Mercury is a soil pollutant of widespread concern, usually derived from airborne deposition. A long–term forest soil monitoring program began in 2002 at five sites (elevation 590–1140 m) in Vermont, USA. Total mercury (THg) and soil organic carbon (SOC) were assessed in the uppermost humified soil layer, either an Oa or A horizon. Sampling occurred every 5 years from ten subplots at each site. Tree species ranged from deciduous, mixed deciduous/conifer, to all conifer. After five samplings, the site means for THg ranged from 167 to 447 µg kg–1 and increased linearly with elevation (R2 = 0.78). One site had a significant temporal increase in THg of 3.9 µg kg–1 yr–1 (p = 0.02). Overall SOC site means ranged from 133 to 434 g kg–1. One site showed a significant temporal increase in SOC (p = 0.01). The mean ratio of THg:SOC was similar at four of the sites (1318–1518 µg kg–1) but much lower (648 µg kg–1) at the site with the highest SOC and the temporal increase in THg. Within individual subplots, THg increased up to a maximum SOC concentration of 320 g kg–1 and usually decreased above that threshold. Although both wet and dry mercury deposition in the northeast USA have declined, no evidence of declining soil THg concentration was found, likely due to strong retention by SOC. Continued monitoring is essential considering ongoing changes in deposition and future changes in the source–sink balance of mercury.

Hydrologic behavior and soil properties across forested landscapes with complex topography exhibit high variability. The interaction of groundwater with spatially distinct soils produces and transports solutes across catchments, however, the spatiotemporal relationships between groundwater dynamics and soil solute fluxes are difficult to directly evaluate. While whole-catchment export of solutes by shallow subsurface flow represents an integration of soil environments and conditions but many studies compartmentalize soil solute fluxes as hillslope vs. riparian, deep vs. shallow, or as individual soil horizon contributions. This potentially obscures and underestimates the hillslope variation and magnitude of solute fluxes and soil development across the landscape. This study determined the spatial variation and of shallow soil base cation fluxes associated with weathering reactions (Ca, Mg, and Na), soil elemental depletion, and soil saturation dynamics in upland soils within a small, forested watershed at the Hubbard Brook Experimental Forest, NH. Base cation fluxes were calculated using a combination of ion-exchange resins placed in shallow groundwater wells (0.3 - 1 m depth) located across hillslope transects (ridges to lower backslopes) and measurements of groundwater levels. Groundwater levels were also used to create metrics of annual soil saturation. Base cation fluxes were positively correlated with soil saturation frequency and were greatest in soil profiles where primary minerals were most depleted of base cations (i.e., highly weathered). Spatial differences in soil saturation across the catchment were strongly related to topographic properties of the upslope drainage area and are interpreted to result from spatial variations in transient groundwater dynamics. Results from this work suggest that the structure of a catchment defines the spatial architecture of base cation fluxes, likely reflecting the mediation of subsurface stormflow dynamics on soil development. Furthermore, this work highlights the importance of further compartmentalizing solute fluxes along hillslopes, where certain areas may disproportionately contribute solutes to the whole catchment. Refining catchment controls on base cation generation and transport could be an important tool for opening the black box of catchment elemental cycling.

Field methods used for collecting data under the Vermont Long-Term Soils Monitoring Program from 2002, 2007, 2012, 2017 and 2022. Includes soil description field form.

The long-term effect of non-native earthworm species on forest soil carbon storage is not clear. While initial invasion into earthworm-free soils stimulates carbon losses, there is evidence that carbon stabilization in soil aggregates is enhanced. Fourteen managed forest sites throughout Vermont were sampled to identify and enumerate earthworms, and determine soil aggregate distribution and physically protected carbon. Most of these sites were northern hardwoods reforested in the mid-20th century after agricultural land use. Soil at six 50 x 50 cm subplots at each site was sampled to a depth of 20 cm below the Oe horizon and the mineral soil fractionated into free microaggregates (<250 mu m), small macroaggregates (250-2000 mu m), and large macroaggregates (>2000 mu m). Microaggregates were then isolated from the macroaggregates and organic carbon determined. Mean earthworm numbers below the Oe horizon at each site ranged from 0 to 313 individuals m(- 2) and ten different species were identified, representing all earthworm ecotypes. Four sites had no earthworms in the soil layer sampled, three sites had earthworms in every subplot with up to six different species, and seven sites had subplots both with and without earthworms. When subplots were grouped by number of species present, there was a trend towards a greater fraction of the mineral soil in macroaggregates containing greater protected carbon, and a lower fraction of free microaggregates. A paired t-test of subplots from the seven sites with variable earthworm presence showed significant differences in the same trends and there was significantly greater mineral soil total carbon and protected carbon in the subplots with earthworms. These results are consistent with recent research into earthworm effects on soil carbon stabilization. While initial invasion likely resulted in a negative carbon balance, our findings suggest that earthworms can enhance transfer of carbon into physically protected pools in these forest soils.

Mineral weathering is an important soil-forming process driven by the interplay of water, organisms, solution chemistry, and mineralogy. The influence of hillslope-scale patterns of water flux on mineral weathering in soils is still not well understood, particularly in humid postglacial soils, which commonly harbor abundant weatherable primary minerals. Previous work in these settings showed the importance of lateral hydrologic patterns to hillslope-scale pedogenesis. In this study, we hypothesized that there is a corresponding relationship between hydrologically driven pedogenesis and chemical weathering in podzols in the White Mountains of New Hampshire, USA. We tested this hypothesis by quantifying the depletion of plagioclase in the fine fraction (≤2 mm) of closely spaced, similar-age podzols along a gradient in topography and depth to bedrock that controls lateral water flow. Along this gradient, laterally developed podzols formed through frequent, episodic flushing by upslope groundwater, and vertically developed podzols formed through characteristic vertical infiltration. We estimated the depletion of plagioclase-bound elements within the upper mineral horizons of podzols using mass transfer coefficients (τ) and quantified plagioclase losses directly through electron microscopy and microprobe analysis. Elemental depletion was significantly more pronounced in the upslope lateral eluvial (E horizon-dominant) podzols relative to lateral illuvial (B horizon-dominant) and vertical (containing both E and B horizons) podzols downslope, with median Na losses of ∼74 %, ∼56 %, and ∼40 %, respectively. When comparing genetic E horizons, Na and Al were significantly more depleted in laterally developed podzols relative to vertically developed podzols. Microprobe analysis revealed that ∼74 % of the plagioclase was weathered from the mineral pool of lateral eluvial podzols, compared to ∼39 % and ∼23 % for lateral illuvial podzols and vertically developed podzols, respectively. Despite this intense weathering, plagioclase remains the second most abundant mineral in soil thin sections. These findings confirm that the concept of soil development as occurring vertically does not accurately characterize soils in topographically complex regions. Our work improves the current understanding of pedogenesis by identifying distinct, short-scale gradients in mineral weathering shaped by local patterns of hydrology and topography.

Harvesting activities are known to decrease forest floor carbon pools, but the response varies with harvest intensity. We examined partial harvesting (33–55% of basal area removed) effects on the forest floor at four northern hardwood sites in Vermont, USA. Six baseline quantitative samples were taken at each site and 9–36 new locations were sampled 1.5–2.6 years after harvesting. Forest soil disturbance was estimated, and basal area was tallied pre- and post-harvest. The forest floor consisted primarily of Oi and Oe horizons. The pre-harvest site means in carbon stock ranged from 6.8 to 12.3 Mg ha-1 and were not significantly different after harvesting. The pre-harvest site means in depth ranged from 2.8 to 4.5 cm and, post-harvest, there was significantly decreased thickness at one site and significantly greater density at two sites postharvest. This compaction was also visually observed in the field. Partial harvesting, which included single-tree and group selection, created highly variable conditions that challenged our experimental design. However, the two sites with the higher number of resampling locations (35–36) had relatively low variability in forest floor metrics and showed significant responses in thickness and density. Continued monitoring is needed to determine long-term trends.

Sequestration and storage of organic carbon (C) in soil is an essential component of climate change mitigation and fundamental in promoting the health and climate resilience of soils. Sources of available soil C data are increasing, which complicates efforts to consolidate the data in forms that can be readily used by stakeholders. Spatial and temporal gaps in data availability also limit the quantification of changes in soil C through space and time. Improved coordination among producers and users of soil C data would provide data compatibility at the spatial and temporal resolution required for C monitoring, accounting, and verification of policy implementation. These challenges can be addressed by forming regional-scale networks to coordinate the collection and use of soil C data by promoting consistency in methods, collecting new data to fill critical gaps, integrating existing data from multiple sources, and providing data interpretation to stakeholders in readily usable forms. Forming networks in regions such as the Northeastern United States would require close coordination with existing programs that are involved in collecting or aggregating soil C within that region. Network formation could be accomplished by (1) producing a planning document, (2) designing a network structure tailored to the region, and (3) acquiring the institutional support to establish and operate the network. Increasing the availability and usage of soil C data through regional networks would support the development of climate change solutions and increased ecosystem services through land management efforts that increase soil C storage.

In soils with a neutral reaction, the metal cations calcium, magnesium, potassium and sodium are electrostatically attracted to soil particles, described as an electric double layer (EDL) of positively charged cations and negatively charged surfaces. In soils dominated by permanently charged clay minerals, cations in the EDL are rapidly and reversibly exchanged with cations in the bulk solution, as described by selectivity constants. In soils low in clay minerals, the major source of negative charge is soil organic matter and variably charged metal oxides. In acidic organic and forest soils, cation exchange between aluminum and hydrogen ions is an important pH buffering process.

In headwater catchments, surface groundwater discharge areas have unique soil biogeochemistry and can be hot spots for solute contribution to streams. Across the northeastern United States, headwater hillslopes with surface groundwater discharge were enriched in soil Mn, including Watershed 3 of Hubbard Brook Experimental Forest, New Hampshire. Soils of this site were investigated along a grid to determine extent of Mn-rich zone(s) and relationships to explanatory variables using ordinary kriging. The O and B horizons were analyzed for total secondary Mn and Fe, Cr oxidation potential, total organic C, moisture content, wetness ratio, and pH. Two Mn hot spots were found: a poorly drained, flowing spring (Location A); and a moderately well-drained swale (Location B). Both had similar to 6,000-9,000 mg Mn kg(-1) soil. However, Location A had high Cr oxidation potential (a measure of Mn reactivity), whereas Location B did not. Location C, a poorly drained seep with slow-moving water, had lower Mn content and Cr oxidation potential. Manganese-rich soil particles were analyzed using X-ray absorption near-edge structure and micro-X-ray diffraction; the dominant oxidation state was Mn(IV), and the dominant Mn oxide species was a layer-type Mn oxide (L-MnO2). We propose input of Mn(II) with groundwater, which is oxidized by soil microbes. Studies of catchment structure and response could benefit from identifying hot spots of trace metals, sourced mainly from parent material but which accumulate according to hydropedologic conditions. Small-scale variation in Mn enrichment due to groundwater and microtopography appears to be more important than regional-scale variation due to air pollution.

Tile drainage (TD) has been identified as a potential non-point source of phosphorus (P) pollution and subsequent water quality issues. Three fields with TD in Vermont USA were monitored to characterize hydrology and P export. Fields were in corn silage and used minimal tillage and cover cropping practices. Preferential flow path (PFP) activity was explored by separating TD flow into flow pathway and source connectivity components using two hydrograph separation techniques, electrical conductivity end member unmixing, and hydrograph recession analysis. TD was the dominant P export pathway because of higher total discharge. Drought conditions during this study limited surface runoff, and possibly resulted in maximum PFP activity in the active clay soils. The non-growing season dominated annual P loading for two of the three study years. Peak P concentrations in TD occurred during events following manure injection in the fall, as well as in the spring post cover crop termination and post-planting. Intra-event analysis of rainfall pulses showed that TD flow and P concentrations were higher because of higher intensity pulses. This study highlights the impacts of current manure management, as well as the potential for climate change to increase P transport to TD.

Wetlands are valuable ecosystems because they are highly productive, support a wide range of wildlife, and serve as hotspots for biogeochemical cycling. Historically, vast areas of wetlands in the United States (US) were drained and converted to agriculture. Efforts are currently underway to restore wetland and floodplain functioning across the US and elsewhere. Re-wetting historically drained and farmed soils can potentially liberate legacy phosphorus (P) to surface waters as soluble reactive P (SRP), offsetting P retained by sedimentation during floods. A better understanding of the controls on SRP release is needed to estimate net P retention in these settings. Soil P saturation ratio (PSR) and soil P storage capacity (SPSC) are two proxies for SRP runoff risk that have shown promise for characterizing restored wetlands but require further testing. In this study, we examined soils at 42 riparian sites ranging from active farms to mature wetlands in the Vermont portion of the Lake Champlain Basin (USA), where phosphorus load reduction is a critical goal to achieve in-lake water quality targets. We additionally quantified potential SRP release to overlying water using intact soil cores from 20 plots spanning 14 sites. Final SRP concentrations in intact cores spanned two orders of magnitude and were predicted well by SPSC and PSR. SRP release was greatest at more recently and frequently farmed sites. Several soil properties, including PSR and SPSC, were correlated with farming frequency and time since farming, indicating that SRP release could be mapped using existing geodata for soils, hydrology and land use. Our findings confirm that soil SRP release during flooding needs to be considered in estimates of net P balance for restored riparian wetlands in agricultural landscapes.

Phosphorus (P) loss from cropland to ground and surface waters is a global concern. In cold climates (CCs), freeze–thaw cycles, snowmelt runoff events, and seasonally wet soils increase P loss potential while limiting P removal effectiveness of riparian buffer zones (RBZs) and other practices. While RBZs can help reduce particulate P transfer to streams, attenuation of dissolved P forms is more challenging. Moreover, P transport studies often focus on either cropland or RBZs exclusively rather than spanning the natural cropland–RBZ–stream gradient, defined here as the cropland–RBZ–stream continuum. Watershed P transport models and agronomic P site indices are commonly used to identify critical source areas; however, RBZ effects on P transport are usually not included. In addition, the coarse resolution of watershed P models may not capture finer-scale soil factors affecting P mobilization. It is clear that site microtopography and hydrology are closely linked and important drivers of P release and transport in overland flow. Combining light detection and ranging (LiDAR) based digital elevation models with P site indices and process-based models show promise for mapping and modeling P transport risk in cropland-RBZ areas; however, a better mechanistic understanding of processes controlling mobile P species across regions is needed. Broader predictive approaches integrating soil hydro-biogeochemical processes with real-time hydroclimatic data and risk assessment tools also hold promise for improving P transport risk assessment in CCs.

Forest soils store a globally important pool of carbon (C) and reforestation has the potential to increase this pool. Past land use, tree species composition, and current management are known to affect carbon storage rates and amounts. Another important factor, especially in the vertical distribution of soil C, is the presence or absence of earthworms. We investigated these interactions in eighteen managed forest sites in Vermont that have varied land use history (cultivation, pasture, woodlot, continuous woodland) and a range of time since reforestation from 60 to 100+ years. Soils were sampled to about 1 m, or shallower if bedrock was reached, and both C and exchangeable cation pools quantified. Earthworms were enumerated to a depth of 20 cm and identified by species. Basal area by tree species was also measured and aboveground C calculated. Detailed land use history was documented using archival sources. Forest type was primarily northern hardwood, with a few lowland spruce-fir sites included. Eleven different species of earthworms were found, representing all ecotypes, and numbers ranged from 0 (at 8 of the 18 sites) to 319 individuals/m2. Carbon pools in the forest floor (1.5?30.1 Mg/ha) were greater with higher site elevation (range of 154?651 masl) and also greater with lower numbers of earthworms. Besides elevation, the best predictor of earthworm presence or absence, and overall numbers, was the soil exchangeable calcium (Ca) pool in the Oa and A horizons (range of 9.4?1288 cmolc/m2). Because soil exchangeable Ca and aluminum (Al) pools were inversely related, higher exchangeable Al was associated with lower earthworm numbers. In the mineral soil, the C pool (82.1?210.8 Mg/ha) was best explained by the thickness of the B horizons with the exchangeable Al pool as a positive, secondary factor. The thickness of the A horizon (0?16.3 cm), and its contribution to the entire profile C pool (0?54.7 Mg/ha), decreased relative to past land use in the order: cultivated < pasture < woodlot. No earthworm metric was strongly related to the C pool in either the mineral soil or the full soil profile. Dramatic differences in the vertical distribution of carbon were related to past land use and earthworm presence, which in turn was negatively correlated with elevation and the presence of conifers. While it is clear that earthworms and prior land-use have a long-lasting effect on the forest floor C pool, it is less clear whether or not there is an effect on full profile C sequestration over time.

Land use can significantly alter soil P forms, which will influence P loss in runoff. Organic P (Po ) compounds are an important component of soil P, but their forms and cycling in soils with different land uses are still poorly understood. In addition, streambanks are potential sources of P loss; P forms and concentrations in streambank soils may vary with land use, affecting potential P loss to water. This study used solution 31 P nuclear magnetic resonance spectroscopy to characterize and quantify P in interior and streambank soils (0-10 cm) under duplicate sites from four different land uses along streams in the Missisquoi River basin (VT, USA): silage corn, hay meadow, emergent wetlands, and forest. Orthophosphate monoesters were the dominant P compound class regardless of land use or landscape position. Forest soils had the lowest Po concentrations, less labile P forms than other soils, and significantly lower concentrations of total inositol hexakisphosphates and total orthophosphate monoesters compared with corn soils. Riparian buffer zones for agricultural soils lowered P concentrations in streambank soils for many soil P pools relative to interior soils. The wetland soils of this study had P concentrations and P forms that were similar to those for interior agricultural soils and generally showed no reduction in P concentrations in streambank soils relative to interior soils. This is consistent with the role of wetlands as P sinks in the landscape but also suggests these wetlands should be carefully monitored to minimize P accumulation, especially in streambank soils.

Long-term monitoring of forest soils is necessary to understand the effects of continued environmental change, including climate change, atmospheric deposition of metals, and, in many regions, recovery from acidic precipitation. A monitoring program was initiated in 2002 at five protected forest sites, primarily Spodosol soils, in Vermont, northeastern USA. Every 5 years, ten soil pits were sampled from random subplots in a 50 × 50-m plot at each site. Samples were taken by genetic horizon and, to reduce variability and improve comparability, from four specific layers: the combined Oi/Oe layer, the combined Oa/A layer, the top 10 cm of the B horizon, and 60–70 cm below the soil surface (usually the C horizon). The samples were archived and a subset analyzed for carbon, nitrogen, and exchangeable cations. After four sampling campaigns, the average coefficients of variation (CVs) at each site had a broad range, 10.7% for carbon in the Oa/A horizon to 84.3% for exchangeable Ca2+ in the B horizon. An investigation of variability within the upper 10 cm of the B horizon across a 90-cm soil pit face showed similar CVs to the entire site, emphasizing the need for consistent and careful sampling. After 15 years, temporal trends were significant in the Oa/A and B horizons at two of the five sites, with one site showing an increase in carbon concentration in both layers along with increases in both exchangeable Ca2+ and Al3+ in the B horizon, perhaps linked to recovery from acidification. The monitoring program plans to continue at 5-year intervals for the next century.

The anthropogenic loading of phosphorus (P) to water bodies continues to increase worldwide, in many cases leading to increased eutrophication and harmful algal blooms [...]

Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ18O and δ15N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.