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.

The Marcell Experimental Forest (MEF) in northern Minnesota, USA may be the longest running research and monitoring program on the hydrology of peatland catchments. The MEF sits astride a continental divide where the headwaters of the Mississippi, St. Lawrence, and Hudson Bay adjoin. When established in 1961, the MEF, with little topographic relief and large fractions of watersheds in peatlands, was distinct from the steep, mountainous catchments that typified other research catchments of the USDA Forest Service. This terrain and the presence of peatlands are representative of vast areas of the Northern Hemisphere, and the research program fills an important role in environmental monitoring and research in hydrology, ecology, biogeochemistry, and environmental change. During the 1960s, six research catchments were established and hydrological, meteorological, and water chemistry monitoring were initiated. Since then, the research and collaborations have proliferated to include new monitoring and ecosystem manipulations, with several paired-watershed studies, that allow the assessment of land management and environmental change effects on forests, water availability, and biogeochemical cycles. Research at the MEF remains vibrant, especially now that the site hosts a large-scale climate manipulation study (the SPRUCE Experiment). Herein, we present information on the site, contacts, long-term monitoring, experiments, and key findings.

Black ash (Fraxinus nigra Marsh) is the dominant hardwood species in many northern forested wetlands, especially in the Great Lakes Region. Black ash is subject to extremely high rates of mortality following the infestation of Emerald ash borer (EAB, Agrilus planipennis Fairmaire). Our research expands upon previous work examining the hydrologic impacts of EAB on black ash wetlands by examining changes in baseflow and response to precipitation using a paired watershed design. To simulate anticipated long-term impacts, all ash stems greater than or equal to 2.54 cm in diameter at breast height were felled and left on site. We hypothesize that 1) the treatment watershed will become more responsive to rainfall events and have higher water yield relative to pre-treatment conditions; and 2) chemical (dissolved organic carbon (DOC), total dissolved nitrogen (TDN), chloride, and sulfate) and isotopic (2H and 18O) tracers in stream water will show a reduced wetland water signature relative to precipitation and local upland groundwater. During the two-year pre-treatment period we observed median water yield to rainfall ratios of 0.033 and 0.022 on an event basis in the control and treatment watersheds, respectively. During the four-year post-treatment period the ratio was 0.013 (-62.3%) for the control watershed and 0.018 (-17.1%) for the treatment watershed. We did not observe an increase in treatment watershed responsiveness relative to the pre-treatment period as expected. However, we did observe a significantly smaller reduction in responsiveness in the treatment watershed relative to the control. Climatic differences and a shift in hydrologic regime in the pre- and post-treatment periods are the likely explanation for the decreased responsiveness of both watersheds to rainfall. Results also show that the relationship between DOC and TDN concentrations in stream water and wetland water were weaker following the treatment. The slope of the relationship between stream water and both wetland surface water and soil pore water in the treatment watershed was reduced by approximately 60% for both DOC and TDN. The relationship did not significantly change within the control watershed. These findings suggest that the loss of black ash will lead to greater responsiveness to rainfall events relative to undisturbed wetlands.