OM removal treatments, we characterized the extent to which the treatments altered the mineral matrix and introduced errors into the estimates of mineral SSA. Pore size distribution was estimated via density functional theory as a complement to the BET analysis. N2-accessible SSA ranged from 9 to 105 m2 g-1 after removal of OM, with muffling yielding higher values than bleaching for most samples. The probable loss of SSA associated with mineral transformations (e.g., of Fe oxides) at high temperatures during muffling was evidently offset by the more thorough removal of OM by that treatment. Although SSA tended to increase with weathering status, relative coverage of SSA by OM was relatively consistent across profiles and tended to be greater on average in A horizons (bleaching: 45% SSA covered, muffling: 51%) than in B horizons (bleaching: 28%, muffling: 34%). The apparent lack of OM coverage of SSA in the B horizon of the most weathered soil (0% of 60 m2 g-1 covered) underscores the overall importance of mineral reactivity in determining OM stabilization. Future work will extend these analyses to examine land-use effects on SSA coverage by OM.

Tropical montane cloud forests (TMCF) are recognized for their capacity to maintain high dry-season baseflow, and a host of other, ecosystem services. Despite their importance, they are endangered with a multidirectional array of land use changes, including conversion to pasture and crops such as coffee, while there are places where forest is being recovered. However, little is known about the effects of this complex dynamic on catchment hydrology. We investigated the effect of land use on rainfall-runoff response in five neighboring headwater micro-catchments in central Veracruz, Mexico, by comparing primary TMCF (PF), young (20 yr-old) and intermediate (40 yr-old) naturally regenerating TMCF (YF and IF, respectively), shaded coffee (SC), and an intensively grazed pasture (IP). We used a 4-year record of high-resolution rainfall and streamflow (10 min) data, collected from 2015 to 2019. These data were analyzed via hydrologic metrics and statistical tests. Results showed no statistical difference in the regulation capacity of high flows after 20 years of natural regeneration, compared to the PF. In terms of baseflow sustenance, our results suggested that PF and IF better promote this hydrological service than the other land uses, although all the catchments showed high mean annual low flows. SC exhibited a high capacity to modulate peak flows comparable to that of PF, and an intermediate capacity to sustain baseflow, suggesting that the integrated functioning of this catchment was largely preserved. Finally, we found that 40 years of pasture management can decrease the soil hydraulic properties in the area, causing a fivefold increase in the peak discharge response, and a much lower baseflow maintenance compared to PF.