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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. [ABSTRACT FROM AUTHOR]

Watershed land use affects nutrient and sediment export, particularly through streambank erosion, which can add to P export and contribute to eutrophication in downstream waterbodies. We characterized P of soils from four different land uses (32 sites) along streams in the Missisquoi River basin (Vermont, USA)—silage corn (Zea mays L.), hay meadow, emergent wetlands, and forest—and their corresponding streambanks. We measured total P (TP), pH 4.8 NH4–acetate P, degree of P saturation (DPS), and soluble P. The latter three measurements were used as predictors of potential P bioavailability. Forest soils were relatively low in TP, whereas soils in corn, hay, and wetland were elevated (>1000 mg kg−1). With the exception of forests, the TP of the corresponding streambanks of each land use was statistically significantly lower than in the interior of the land use, while still higher than those in forests, suggesting a possible influence of land use on its adjacent streambank. The pH 4.8 NH4–acetate P was low in nonagricultural land uses and all streambanks of different land uses, but higher than optimum for soils in cornfields and hayfields. The DPS averaged 36% in the cornfields, but <21% in all of the streambanks. Mean soluble P was 0.14 mg kg−1 for corn‐ and hay‐associated streambanks with a DPS <10% but was as high as 3.2 mg kg−1 in the agricultural fields. The combination of low bioavailable P measurements indicates that most streambank soils are likely low contributors to P enrichment downstream. However, the elevated TP in some agricultural streambank soils suggests an accumulation of legacy P. Core Ideas: Soil P concentrations varied widely among four watershed land uses.Agriculture and wetland streambank soils had less P than adjacent land uses.Streambank soils from the four different land uses had low P release potential. [ABSTRACT FROM AUTHOR]

Movement of sediment, and associated phosphorus, from stream banks to freshwater lakes is predicted to increase with greater frequency of extreme precipitation events. This higher phosphorus load may accelerate harmful algal blooms in affected water bodies, such as Lake Champlain in Vermont, New York, and Québec. In the Mad River, a subwatershed in central Vermont's Lake Champlain Basin, extreme flooding from Tropical Storm Irene in 2011 caused extensive erosion. We measured stream channel change along the main stem between 2008 and 2011 by digitizing available prestorm and poststorm aerial imagery. Soils were sampled post Irene at six active stream erosion sites, using an experimental design to measure differences in soil texture and phosphorus both with depth (90 cm) and distance from the stream. In addition to total phosphorus (TP), we determined bioavailable (soil test) phosphorus (STP) and the degree of phosphorus saturation (DPS). The six sites represented a 0.87‐km length of stream bank that contributed an estimated 17.6 × 103 Mg of sediment and 15.8 Mg of TP, roughly the same as average annual watershed export estimates. At four sites, the STP and DPS were low and suggested little potential for short‐term phosphorus release. At two agricultural sites where the lateral extent of erosion was high, imagery showed a clear loss of well‐established riparian buffer. Present‐day near‐stream soils were elevated in STP and DPS. An increase in these extreme events will clearly increase sediment loads. There will also be increasing concentration of sediment phosphorus if stream banks continue to erode into actively managed agricultural fields. Plain Language Summary: Extreme precipitation events can cause streams to swell above their banks and erode near‐stream soil. Because stream bank soils contain phosphorus, increased erosion can increase the transport of phosphorus into lakes, leading to harmful algal blooms. Our study occurred in central Vermont along the Mad River, which flows into Lake Champlain. Tropical Storm Irene delivered a record amount of rainfall to much of Vermont. Soils were already wet and streams quickly flooded, causing drastic stream channel changes. We used aerial photography to measure the change in stream channel and calculated the amount of soil lost. We also sampled soil at six erosion sites, measuring various forms of phosphorus. These measurements suggest that one extreme event delivered as much sediment and phosphorus into the stream as is normally exported over an entire year. Bioavailable phosphorus was low in most soil samples. However, high erosion along two agricultural fields removed large trees that formed a buffer with the stream and pushed present‐day stream banks well back into the farmland. These soils had elevated amounts of bioavailable phosphorus and would likely contribute more toward downstream eutrophication if erosion continues. The effect of this one extreme event will continue to be felt for years to come. Key Points: Extreme event‐induced stream bank movement along six eroding sites generated sediment loads equivalent to average annual watershed exportsPhosphorus loads were also high, but short‐term phosphorus release may be limited because of low bioavailability in the eroded sedimentStripping of riparian buffers and lateral bank retreat into actively managed agricultural fields may enhance future phosphorus losses [ABSTRACT FROM AUTHOR]

Emerging evidence shows that interactions between soils and subsurface flow paths contribute to spatial variations in stream water chemistry in headwater catchments. However, few have yet attempted to quantify chemical variations in soils at catchment and hillslope scales. Watershed 3 (WS3) at Hubbard Brook Experimental Forest, New Hampshire, USA, was studied in order to better understand pedogenesis and its relationship to subsurface water dynamics. In WS3, 99 soil profiles were described, sampled by horizon, and assigned to a hydropedologic unit (HPU), a functional classification previously developed using landscape and morphological metrics which corresponded with distinct water table regimes. Soil samples were extracted with 1) citrate-dithionite (d) and analyzed for Fe d and Mn d ; and 2) acid ammonium oxalate (o) and analyzed for Al o , Fe o and the rare earth elements La o , Ce o , and Pr o . Total organic C was also measured. These elements were redistributed via vertical and lateral podzolization. Typical (horizontally layered) podzols developed in the majority of the catchment due to predominantly vertical, unsaturated flow. However, lateral flow produced four other podzol types with distinct chemistry; thicker spodic horizons of laterally accumulating soils generally reflected larger pools of trace metals and subsoil organic C. The spatial distribution of positive cerium-anomalies (Ce/Ce*) in soil profiles proved to be a consistent hydropedologic indicator of lateral flow and seasonally high water table in three hillslopes. Despite occasional high water table in some of the HPUs, they were not hydric soils and were distinct from wetter podzols of coastal plains due to their high Fe content. This study suggests that vertical and lateral spatial variation in soil chemical composition, including the complexity of Ce distribution, as it relates to subsurface water dynamics should be considered when studying or predicting catchment scale functions such as stream solute export and biogeochemical processes. [ABSTRACT FROM AUTHOR]

Measurements of soil water potential and water table fluctuations suggest that morphologically distinct soils in a headwater catchment at the Hubbard Brook Experimental Forest in New Hampshire formed as a result of variations in saturated and unsaturated hydrologic fluxes in the mineral soil. Previous work showed that each group of these soils had distinct water table fluctuations in response to precipitation; however, observed variations in soil morphology also occurred above the maximum height of observed saturation. Variations in unsaturated fluxes have been hypothesized to explain differences in soil horizon thickness and presence/absence of specific horizons but have not been explicitly investigated. We examined tensiometer and shallow groundwater well records to identify differences in unsaturated water fluxes among podzols that show distinct morphological and chemical differences. The lack of vertical hydraulic gradients at the study sites suggests that lateral unsaturated flow occurs in several of the soil units. We propose that the variations in soil horizon thickness and presence/absence observed at the site are due in part to slope-parallel water flux in the unsaturated portion of the solum. In addition, unsaturated flow may be involved in the translocation of spodic material that primes those areas to contribute water with distinct chemistry to the stream network and represents a potential source/sink of organometallic compounds in the landscape. [ABSTRACT FROM AUTHOR]

Classical podzolization studies assumed vertical percolation and pedon-scale horizon development. However, hillslope-scale lateral podzolization also occurs where lateral subsurface water flux predominates. In this hydropedologic study, 99 podzols were observed in Watershed 3, Hubbard Brook experimental forest, new Hampshire. Soil horizon samples were extracted with citrate--dithionite (d) and acid ammonium oxalate (o) to quantify fed, Mnd, Alo, and Feo. Optical density of oxalate extract (ODOe) was measured to assess spodic c. Amorphous organometallic complexes (AOC) were observed in thin section, for which Al, Fe, Mn, and c were quantified using scanning electron microscopy--energy dispersive X-ray spectroscopy. Porosity and AOC/mineral ratio were calculated for thin section images using ImageJ. Laterally developed spodic horizons were twice as thick as vertically developed spodic horizons and contained higher concentrations of Al and Mn but lower fe and c. Vertically developed spodic horizons had crumb microstructure with higher porosity, while laterally developed spodic horizons were more infilled. Aluminum + 0.5Fe and ODOE in the surface of laterally developed podzols were high and lacked contrast with the spodic horizon, making spodosol classification problematic. Vertically developed spodic horizons form by solutional translocation and precipitation of AOC under unsaturated conditions. However, laterally developed spodic horizons could form via lateral translocation of solutes or physical transport and deposition of colloidal AOC with unsaturated or saturated flow. This study demonstrates the importance of lateral podzolization in producing soils with distinctive morphology, composition, and classification. future studies or mapping efforts in podzolized catchments should incorporate these different pedogenic processes. [ABSTRACT FROM AUTHOR]

Abstract: Soil nitrification rates in northeastern USA forested ecosystems appear to be regulated by a number of factors and are likely influenced by continuing N deposition. Among other factors, rates across small watersheds have been found to relate to the soil C/N ratio and tree species composition. We measured potential net nitrification rates in the Ranch Brook watershed, a relatively large (9.6km2) forested basin in north central Vermont, to determine if relationships found in smaller catchments were evident at a larger scale. The stream network was divided into eight reaches to determine the variability within the watershed. Sampling points (6–15 along each reach, total of 74) were established along transects that paralleled the major watershed tributaries. At each point, we measured net rates of nitrification and ammonification in the uppermost humified soil horizon (Oa or A), using a one-day lab incubation. The basal area and density of all tree species were measured in a 10-m radius plot, along with a number of topographic metrics such as slope, aspect and elevation. In a stepwise regression, 39% of the variability in net nitrification rates was explained by the density of Picea rubens, elevation and the thickness of the forest floor. When net nitrification rates were normalized to soil C concentration, 60% of the variability was explained by soil N concentration, C/N ratio, elevation and the density of P. rubens. The significant negative influence of P. rubens density, and not basal area, was consistent with a previous cross-site study of 10 smaller northeastern USA watersheds. No influence of sugar maple basal area or density was found. Other relationships, similar to those found in smaller watersheds, were net nitrification rates predicted by the fraction of inorganic N as , net nitrification predicted by the C/N ratio and the C/N ratio predicted by tree species. The consistent influence of tree species on potential net nitrification rates demonstrates a role for future forest management in influencing ecosystem processes. [Copyright &y& Elsevier]

Stable isotope signatures in soil-derived N03 provide an opportunity for environmental source tracing, but rapid changes in N transformation rates caused by sampling disturbance may create an artifact in the signature of extracted N03. To study this, we measured net nitrification and ammonification rates and the ~`5N and ~18Q of N03 in soils from a water- shed in Camels Hump State Forest, Vermont, known to be sensitive to sampling disturbance. Eleven Oa and one A horizon samples (C/N ratio 15-25, pH 3.3-4.2) had N03 extracted for isotope analysis approximately 30 mm after sampling and again after a 2- to 3-d incuba- tion period at 10°C. Net nitrification rates during incubation were rapid and linear (0.9- 15.3 pmol L1 soil h'), with increases in N03 detectable within 1 h after sampling. The ~15N and ~18O of soil N03 changed dramatically between field extraction and extraction after the incubation period. Soils that were initially relatively enriched in 615N03 became more depleted after incubation, as much as l3.5%o lighter. Soils that were initially relatively depleted in 6'5N03 became more enriched after incubation, as much as 16.5%o heavier. The latter had high net nitrification rates and low final NH4~ concentrations, probably caus- ing the enrichment of 15N in N03 due to a diminishing substrate pool. The 6180 of soil N03 showed little change during incubation and changes were not significantly related to changes in the 15N of N03, suggesting that denitrification was not a primary mechanism. Added enriched 15NH4CI (11 l.9%o) was rapidly incorporated into the soil N03 pool in a pattern supportive of a mechanism due to increased nitrification rates. Soil sampling disturbance can dramatically alter the isotopic signature of soil N03, and the isotopic signature of extracted N03 may not be a reliable environmental tracer. [ABSTRACT FROM AUTHOR]

Details a series of experiments illustrating the extent of net nitrification increases in the Brush Brook watershed in Vermont. Small-scale impact disturbance in the field resulting in a tripling of nitric oxide concentrations after a two-week incubation; Ammonium concentrations increasing over the first ten hours.

Abstract: Upland headwater catchments, such as those in the Appalachian Mountain region, are typified by coarse textured soils, flashy hydrologic response, and low baseflow of streams, suggesting well drained soils and minimal groundwater storage. Model formulations of soil genesis, nutrient cycling, critical loads and rainfall/runoff response are typically based on vertical percolation, development of soil horizons parallel to the land surface, mineral weathering inputs limited to the rooting zone and drainage from lumped catchment reservoirs (e.g., the subsoil) as the dominant source of stream flow. However, detailed study of the hydrologic reference catchment at Hubbard Brook Experimental Forest, NH, USA shows striking spatial patterns of soil development that reflect the influence of transient water tables within the solum in nearly all landscape positions. Shallow bedrock and variably low hydraulic conductivity in the subsoil promote lateral flow and development of soil horizons along hillslope flowpaths rather than in vertical profiles. We distinguished several morphologic units based on the presence of diagnostic horizons indicative of differing patterns of podzolization and carbon storage. The distribution of soils appears to be highly dependent on local drainability and frequency and duration of transient saturation within the solum. As such, monitoring of hydropedologic groups and transient water table fluctuations may prove to be a sentinel for the effects of climate change on spatial distribution of soils and retention/release of solutes from upland catchments. [Copyright &y& Elsevier]